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Carbonate and Phosphate Precipitation byChromohalobacter marismortuiMaría Angustias Rivadeneyra a; Agustín Martín-Algarra b; Antonio Sánchez-Navasc; Daniel Martíin-Ramos ca Departamento de Microbiología, Facultad de Farmacia, Universidad de Granada,Campus Universitario de Cartuja, 18071, Granada, Spainb Departamento de Estratigraía y Paleontología, Facultad de Ciencias, Universidadde Granada, Campus Universitario de Fuentenueva, 18071, Granada, Spainc Departamento de Mineralogía y Petrología, Facultad de Ciencias, Universidad deGranada, Campus Universitario de Fuentenueva, 18071, Granada, Spain

Online Publication Date: 01 March 2006To cite this Article: Rivadeneyra, María Angustias, Martín-Algarra, Agustín,

Sánchez-Navas, Antonio and Martíin-Ramos, Daniel (2006) 'Carbonate and Phosphate Precipitation byChromohalobacter marismortui', Geomicrobiology Journal, 23:2, 89 - 101To link to this article: DOI: 10.1080/01490450500533882URL: http://dx.doi.org/10.1080/01490450500533882

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© Taylor and Francis 2007

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Geomicrobiology Journal, 23:89–101, 2006

Copyright c© Taylor & Francis Group, LLC

ISSN: 0149-0451 print / 1521-0529 online

DOI: 10.1080/01490450500533882

Carbonate and Phosphate Precipitation byChromohalobacter marismortui

Marıa Angustias RivadeneyraDepartamento de Microbiologıa, Facultad de Farmacia, Universidad de Granada, Campus Universitario

de Cartuja, 18071-Granada, Spain

Agustın Martın-AlgarraDepartamento de Estratigrafıa y Paleontologıa, Facultad de Ciencias, Universidad de Granada,

Campus Universitario de Fuentenueva, 18071-Granada, Spain

Antonio Sanchez-Navas and Daniel Martın-RamosDepartamento de Mineralogıa y Petrologıa, Facultad de Ciencias, Universidad de Granada,

Campus Universitario de Fuentenueva, 18071-Granada, Spain

The ability of Chromohalobacter marismortui to precipitate car-bonate and phosphate minerals has been demonstrated for the firsttime. Mineral precipitation in both solid and liquid media at dif-ferent salts concentrations and different magnesium/calcium ratiosoccurred whereas crystal formation was not observed in the control.The precipitated minerals were studied by X-ray diffraction, scan-ning electron microscopy and EDX, and were different in liquidand solid media. In liquid media aragonite, struvite, vaterite andmonohydrocalcite were precipitated forming crystals and bioliths.Bioliths accreted preferentially close to organic pellicles, whereasstruvite preferentially grows in microenvironments free of such pel-licles. Magnesian calcite, calcian-magnesian kutnahorite, “proto-dolomite” and huntite were formed in solid media. The Mg contentof the magnesian calcite and of Ca-Mg kutnahorite also varied de-pending on the salt concentration of the culture media. This is thefirst report on bacterial precipitation of Ca-Mg kutnahorite andhuntite in laboratory cultures. The results of this research showthe active role played by C. marismortui in mineral precipitation,and allow us to compare them with those obtained previously usingother taxonomic groups of moderately halophilic bacteria.

Keywords biomineralization, aragonite, huntite, Ca-Mg kutnahorite,magnesian calcite, monohydrocalcite, struvite, vaterite,C. marismortui

Received 22 March 2005; accepted 9 August 2005.We acknowledge A. Gonzalez-Segura (CIC, Univ. of Granada) for

her help along different phases of the SEM laboratory work. We thankthe critical reading of two anonymous reviewers for their commentsand suggestions. Financial support for this research was provided bythe Spanish Project BTE 2001-2852 MEC-CICYT, and by the ResearchGroup No.164 (4089) of the Junta de Andalucıa.

Address correspondence to Agustın Martın-Algarra, Departamentode Estratigrafıa y Paleontologıa, Facultad de Ciencias, Campus Univer-sitario de Fuentenueva, 18071, Granada, Spain. E-mail: agustin@ugr.es

INTRODUCTION

It is widely accepted that microorganisms, mainly bacte-

ria, contribute to precipitation of a wide variety of minerals,

including carbonates, phosphates, sulphides, oxides and sili-

cates (Ehrlich 2002). This is confirmed by numerous laboratory

studies, which demonstrate precipitation of different minerals

in bacterial cultures (Beavon and Heatley 1962; Morita 1980;

Ferris et al. 1991; Castanier et al. 1999; Rivadeneyra et al. 1983,

1985, 1992a, 1992b, 1998, 2004; Van Lith et al. 2003), and also

by observation of fossil and recent microbial accretions (stro-

matolites, thrombolites, mud mounds) and of some allochems

(pisoids, oncoids, ooids, peloids, spherulites, etc.) of diverse

mineralogy commonly found in sediments, from continental to

shallow marine to deep marine environments (Buczynski and

Chafetz 1991; Chafetz and Buczynski 1992; Martın-Algarra and

Sanchez-Navas 2000; Sanchez-Navas and Martın-Algarra 2001;

Braissant et al. 2003; Cailleau et al. 2005).

Several mechanisms have been proposed for carbonate pre-

cipitation by bacteria in natural habitats (Castanier et al. 1999;

Ehrlich 2002). Nevertheless, the exact role of bacteria in biomin-

eralization is not known in many cases, and some controversy

exists about if they play a passive or an active role, or about if

they can directly influence, or not, the mineralogy of precipi-

tates (Reitner 1993; Peckman et al. 1999; Bosak et al. 2004).

In many cases, the optimal conditions for bacterial precipita-

tion of minerals are not exactly known, and precipitation can

be certainly influenced by abiogenic factors. Among them salt

concentration and ionic composition of the medium are the most

important factors for carbonate precipitation (Rivadeneyra et al.

1985; Ferrer et al. 1988; Fernandez-Dıaz et al. 1996) but also

fluid chemistry and fluid flow (Gonzalez et al. 1992) or viscosity

(Buczinski and Chafetz 1991). Moderately halophilic bacteria

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90 M. A. RIVADENEYRA ET AL.

are microorganisms highly useful to determine how the ionic

composition of the environment affects the bacterial precipi-

tation of minerals, because they grow under widely changing

saline concentrations. Previous studies found significant differ-

ences in the mineralogy of the precipitates by different, moder-

ately halophilic bacteria (Ferrer et al. 1988; Rivadeneyra et al.

1993, 1998, 2004). A sound investigation of these differences

could help to further our knowledge regarding the mechanisms

of formation of these biominerals and the role of bacteria in car-

bonate and phosphate precipitation either in laboratory cultures

or in natural habitats.

Chromohalobacter marismortui (Ventosa et al. 1989) is a

moderately halophilic bacterium isolated in 1940 by Elazari-

Volcani from the Dead Sea, and originally described as Chro-

mobacterium marismortui. In this paper we study mineral

precipitation by this bacterium, which has not been described

before. We have studied: (1) its ability to form biominerals in

both liquid and solid media and at different salts concentrations;

(2) the mineralogy, morphology and texture of the precipitates;

and (3) the extent of the precipitation of different mineral phases

and their kinetic control.

MATERIAL AND METHODS

Microorganism, Culture Media and Mineral Precipitation

Chromohalobacter marismortui ATCC 17056 (=CCM 3518)

was used in this study. It is a gram-negative, nonsporeforming,

rod-shaped (sometimes slightly curved), chemoorganotrophic

and strictly aerobic bacterium. Its growth margins are wider

than those of other moderately halophilic bacteria because it

can grow in solid media containing from 2 to 30% (wt/vol) total

salts, with an optimum at about 10% (wt/vol) salts. In liquid

media it produces turbidity and pellicles (Ventosa et al. 1989).

A modified halophilic bacteria medium of the following com-

position (wt/vol) was used: 1% yeast extract, 0.5% proteose-

peptone, 0.1% glucose, supplemented with a balanced mixture

of sea salts to final concentrations of 2.5%, 7.5%, 15% and 20%

(wt/vol). The medium was amended with 0.4% calcium acetate

and the pH adjusted to 7.2 with 1 M KOH. To obtain solid media,

20 g/l “Bacto-Agar” was added.

C. marismortui was surface inoculated onto solid media at

2.5, 7.5, 15 and 20% of salts concentrations, and incubated at

32◦C for 40 days. The plates were examined periodically by

optical microscopy for the presence of precipitates. These ex-

periments were carried out in triplicate. For textural and min-

eralogical analysis, precipitates formed at different salt concen-

tration were removed from the medium out of agar blocks by

placing them in a boiling water bath until the agar dissolved.

The supernatants were decanted and the sediments resuspended

and washed in distilled water until the precipitates were free of

impurities, to be finally air-dried at 37◦C.

C. marismortui was also inoculated into flasks containing

150 ml of liquid culture medium at different salt concentration

(2.5%, 7.5%, 15% and 20%), and incubated at 32◦C. After 40

days of incubation, precipitates were collected from the medium,

transferred to distilled water, washed to become free of impu-

rities, air-dried at 37◦C. To evaluate the possible influence of

heating in mineral composition of precipitated Ca-Mg carbon-

ate phases, some precipitates obtained in liquid media were di-

vided in two fractions, and one of them washed following sim-

ilar procedures used for solid media, but no mineral changes in

the carbonate fraction were observed with respect to nonheated

samples.

The obtained precipitates were observed under binocular

glass before X-ray diffraction study. In all experiments, controls

consisted of uninoculated culture media and media inoculated

with heat-killed cells were also included.

Analytical Techniques

The purified precipitates were examined by powder X-ray

diffraction (PXRD) using a Philips PW 1710/00 diffractometer,

with graphite monochromator, automatic slit, CuKα radiation

and on-line connection with a microcomputer. Data were col-

lected for 0.4 seconds integration time in 0.02◦ 2θ steps at 40 kV

and 40 mA in a 2θ interval between 3◦–80◦. Data processing was

performed using the XPowderTM program in order to obtain the

qualitative and quantitative mineral composition (Martın 2004).

Data processing and indexation of PXRD patterns, and crys-

tallinity measures on them, allowed detecting precisely the na-

ture of the mineral phases. XPowder 〈http://www.xpowder.com〉

uses least square methods to refine the unit-cell parameters of

crystalline phases in order to determine, with the required pre-

cision, the exact term of any isomorphic series of minerals. Al-

though the program allows refining the 2θ angle, a standard

(metallic silicon) was used to fit experimentally the zero scale.

The molar proportion (mol %) of CaCO3 in Ca-Mg phases was

obtained from cell parameter refinement. The crystalline mo-

saic size on hkl reciprocal vectors was obtained from full width

at half of the maximum intensity (FWHM) after instrumental

broadening and Kα2 corrections. When small amounts of pre-

cipitate were obtained, diffractograms were performed on the

whole sample, in the other cases also after powdering a fraction

of the sample in an agate mortar to guarantee a polycrystalline

powder, but no significant differences were detected in PXRD

analyses from powdered and not powdered samples.

Secondary electron micrographs of bacterial precipitates ob-

tained under different culture conditions were performed on

gold-coated samples with a Zeiss DMS scanning electron mi-

croscope, operated at an acceleration voltage of 20 kV. Some

selected samples were coated with carbon for energy dispersion

X-ray microanalysis (EDX). High resolution SE images were

obtained with a field emission scanning electron microscope

(FESEM) LEO 1525, under 2–3 kV on minor carbon coated

samples.

RESULTS

In all salt concentrations tested, C. marismortui precipitated

carbonate in solid media, and both carbonate and phosphate

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MINERAL PRECIPITATION BY C. Marismortui 91

TABLE 1

Times for growth and precipitation by C. marismortui in

solid media

Time (in days) for

Salts

(%)

Mg

(ppm)

Mg/Ca

Molar

relation Growth

Starting

precipitation

Widespread

precipitation

2.5 906 1.5 1 1 3

7.5 2717 4.1 1 2 4

15 5435 7.7 1 8 10

20 7247 9.8 2 10 14

minerals in liquid media (Tables 1–2 and Figure 1), whereas no

precipitate formation was observed in the control cultures. Table

1 shows times required for C. marismortui to growth, to start with

precipitation and to produce widespread precipitation in solid

media. In both solid and liquid media time required for starting

and widespread precipitation increases with higher salt concen-

tration. When observed under binocular glass and SEM, mineral

precipitates form either: a) perfect single or twinned transparent

to translucent crystals with vitreous lustre, of struvite (Figure 2);

TABLE 2

Mineralogy of C. marismortui precipitates

Unit-cell axes(A)Culture

medium

Salts

(%)

Minerals

determined by PXRD

Mol%

CaCO3 a b c

Crystalline mosaic

size (in nm)

%

weight

Solid 2.5 Magnesian calcite 91–92 4.95–4.96 4.95–4.96 16.78–16.95 22–26 100

Solid 7.5 Magnesian calcite 68–83 4.92 4.92 16.89 12 17

Solid 7.5 Ca-Mg kutnahorite(∗) 58–68 4.87 4.87 16.45 12 83

Solid 15 Ca-Mg kutnahorite(∗) 74–82 4.89–4.91 4.91–4.89 16.57–16.45 9–13 100

Solid 20 “Proto-dolomite”(∗∗) 58 4.84 4.84 16.23 7 89

Solid 20 Huntite N.D. 9.51 9.51 7.83 7 11

Liquid 2.5 Struvite — 6.95 11.20 6.13 128 78

Liquid 2.5 Aragonite 100 4.96 7.97 5.74 64 22

Liquid 7.5 Struvite(1) — 6.94–6.95 11.20–11.21 6.13–6.14 77–123 100

Liquid 7.5 Aragonite(2) 100 4.97 7.95 5.74 17 100

Liquid 15 Struvite(3) — N.D. N.D. N.D. N.D. N.D.

Liquid 15 Poorly crystalline phosphates(2) — — — — — 33

Liquid 15 Vaterite(2)(∗∗) 100 7.15 7.15 16.94 12 43

Liquid 15 Aragonite(2) 100 4.96 7.96 5.76 13 24

Liquid 20 Aragonite 100 4.96 7.97 5.75 18 68

Liquid 20 Monohydrocalcite(∗∗∗) 100 10.56 10.56 7.55 34 32

(1)Only analysed the struvite fraction accumulated in the bottom of culture flasks, without bioliths.(2)Only analysed the biolith fraction accumulated in pellicles, without struvite crystals.(3)Struvite not determined by PXRD, but optically from crystals accumulated in the bottom of the culture flasks.(∗)Reference Intensities Ratio (RIR = Imaximum/Icorundum) of kutnahorite is estimated as 1, because experimental RIR of kutnahorite is not available

in the literature.(∗∗)“Proto-dolomite” includes a mixture of Ca-Mg carbonate phases among which Ca-Mg kutnahorite, dolomite and other badly characterised

Ca-Mg carbonate phases are present.(∗∗∗)RIRs of monohydrocalcite and vaterite are estimated as 1 = RIRaragonite, because their chemical composition and massic absorption

coefficients are similar, and because their experimental RIRs are not available in the literature.

N.D.: Not determined.

FIG. 1. Indexed XRD pattern of Ca-Mg kutnahorite, with minor amounts of

magnesian calcite and amorphous substances, precipitated by C. marismortui in

solid medium 15%.

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92 M. A. RIVADENEYRA ET AL.

FIG. 2. Texture of struvite crystals precipitated by C. marismortui in liquid medium 7.5%. (A): General view of struvite crystals. (B): Twinned struvite crystal

with prismatic and pinacoidal faces. (C): Close-up of the upper right pinacoidal face of the crystal shown in photo (B). (D): Calcified bacterial cells on top of a

struvite crystal. (E): Same as (D) with higher magnification; note the numerous small discoidal to spheroidal units attached to the crystal surface and to bacteria.

(F): Close-up of (E).

b) isolated or aggregated carbonate objects with (sub)spherical

shape, hereafter called bioliths (Figures 3–5); c) fine-grained

powdery white masses (Figures 3–4). In different cultures, the

amount and size of carbonate bioliths decreases with increas-

ing salt concentration, although bioliths of different size were

usually observed in the same culture (Figures 3A–B, 4A-F, 5A).

Bioliths formed in liquid media preferentially appear within pel-

licles floating onto the surface of the medium, whereas struvite

crystals formed mainly in the bottom of the flasks. Both frac-

tions were selectively sampled from liquid medium 7.5%, and

independently analysed by PXRD. In samples from liquid me-

dia 15% and 20%, struvite crystals were optically identified and

only the fractions composed of carbonate bioliths were analysed

by PXRD.

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MINERAL PRECIPITATION BY C. Marismortui 93

FIG. 3. Texture of C. marismortui precipitates formed in liquid medium 2.5% (photos A–C), 7.5% (photos D–E) and 15% (photo F). (A): General view of

aragonite bioliths, and of struvite crystals. (B): Close-up of (A), showing lumps of aragonite bioliths, and plate-shaped bioliths on both sides, with flat (f) and

mammilated (m) surfaces. (C): Detailed view of the spiny surface of the biolith arrowed in (B) showing needles formed by piling up of aragonite nanocrystals. (D):

Detailed view of an isolated and compact spheroidal aragonite biolith; arrow points to location of the calcified bacterial moulds shown in photo (E). (E): Calcified

bacteria in the surface of biolith shown in photo (D). (F): Cluster of spheroidal bioliths attached together by partially mineralized organic pellicles (arrows) and

surrounded by fragments of powdery carbonate precipitates.

Mineralogy

The high background of PXRD patterns obtained for the dif-

ferent precipitates formed in C. marismortui cultures reveals

a general low crystallinity (small crystal size, strong lattice

strain and presence of amorphous substances) of these precip-

itates which are, however, associated with crystalline phases

evidenced by reflections. The mineralogy of the precipitates ob-

tained from cultures at different salt concentrations, according

to the PXRD study, is reproduced in Table 2, which also in-

cludes quantitative analysis of the phases, the molar proportion

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94 M. A. RIVADENEYRA ET AL.

FIG. 4. Texture of C. marismortui precipitates formed in solid medium 2.5% (photo A), 7.5% (photos B–C) and 15% (photos D–F). (A): Isolated subspherical

biolith, resulting from accretion of two crossed dumbbell shaped objects like those arrowed in (C). (B): General view of isolated and grouped bioliths with different

size. (C): Close up of photo (B); the big arrow points to a broken biolith with fibrous-radiating internal structure; small arrows point to dumbbell shaped objects.

(D): Mass of bioliths sticked by a gelly-like matrix rich in organic matter; the squared area corresponds to the inset shown in (F); the arrow points to the site where

photo (D) of Figure 5 was obtained. (E): Group of bioliths sticked by partially mineralized gelly-like organic substances; the asterisk indicates the location of the

EDX analysis reproduced in Figure 6F. (F): Close up of the area squared in (D), showing a biolith with porous surface covered by a partially mineralized thin

pellicle rich in organic matter; note also the presence of some calcium sulphate crystals.

(mol %) of CaCO3 in Ca-Mg phases, the crystalline mosaic

size and the unit-cell parameters. Important differences are ob-

served in the mineralogy of precipitates formed in liquid and in

solid media. In liquid media aragonite and struvite were precip-

itated at all salt concentrations, whereas vaterite and monohy-

drocalcite were also produced at higher salinity (15% and 20%,

respectively). In solid media, magnesium calcite was formed,

together with a poorly ordered Ca-Mg carbonate mineral which,

as deduced by its unit-cell parameters (compare with card n◦

19-034 of the ASTM X-ray Powder Data File), correlates with

kutnahorite although the studied phase is free of Mn and Fe (Fig-

ure 1, Table 2). This Ca-Mg phase, hereafter named as Ca-Mg

kutnahorite, starts to form at salt concentration of 7.5% and it

is the only mineral precipitated at 15%. In cultures at 20% salt

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MINERAL PRECIPITATION BY C. Marismortui 95

FIG. 5. Texture of C. marismortui precipitates in solid medium 15%. (A): General view. (B): Two bioliths with different surface texture. (C): Close up of the biolith

on the left-hand side of B. (D): Close up of the surface of a biolith similar to that shown on the right-hand side of (B) and located in the point arrowed in Figure 4D.

concentration, Ca-Mg kutnahorite changes to other Ca-Mg car-

bonate phases which are mostly assigned to a “proto-dolomite”

(89%) on the base of a and c values obtained from refinement

of unit-cell parameters (a = 4.84 A and c = 16.23 A, Table 2).

Small amounts of poor crystalline huntite (11%) are also present

(Table 2).

CaCO3 content (mol %) ranges from 83% to 92% in magne-

sian calcite, from 68 to 80 percent in Ca-Mg kutnahorite and of

58% in “proto-dolomite”, and correlates negatively with salinity

and crystallinity (Table 2). Mosaic size ranges from 77 to 128

nm for struvite and from 13 to 64 nm for aragonite formed in

liquid media (Table 3); it ranges from 12 to 26 nm for magnesian

calcite and from 9 to 13 nm for Ca-Mg kutnahorite and is 7 for

“proto-dolomite” and huntite formed in solid media (Table 2).

Mosaic size also correlates negatively with increasing salinity

in both media (Table 2).

Morphology and Texture

Secondary electron images show the morphology and tex-

ture of C. marismortui precipitates obtained in liquid media

(Figures 2–3) and in solid media (Figures 4–5). Figures 2A and

3A show general panoramas of the struvite crystals and of the

aragonite bioliths obtained in liquid media at 7.5% and at 2.5%

salt concentration, respectively. Figures 4B and 4D show gen-

eral views of bioliths formed in solid media at 7.5% and 15%

salts, respectively.

Struvite crystals have a maximum length usually comprised

between 150 µm and 400 µm (Figures 2A and 3A). They are

frequently twinned and develop prismatic and pinacoidal faces

(Figure 2B). A detailed observation of crystal faces let to ob-

serve stepped surfaces (Figure 2C) and numerous calcified bac-

teria attached onto them (Figure 2D). According to Ventosa

et al. (1989), C. marismortui is rod-shaped, sometimes slightly

curved, with 0.6 to 1.0 by 1.5 to 4.0 µm in size; the mineralized

bacteria observed here have similar shapes and sizes (Figure 2D,

2E). Numerous discoidal (sometimes hexagonal) to subspheri-

cal small units, with diameters lesser than 100 nm up to around

200 nm, are attached either to the growth steps of struvite crystal

faces or to bacteria (Figures 2E and 2F).

In addition to fine grained carbonate masses, perfectly spher-

ical bioliths, sometimes hemispherical and with a hole on one

side, are the dominant component of the carbonatic fraction pre-

cipitated either in liquid (Figures 3A, 3B, 3D, 3F) or in solid me-

dia (Figures 4A–F and 5A–B). Their main size is submillimetric

(several tens to several hundreds of microns), but it largely

changes, even in samples from the same culture (Figures 4B

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96 M. A. RIVADENEYRA ET AL.

and 4C), from few µm up to 1 mm (sometimes and exception-

ally). In solid media, small bioliths with dumbbell shape and

longer dimension of around 20 µm sometimes appear (Figure

4C, small arrows), and finally close to give spherical bioliths

(Figure 4A). These frequently lump together to form clusters

(Figures 3F and 4C) and, sometimes, plates with a flat surface

and a mammilated hemispheroidal surface (f and m, respec-

tively, in Figure 3B). Bioliths formed in liquid media can be

found sticked by thin partially mineralized pellicles rich in or-

ganic matter (arrows in Figure 3F), but these are usually much

more abundant around bioliths formed in solid media (Figures

4D, 4E and 4F). Some bioliths have yellowish colours and over-

polite adamantine lustre but, usually, most of them have white

colour, are formed by aggregates of very small crystalline parti-

cles (Figures 3E, 5C and 5D) and, when broken, they can have

a laminated and/or fibrous-radiated internal structure (Figure

4C, big arrow). Bioliths usually have rough surfaces with small

holes, and high porosity (Figures 3D, 4F, 5B and 5D). Many

holes have bacteria-like size and shape and, in some cases, min-

eralized bacteria are clearly appreciated (Figures 3E and 5D,

arrow).

EDX Study

EDX microanalyses of isolated single and twinned struvite

crystals and of bioliths from samples with different mineralogy

confirm PXRD results (Figure 6). EDX spectra of carbonate

FIG. 6. EDX spectra of C. marismortui precipitates. (A): aragonite (liquid medium 2.5%). (B): Typical magnesian calcite (solid medium 7.5%). (C): struvite

(liquid medium 7.5%). (D): Ca-Mg kutnahorite biolith (solid medium 15%). (E): Kutnohorite biolith with abundant organic pellicles (solid medium 15%).

(F): Mineralized organic pellicle rich in residues of the culture medium (see location on Figure 4E; solid medium 15%).

bioliths indicate that some of them (Figure 6A) are composed

only by calcium carbonate (those precipitated in liquid media,

formed by aragonite, vaterite or monohydrocalcite), whereas

variable amounts of Mg and Ca are present in those of solid

media, formed by magnesian calcite and Ca-Mg kutnahorite

(Figures 6B, 6D, 6E). The stoichiometry of struvite crystals is

well defined by a nearly constant intensity ratio of P and Mg

peaks in EDX spectra (Figure 6C). Small amounts of P, Cl, S, K

and Na, as well as NaCl and CaSO4 crystals, are usually present

together with Ca and Mg in the microanalyses of the carbonate

bioliths formed in solid (Figures 6E and 6F) or in liquid media,

but their occurrence is interpreted to be directly derived from

the elements of the culture medium, due to incomplete washing

of dead bacterial remains and pellicles together with residues of

the culture medium, and to contamination during sample drying

by salts remaining yet in dissolution.

DISCUSSION

Mineral Precipitation by C. marismortui

The precipitation of minerals within laboratory cultures of C.

marismortui is reported for the first time in this paper. Aragonite,

magnesian calcite, vaterite, monohydrocalcite and struvite are

minerals known to be precipitated by different halophilic and

non-halophilic bacteria (Rivadeneyra et al. 1983, 1991, 1992a,

1992b, 1993, 1998, 2004; Ferrer et al. 1988; Ben Chekroun et al.

2004), but no reference to kutnahorite nor huntite precipitation

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MINERAL PRECIPITATION BY C. Marismortui 97

by bacteria has been noticed. Although some microbial contri-

bution to the formation of both minerals has been supposed for

sedimentary kutnahorite formed in ancient deep marine black

shales (Calvert and Pedersen 1996; Gutzmer and Beukes 1998)

and in marine fish intestines (calcian kutnahorite: Walsh et al.

1991), and for sedimentary huntite formed in alkaline lakes

(Mutlu et al. 1999), duricrusts (Dill et al. 2002) and cave walls

(Canaveras et al. 2001), this is the first study in which bacterial

precipitation of both minerals in laboratory cultures has been

reported.

Mineral precipitation by bacteria is a complex phenomenon

and, as biomorphic structures formed by Ca and Mg carbonates

of different mineralogy have been precipitated in laboratory cul-

tures under purely abiogenic conditions (Gonzalez et al. 1992;

Fernandez-Dıaz et al. 1996; Bosak et al. 2004), some authors

doubt about a true bacterial origin of many natural carbonate

precipitates, whereas other studies conclude that bacteria are

not capable of influencing on the mineralogy of the associated

precipitates (Reitner 1993; Peckman et al. 1999). Nevertheless,

our results show that C. marismortui plays an important role in

mineral precipitation because the proportions of aragonite, mag-

nesian calcite, vaterite, monohydrocalcite and struvite, as well

as the magnesium content in the magnesian calcite, are different

to those obtained from precipitates of other halophilic bacte-

ria in the same culture media (Ferrer et al. 1988; Rivadeneyra

et al. 1993, 1998, 2004). In addition, it has also induced pre-

cipitation of Ca-Mg kutnahorite and huntite. Our interpretation

is reinforced by the absence of precipitation observed in the

control cultures inoculated with heat-killed cells, which demon-

strates that C. marismortui acts not only as heterogeneous nu-

cleation site for precipitation but also plays an active role in

the precipitation of carbonate minerals as previously shown for

other bacteria (Morita 1980; Ferrer et al. 1988; Rivadeneyra

et al. 1993, 1998, 2004; Braissant et al. 2003; Cailleau et al.

2005).

Role of Cell Surfaces and Bacterial Metabolismin Carbonate Precipitation

Bacteria can serve as a nucleus for mineral precipitation

upon adsorbing Ca2+, Mg2+ and other metallic cations onto

the cell surface: membranes, walls and extracellular polymeric

substances or EPS (Morita 1980; Beveridge and Fyfe 1985;

Ferris et al. 1991; Braissant et al. 2003; Van Lith et al. 2003;

Cailleau et al. 2005). Bacteria pump Ca2+ towards the exterior

of the cell and Mg2+ towards the interior (Rosen 1987) and,

on the negatively charged surfaces of their cellular envelopes,

Ca2+ is adsorbed with greater intensity than Mg2+ (Wolt 1994;

Maier et al. 2000). So, when purely inorganic chemical pre-

cipitation is difficult in natural or in laboratory sterile envi-

ronments, the presence of bacteria can induce precipitation of

minerals around them, in particular of carbonates, by lower-

ing Mg/Ca ratio relative to that of the environment where they

live.

Bacteria have different types of cell surface charge (Van der

Mei et al. 2000). In addition, the bacterial metabolic activity has

great importance of in the mineralization process of Ca and Mg

minerals (Ehrlich 2002): it changes pH, ionic strength and ionic

make-up of the medium, and influences cell surface charges

(Ahimou et al. 2002; Lytle et al. 2002). Because of that we think

that differences between minerals precipitated by C. marismor-

tui and other bacteria might be related to their different capacity

to selectively adsorb diverse concentrations of Mg2+ and Ca2+

onto its cellular envelope, as previously described in other bac-

terium (Rivadeneyra et al. 2004).

The results presented above induce to interpret that the mech-

anism of carbonate formation by C. marismortui involves ad-

sorption of Ca2+ and Mg2+ cations and the production of CO2

and NH3 during metabolization of organic nutrients. This would

drive local supersaturation gradients and precipitation of carbon-

ates around bacterial surfaces using them as nucleation sites,

because supersaturation of a biomineral in a soluble medium

is a key factor that makes biomineralization possible (Simkiss

and Wilbur 1989). A similar mechanism was also described by

Ehrlich (2002) and Castanier et al. (1999) for bacterial carbon-

ate precipitation in natural habitats, and by Rivadeneyra et al.

(1998, 2004) in laboratory cultures of other halophilic bacteria.

Influence of the Environmental Conditions in MineralPrecipitation

An excess of salts and high concentration of magnesium

ions has a negative influence on bacterial carbonate precipita-

tion (Rivadeneyra et al. 1985, 1991; Ferrer et al. 1988) in the

same way as in inorganic carbonate precipitation (Berner 1975;

Morse and Mackenzie 1990). Our results confirm these obser-

vations since C. marismortui precipitates optimally at 2.5% salt

concentration (wt/vol) in spite of the fact that this bacterium

grows optimally at 10% (Ventosa et al. 1989). However, only

slight differences on crystals formation were found between 2.5

and 7.5% salts. Furthermore, C. marismortui precipitated well

at 15% and 20% salt concentration. Our results confirm: (a) that

magnesium ions have less inhibitory effect in bacterial than in

chemical precipitation (Rivadeneyra et al. 1985); (b) that this

effect is significantly less in moderately halophilic bacteria than

in non-halophilic ones (Ferrer et al. 1988); and (c) that bacteria

can induce precipitation of carbonate minerals in environments

where precipitation is not possible.

Important differences in biomineralization by C. marismor-

tui have been found depending on culture in liquid or in solid

media. In artificial culture media in laboratory, and in natural

environments, several authors have found that the precipita-

tion of aragonite or calcite took place depending on the viscos-

ity of the medium (Rivadeneyra et al. 1985, 1998; Buczynski

and Chafetz 1991). More liquid environments tend towards the

precipitation of aragonite, while, in more viscous areas of the

same environment, calcite is precipitated. Results obtained from

C. marismortui cultures confirm these observations because

aragonite was only precipitated in liquid media.

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98 M. A. RIVADENEYRA ET AL.

In inorganic precipitation of carbonates, calcite is inhib-

ited and aragonite increased with a higher concentration of

Mg2+ (Cailleau et al. 1977; Kitano and Hood 1962; Morse and

Mackenzie 1990). Aragonite is generally precipitated in nat-

ural fluid media with Mg2+/Ca2+ molar ratio greater than 1

(Gonzalez 1989). In our study, the Mg2+/Ca2+ ratio was always

greater than 1 (Table 1). Rivadeneyra et al. (2004) applied the

geochemical computer program PHREEQC to ionic composi-

tion data of culture media similar to those used in this study,

and concluded that anhydrite, aragonite, calcite, dolomite, gyp-

sum, and halite could be precipitated inorganically in the media

assayed. Nevertheless, in C. marismortui cultures, struvite, va-

terite, monohydrocalcite, Ca-Mg kutnahorite and huntite were

precipitated in addition to aragonite and magnesian calcite. This

confirms again that precipitation was actively influenced by this

bacterium, although this influence was more active in solid me-

dia than in the corresponding liquid media, where great amounts

of aragonite were precipitated.

Regarding vaterite and monohydrocalcite, liquid media and

high concentration of salts favour the formation of both minerals

by halophilic bacteria (Rivadeneyra et al. 1991, 1998, 2004).

Our results confirm this conclusion since both minerals were

only found in C. marismortui cultures in liquid media with,

respectively, 15% and 20% salts concentration.

Bioprecipitation of Ca-Mg Kutnahorite, “Proto-Dolomite”and Huntite

Kutnahorite and huntite are minerals that belong to the

dolomite group, with chemical formulae Ca (Mn0.6 Mg0.3 Fe2+0.1)

(CO3)2 and Ca Mg3 (CO3)4, respectively. Although kutnahorite

is a Ca-Fe(II)-Mg-Mn(II) carbonate, the mineral precipitated by

C. marismortui is, in this case, exclusively calcian-magnesium,

and comparatively enriched in Ca. It has been named Ca-Mg kut-

nahorite because it does not contain any Mn or Fe, which were

absent in the culture media and because Mg- and Ca-kutnahorite

types have been described previously: Mg-kutnahorite may pre-

cipitate in veins from hydrothermal fluids released after de-

hydration reactions in fault zones (Kastner et al. 1997) and

Ca-kutnahorite has been found in relation to mineralization

within the intestine of marine teleosts (Walsh et al. 1991).

The formation and persistence of metastable Ca-Mg carbon-

ates with complex ordering structures in relation to their sta-

ble counterparts are common facts in low-temperature inorganic

carbonate precipitation (Deelman 2005), and even amorphous

Ca-Mg phases are increasingly recognized as biomineraliza-

tion products (Raz et al. 2000; Addadi et al. 2003; Politi et al.

2004).

Ordering of Ca and Mg in cation layers give rise to many

structures and superstructures with their corresponding diffrac-

tions characteristic. Although ideal structures of common car-

bonate minerals (calcite, dolomite, aragonite) have been well

characterized (Reeder 1983), early X-ray diffraction studies of

low-temperature Ca-Mg-Fe-Mn carbonates revealed consider-

able complexity (e.g., Graf and Goldsmith 1956; Goldsmith and

Graf 1957). However, structural refinements of unit cell parame-

ters by PXRD constitute a quite good approximation not only to

polymorphic changes but also to chemical changes in a structure,

and constitute a quite reliable method for mineralogical charac-

terization. This structural approximation allows, in our case, to

assign the Ca-Mg carbonates precipitated in solid media to the

mineral species kutnahorite, in spite of the absence of Mn and

Fe in its structure, because the lattice parameters of the studied

mineral correspond to it (a = 4.87 A and c = 16.35 A, and

a = 4.89 A and c = 16.50 A for disordered and substantially

ordered kutnahorite respectively: Peacor et al. 1987).

In the studied Ca-Mg kutnahorite, the Mg plays the same elec-

tronic role than Mn plays, being the most electronegative cation,

in the normal kutnahorite mineral s.s. Metastable arrangements

occur when minerals are grown rapidly at low temperatures

(Fyfe 1964), and Mg content in metastable Ca carbonates in-

creases with growth rate (Morse and Mackenzie 1990; Deel-

man 2005). Increasingly high supersaturation in CO2−3 , Ca2+

and Mg2+ induced by C. marismortui are responsible of the

precipitation of the mineral sequence observed, (magnesian cal-

cite, Ca-Mg kutnahorite and “proto-dolomite”), which are pro-

gressively richer in Mg. Although the term “proto-dolomite”

must be avoided (Kelleher and Redfern 2002) we use it between

quotation marks to emphasize the structural importance of in-

termediate phases in the mineral sequence towards the dolomite

formation. So, under certain physicochemical conditions, appro-

priate Ca2+ and Mg2+ supersaturation may precipitate a calcium

and magnesium carbonate isostructural with kutnahorite instead

of other Ca-Mg carbonate minerals. Its precipitation within C.

marismortui cultures confirms that, as hypothesised by Walsh

et al. (1991), at least some bacteria can be directly involved in

its precipitation in natural environments, in the same way as

precipitation of other carbonate minerals by bacteria has been

demonstrated.

Davies et al. (1977) have described formation of huntite

together with other carbonate minerals (dolomite, magnesium

hydroxide carbonate, calcite and monohydrocalcite) in hyper-

saline brines with abundant microbial population. They affirm

that high CO2 concentration caused by the intense microbial ac-

tivity would have been responsible for the formation of huntite.

The synsedimentary and diagenetic formation of huntite has also

been described forming, respectively, massive deposits related

to mud-flat lacustrine dolomites and dolomitic marls and open-

lake limestone sediments related to arid or semiarid conditions,

and fillings of fenestral dissolution voids within the same sed-

iments that flocculated under pluvial climatic regime (Akbulut

and Kadir 2003). The precipitation of huntite in brine-soaked

mud-flats of lakes (Mutlu et al. 1999), in speleothems (Canaveras

et al. 2001) and in recent hardpans developed on mining residues

of sulphide ore deposits (Dill et al. 2002) has been considered a

product of microbial activity. Our results on poorly crystalline

huntite precipitation in pure culture of C. marismortui (solid me-

dia) at the highest salinity (20%) agree with this hypothesis, and

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MINERAL PRECIPITATION BY C. Marismortui 99

let us to think that the presence of this mineral in many natural

habitats is related to halophilic bacterial mediation.

Bioprecipitation of Struvite and Influence of Pellicles

In all liquid media, C. marismortui has also precipitated abun-

dant struvite. This mineral is only present, in nature, in very pe-

culiar environments always rich in organic matter, such as guano

deposits, old graveyards, under the floors of stables, and in uri-

naries (Robinson 1898; Nriagu and Moore 1984). It is also a

common mineral component of renal calculi (Rivadeneyra et al.

1999). Because of that, struvite precipitation has usually been

related to bacterial activity, and this has been confirmed in many

laboratory studies (Beavon and Heatley 1962; Rivadeneyra et al.

1983, 1992a, 1992b; Da Silva et al. 2000). A bacterial precip-

itation mechanism of struvite in confined microenvironments

by adsorption of Mg2+ and PO3−4 ions together with produc-

tion of NH+4 ions during metabolization of organic nutrients

was proposed by Rivadeneyra et al. (1992b). A similar mech-

anism can operate in C. marismortui cultures, because the or-

ganic matter of the medium (peptone and yeast extract) can

provide enough amount of NH+4 and PO3+

4 ions to allow struvite

precipitation.

As a proof of this, similar growth units with 100–200 nm in

size are clearly visible on the surface of struvite crystals and

on mineralized bacteria (Figures 2D–2F). The medium is, how-

ever, rich in Ca, which inhibits struvite precipitation (Beavon and

Heatley 1962; Rivadeneyra et al. 1983) and in Mg, which inhibits

carbonate precipitation (Berner 1975; Morse and Mackenzie

1990, and references therein). To explain the precipitation of

struvite and carbonate minerals we invoke the role played by

turbidity and pellicles produced C. marismortui in liquid media,

which would contribute to create two different microenviron-

ments in the culture: in one of them, within and around organic

pellicles (Figure 3F) formed preferentially in the surface of liq-

uid media which was, indeed, better oxygenated, bacteria would

accumulate more Ca, thus favouring CaCO3 supersaturation and

accretion of bioliths.

Organic matter (dead bacterial remains and exopolymers, or

EPS) may be responsible for Ca-Mg carbonate supersaturation

by forming complexes with the ionic species, by inhibiting its

crystallization, and by desolvation processes through incorpo-

ration of water to polymer surfaces (Cody, 1991; Kuhl et al.

1996; Iannace et al. 1997). The remaining liquid media, es-

pecially in the bottom of the flasks, would be relatively en-

riched in Mg allowing to precipitation of struvite. The pres-

ence of Mg in EDX spectra of bioliths covered with organic

pellicles (whereas PXRD indicates that only calcium carbonate

minerals are present in these bioliths, exceptionally associated

with small amounts of poorly crystalline phosphates: Table 3),

demonstrates that Mg plays a less inhibiting role in calcium car-

bonate precipitation within pellicles formed in liquid media. It

also shows that the relative increase of the Mg2+/Ca2+ ratio

in the remaining solution, together with bacterial production of

NH4+ ions, favoured struvite nucleation on bacterial surfaces,

in the same way as previously discussed for carbonate minerals.

Sequential Precipitation of Biominerals

It is well demonstrated that bacterial carbonate precipitation

is a sequential process (Rivadeneyra et al. 1998, 2004). It starts

with supersaturation around the cell immediately followed by

precipitation in the bacterial envelopes. The systematic pres-

ence of partially collapsed to incipiently calcified bacteria, or

of their empty moulds within fine-grained powdery precipitates

and on the surface of struvite crystals and of carbonate bioliths,

as well as within some bioliths, evidences that precipitation of

carbonates and of struvite starts with precipitation on the sur-

face of C. marismortui. A detailed observation of bioliths formed

in diverse cultures evidences the existence of different surface

structures (Figures 3C, 4F, 5B, 5C and 5D) which suggest that

significant differences could exist in the intimate mechanisms of

precipitation, growth and/or recrystallization of the diverse min-

erals found in C. marismortui bioliths. Nevertheless, in spite of

these possible differences in detail, we consider that all pre-

cipitation processes either of carbonates or of phosphates made

by C. marismortui could be described as biologically induced

biomineralization, that is, processes occurring in open microen-

vironments, without an specific link to any special organic matrix

of macromoleculae (see Lowenstan and Weiner 1989, or Chafetz

and Buczynsky 1992, among many others).

Finally, it seems to us significant that coeval precipitation

of struvite and carbonate minerals occurs in the same culture.

These results connect bacterial precipitation of carbonate and

phosphate minerals, which is also commonly found in phos-

phate and carbonate stromatolites of the fossil record (Martın-

Algarra and Sanchez-Navas 2000). A more intense research on

these processes would help us to understand how precipitation

of each mineral can allow, facilitate or inhibit precipitation of

other minerals according to changing conditions. We also con-

sider that, in the same way as in laboratory cultures, a contin-

uous research in this field would help us to understand similar

processes occurring in natural sedimentary environments, and

to better recognise their interactions with the carbon and phos-

phorous cycles in natural habitat as well as the relations and/or

interferences between them.

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