Biotic controls on microbial communities in estuarine and

riverine habitats

Trisha Pascal-Lopez

Microbial Ecology Lab,Mentors: Rima Franklin, Ph.D,

and Amy Jenkins

•Nutrient Input (ex. farm runoff)

•Algal Bloom (consumed by bacteria)

•Bacteria (hypoxia)

•Marine creature die

•Trophic cascade

www.jerrygreerphotography.com/james_river_summer_reflection.htm

A. Purpose

– Study the ecological relationships that occur in an aquatic microbial food web

– To understand the interdependence of three levels of the food web (phytoplankton, bacteria, and grazers)

– Focus on response of bacteria to other community members

1) Two different initial environment samples:

• James River mile 21 (Estuarine)

• James River Rice Center (Riverine)

B. Setup

Temp

(C)

Salinity (ppt)

Turbidity (NTU)

pH DO (mg/L)

Estuarine site (RM 21)

26.6 16.5 10.5 8.1 8.1

Rice Center (RM 75)

27.2 0.2 15.2 7.9 8.9

Courtesy of Paul Bukaveckas & River Ecology lab

i) Environment Phytoplankton Bacteria Grazers

ii) Environment + cycloheximide

Phytoplankton Bacteria -------------

iii) Environment + dark ------------ Bacteria Grazers

iv) Environment + cylcoheximide + dark

------------ Bacteria ------------

2) Four treatments:

• Manipulated light and dark levels

• Eukaryotic growth inhibitor (cycloheximide)

• 3 replicate of each treatment per environment, 3 day incubation

B. Setup

1) Phytoplankton community:

• Fluorescence detection of chlorophyll levels

2) Bacterial community:

• Abundance (microscopic direct counts)

• Culturability (counts of solid media/agar)

• Genetic profiling (RAPD DNA fingerprints)

3) Grazer community:

• Abundance and diversity (microscopic determination)• Mainly protozoa, rotifers and larvae

Methods

www.compucyte.com/pubbacterialdetection.htm

C. ResultsInitial comparison

• Distinct communities at start of experiment

• Estuary had lower phytoplankton, grazer and bacteria abundance than riverine site (Rice).

EstuaryEstuarysitesite

RiceRiceCenterCenter

Were the treatments effective?

Estuary Rice Center

Ch

loro

ph

yll

(RF

U)

0

2

4

6

8

LightLight + cycloheximide DarkDark + cycloheximide

Estuary Rice Center

Gra

zer

cou

nts

(o

rgan

ism

s/m

l)

0

20

40

60

80

100

120

LightLight + cycloheximideDarkDark + cycloheximide

Estuary Rice Center

Gra

zer

div

ersi

ty (

nu

mb

er o

f ty

pes

)

0

1

2

3

4

5

LightLight + cycloheximideDarkDark + cycloheximide

• Changes to the phytoplankton population (greater for estuary)

• Affected estuary & riverine phytoplankton differently

• Reduced grazer abundance

• Grazers linked to phytoplankton?

• Decrease in grazer diversity

How did bacteria respond to the changes in the phytoplankton and grazer abundance/ diversity?

Estuary Rice Center

Bac

teri

al c

ou

nts

(ce

lls/

ml)

0

2e+6

4e+6

6e+6

8e+6

1e+7

LightLight + cycloheximideDarkDark + cycloheximide

Estuary Rice Center

Cu

ltu

rab

ilit

y o

n s

ite-

spec

ific

med

ia (

%)

0

2

4

6

50

75

100

125

150

LightLight + cycloheximideDarkDark + cycloheximide

Estuary: Bacteria abundance not linked to phytoplankton or grazers abundances (unaffected in all treatments).

Rice Center (Riverine): Treatments had an effect on bacterial community.

Changes in bacterial community composition?

Estuary: Community composition similar in all treatments

LightDark

+ cyclohex.Dark

Light+ cyclohex.

River: Changes in bacterial community with each treatment.

LightDark

+ cyclohex.Dark

Light+ cyclohex.

Implications

• Microbial communities in the Estuarine and Riverine environments are different.– Estuarine environment bacterial communities

are not linked to grazer or phytoplankton abundance

– Riverine environment shows bacterial interactions with phytoplankton and grazer communities

• Microbial community interactions vary depending on the environment which controls their response to environmental stress.

D. Future Goals– Statistical analysis of data

– Chemistry (Nitrogen and DOC)

– Run the experiment again with changes

• Add another mile marker • Collect water sample at different seasons or

times of day• Manipulate nutrient concentration

Thank You

Questions?