© 2006 thomson-brooks cole chapter 7 multicellular primary producers

© 2006 Thomson-Brooks Cole

Chapter 7

Multicellular Primary Producers


Key Concepts

• Multicellular marine macroalgae, or seaweeds, are mostly benthic organisms that are divided into three major groups according to their photosynthetic pigments.

• The distribution of seaweeds depends not only on the quantity and quality of light but also on a complex of other ecological factors.


Key Concepts

• Marine algae supply food and shelter for many marine organisms.

• Flowering plants that have invaded the sea exhibit adaptations for survival in saltwater habitats.

• Seagrasses are important primary producers and sources of detritus, and they provide habitat for many animal species.


Key Concepts

• Salt marsh plants and mangroves stabilize bottom sediments, filter runoff from the land, provide detritus, and provide habitat for animals.


Multicellular Algae

• Seaweeds are multicellular algae that inhabit the oceans

• Major groups of marine macroalgae:– red algae (phylum Rhodophyta)– brown algae (phylum Phaeophyta)– green algae (phylum Chlorophyta)

• Scientists who study seaweeds and phytoplankton are called phycologists or algologists


Distribution of Seaweeds

• Most species are benthic• Benthic seaweeds define the inner

continental shelf, where they provide food and shelter to the community– compensation depth—the depth at which

the daily or seasonal amount of light is sufficient for photosynthesis to supply algal metabolic needs without growth

• Distribution is governed primarily by light and temperature



• Effects of light on seaweed distribution– chromatic adaptation, proposed in the

1800s, was accepted for 100 years• chromatic adaptation—the concept that the

distribution of algae was determined by the light wavelengths absorbed by their accessory photosynthetic pigments, and the depth to which these wavelengths penetrate water

– such zonation does not occur– distribution depends more on herbivory,

competition, pigment concentration, etc.



• Effects of temperature on seaweed distribution– diversity of seaweeds is greatest in

tropical waters, less in colder latitudes– many colder-water algae are perennials

(living more than 2 years)• only part of the alga survives colder seasons• new growth is initiated in spring

– intertidal algae can be killed if temperatures become too hot or cold


Structure of Seaweeds

• Thallus—the seaweed body, usually composed of photosynthetic cells– if most of it is flattened, it may be called a

frond or blade

• Holdfast—the structure attaching the thallus to a surface

• Stipe—a stem-like region between the holdfast and blade of some seaweeds


Biochemistry of Seaweeds

• Photosynthetic pigments– Color of thallus = wavelengths of light not

absorbed by the seaweed’s pigments– All have chlorophyll a plus:

• chlorophyll b in green algae• chlorophyll c in brown algae• chlorophyll d in red algae

– Chlorophylls absorb blue/red, pass green– Accessory pigments absorb various colors

• e.g. carotenes, xanthophylls, phycobilins



• Composition of cell walls– Primarily cellulose– May be impregnated with calcium

carbonate in calcareous algae– Many seaweeds secrete slimy mucilage

(polymers of several sugars) as a cell covering• holds moisture, and may prevent desiccation• can be sloughed off to remove organisms

– Some have a protective cuticle—a multi-layered protein covering



• Nature of food reserves– Excess sugars are converted into polymers– Stored as starches– Unique sugars and alcohols may be used

as antifreeze substances by intertidal seaweeds during cold weather


Reproduction in Seaweeds

• Fragmentation—asexual reproduction in which the thallus breaks up into pieces, which grow into new algae– drift algae—huge accumulations of seaweeds

formed by fragmentation

• Asexual reproduction through spore formation– haploid spores are formed within an area of the

thallus (sporangium) through meiosis– sporophyte—stage of the life cycle that produces

spores, which is diploid


Reproduction in Seaweeds

• Sexual reproduction– gametes fuse to form a diploid zygote– gametophyte—stage of the life cycle that

produces gametes– gametangia—structures where gametes

are typically produced

• Alteration of generations—the possession of 2 or more separate multicellular stages (sporophtye, gametophyte) in succession


Green Algae

• Structure of green algae– Most are unicellular or small multicellular

filaments, tubes or sheets– Some have a coenocytic thallus consisting

of a single giant cell or a few large cells containing more than 1 nucleus and surrounding a single vacuole• the cell grows and the nucleus divides

– There is a large diversity of forms among green algae


Green Algae

• Response of green algae to herbivory– Tolerance: rapid growth and release of

huge numbers of spores and zygotes – Avoidance: small size allows them to

occupy out-of-reach crevices– Deterrence:

• calcium carbonate deposits require strong jaws and fill stomachs with non-nutrient minerals

• many produce repulsive toxins


Green Algae

• Reproduction in green algae– the common sea lettuce, Ulva, has a life

cycle that is representative of green algae– basic alternation of generations between

the sporophyte and gametophyte stages• sporophytes and gametophytes are nearly

identical• spores and gametes are similar, but spores

have 4 flagella while gametes have 2• gametes of opposite mating types must fuse

for fertilization to occur


Red Algae

• Primarily marine and mostly benthic• Red color comes from phycoerythrins

– Thalli can be many colors, yellow to black

• Structure of red algae– Almost all are multicellular– Thallus may be blade-like, composed of

branching filaments, or heavily calcified• algal turfs—low, dense groups of filamentous

and branched thalli that carpet the seafloor over hard rock or loose sediment


Red Algae

• Response of red algae to herbivory– making their thalli less edible by

incorporating calcium carbonate– changing growth patterns to produce

hard-to-graze forms like algal turfs– evolving complex life cycles which allow

them to rapidly replace biomass– avoiding herbivores by growing in crevices


Red Algae

• Reproduction in red algae– 2 unique features of their variety of life

cycles:• absence of flagella• occurrence of 3 multicellular stages: 2

sporophytes in succession and one gametophyte


Red Algae Life Cycle

• sperm from male gametophyte forms zygote on part of female gametophyte, then divides– carposporophyte—unique red algae stage which

develops from the female gametophyte once the attached zygote begins to divide

• carposporophyte produces non-motile diploid spores called carpospores

• carpospores settle, germinate, and grow into an adult alga called a tetrasporophyte

• tetrasporophyte releases non-motile haploid tetraspores which grow into gametophytes


Red Algae

• Ecological relationships of red algae– a few smaller species are:

• epiphytes—organisms that grow on algae or plants

• epizoics—organisms that grow on animal hosts

– consolidation—process of cementing loose bits and pieces of coral together• red coralline algae precipitate calcium

carbonate from water and aid in consolidation of coral reefs


Red Algae

• Commercial uses of red algae– phycocolloids (polysaccharides) from cell

walls are valued for gelling or stiffening• e.g. agar, carrageenan

– Irish moss is eaten in a pudding– Porphyra are used in oriental cuisines

• e.g. sushi, soups, seasonings

– cultivated for animal feed or fertilizer in parts of Asia


Brown Algae

• Familiar examples:– rockweeds– kelps– sargassum weed

• 99.7% of species are marine, mostly benthic

• Olive-brown color comes form the carotenoid pigment fucoxanthin


Brown Algae

• Distribution of brown algae– more diverse and abundant along the

coastlines of high latitudes– most are temperate– sargassum weeds are tropical


Brown Algae

• Structure of brown algae– bladders—gas-filled structures found on

larger blades of brown algae, and used to help buoy the blade and maximize light

– cell walls are composed of cellulose and alginates (phycocolloids) that lend strength and flexibility

– trumpet cells—specialized cells of kelps that conduct photosynthetic products (e.g. mannitol) to deeper parts of the thallus


Brown Algae

• Reproduction in brown algae– usual life cycle = alternation of

generations between a sporophyte (often perennial) and a gametophyte (usually an annual)

– rockweed (Fucus) eliminates gametophyte stage; meiosis occurs on inflated tips of the sporophyte, fertilization in the water

– rhizoids—root-like structures which attach the fertilized egg and grow into a holdfast


Brown Algae

• Brown algae as habitat– kelp forests house many marine animals– sargassum weeds form floating clumps

that provide a home for unique organisms

• Commercial products from brown algae– thickening agents are made from alginates– once used as an iodine source– used as food (especially in the Orient) and

cattle feed


Marine Flowering Plants

• General characteristics of marine flowering plants– vascular plants are distinguished by:

• phloem—vessels that carry water, minerals, and nutrients

• xylem—vessels that give structural support

– seed plants reproduce using seeds, structures containing an embryonic plant and supply of nutrients surrounded by a protective outer layer


Marine Flowering Plants

– 2 types of seed plants:• conifers (bear seeds in cones)• flowering plants (bear seeds in fruits)

– all conifers are terrestrial– marine flowering plants are halophytes,

meaning they are salt-tolerant


Invasion of the Sea by Plants

• Flowering plants evolved on land and then adapted to the marine environment

• Flowering plants compete with seaweeds

• Their bodies are composed of polymers like cellulose and lignin that are indigestible to most marine organisms

• A single species may dominate long-term; other organisms depend on it


Seagrasses

• Seagrasses are hydrophytes (they generally live beneath the water)

• Classification and distribution of seagrasses– 12 genera in 5 families of 3 clades (groups

with a common ancestor)• 1 clade = eelgrasses and surf grasses• 2nd clade = paddle grasses (Halophila), turtle

grasses, and Enhalus • 3rd clade = paddle grass (Ruppia), manatee

grasses, and shoal grasses


Seagrasses

– ½ of the species inhabit the temperate zone and higher latitudes; other ½ are tropical and subtropical

• Structure of seagrasses– vegetative growth—growth by extension

and branching of horizontal stems (rhizomes) from which vertical stems and leaves arise

– 3 basic parts: stems, roots and leaves


Seagrasses (Structure)

– stems • have cylindrical sections called internodes

separated by nodes (rings)• rhizomes—horizontal stems with long

internodes with growth zones at the tips, usually lying in sand or mud

• vertical stems arise from rhizomes, usually have short internodes, and grow upward toward the sediment surface

– roots• arise from nodes of stems and anchor plants• usually bear root hairs—cellular extensions• allow interaction with bacteria in sediments



– leaves• arise from nodes of rhizomes or vertical stems• scale leaves—short leaves that protect the

delicate growing tips of rhizomes• foliage leaves—long leaves from vertical

shoots with 2 parts– sheath that bears no chlorophyll– blade that accomplishes all photosynthesis using

chloroplasts in its epidermis (surface layer of cells)

• undergo periods of growth and senescence– blade life cycles affect epiphytes on seagrasses



– aerenchyme—an important gas-filled tissue in seagrasses• lacunae—spaces between cells in aerenchyme

tissues throughout the plant– provide a continuous system for gas transport

• aerenchyme is reduced to microscopic pores at nodes and where parts join to keep water out

• provides buoyancy to the leaves so they can remain upright for sunlight exposure

• tannins—antimicrobials produced as a chemical defense against invasion of the aerenchyme by fungi or labyrinthulids


Seagrasses

• Reproduction in seagrasses– some use fragmentation, drifting and re-

rooting and do not flower– flowers are usually either male or female

and borne on separate plants– hydrophilous pollination

• sperm-bearing pollen is carried by water currents to stigma (female pollen receptor)

– a few species produce seedlings on the mother plant (viviparity)


Seagrasses

• Ecological roles of seagrasses– role of seagrasses as primary producers

• less available and digestible than seaweeds• contribute to food webs through fragmentation

and loss of leaves – sources of detritus

– role of seagrasses in depositing and stabilizing sediments• blades act as baffles to reduce water velocity• decay of plant parts contributes organic matter• rhizomes and roots help stabilize the bottom• reduce turbidity—cloudiness of the water


Seagrasses (Ecological Roles)

– role of seagrasses as habitat• create 3-dimensional space with greatly

increased area on which other organisms can settle, hide, graze or crawl

• rhizosphere—the system of roots and rhizomes along with the surrounding sediment

• the young of many commercial species of fish and shellfish live in seagrass beds


Salt Marsh Plants

• Much less adapted to marine life than seagrasses; must be exposed to air

• Classification and distribution of salt marsh plants– salt marshes are well developed along the

low slopes of river deltas and shores of lagoons and bays in temperate regions

– salt marsh plants include:• cordgrasses (true grasses)• needlerushes• many kinds of shrubs and herbs


Salt Marsh Plants

• Structure of salt marsh plants– smooth cordgrass, which initiates salt

marsh formation, grows in tufts of vertical stems connected by rhizomes• culm—vertical stem• tillers—additional stems produced by a culm at

its base which give a tufted appearance

– aerenchyme allows diffusion of oxygen– flowers are pollinated by the wind– seeds are dispersed by water currents


Salt Marsh Plants

• Adaptations of salt marsh plants to a saline environment– facultative halophytes—plants that can

tolerate salty as well as fresh water– leaves covered by a thick cuticle to retard

water loss– well-developed vascular tissues for

efficient water transport– Spartina alterniflora have salt glands– shrubs and herbs have succulent parts


Salt Marsh Plants

• Ecological roles of salt marsh plants– contribute heavily to detrital food chains– help stabilize coastal sediments and

prevent shoreline erosion– rhizomes of cordgrass help recycle the

nutrient phosphorus through transport from bottom sediments to leaves

– remove excess nutrients from runoff– are consumed by terrestrial animals (e.g.

insects)


Mangroves

• Classification and distribution of mangroves– mangroves include 54 diverse species of

trees, shrubs, palms and ferns in 16 families

– ½ of these belong to 2 families:• red mangrove (Rhizophora mangle)• black mangrove (Avicennia germinans)

– others are white mangroves, buttonwood, and Pelliciera rhizophoreae


Mangroves (Distribution)

– thrive along tropical shores with limited wave action, low slope, high rates of sedimentation, and soils that are waterlogged, anoxic, and high in salts

– low latitudes of the Caribbean Sea, Atlantic Ocean, Indian Ocean, and western and eastern Pacific Ocean

– mangal—a mangrove swamp community


Mangroves

• Structure of mangroves– representative of mangroves are trees

with simple leaves, complex root systems– roots: many are aerial (above ground) and

contain aerenchyme• stilt roots of the red mangrove arise high on

the trunk (prop roots) or from the underside of branches (drop roots)

• lenticels—scarlike openings on the stilt root surface connecting aerenchyme with the atmosphere


Mangroves (Structure)

• anchor roots—branchings from the stilt root beneath the mud

• nutritive roots—smaller below-ground branchings from anchor roots which absorb mineral nutrients from mud

• black mangroves have cable roots which arise below ground and spread from the base of the trunk

• anchor roots penetrate below the cable root• pneumatophores—aerial roots which arise from

the upper side of cable roots, growing out of sediments and into water or air


Mangroves (Structure)

– leaves• mangrove leaves are simple, oval, leathery

and thick, succulent like marsh plants, and never submerged

• stomata—openings in the leaves for gas exchange and water loss

• salt is eliminated through salt glands (black mangroves) or by concentrating salt in old leaves and then shedding them


Mangroves

• Reproduction in mangroves– simple flowers pollinated by wind or bees– mangroves from higher elevations have

buoyant seeds that drift in the water– mangroves of the middle elevation and

seaward fringe have viviparity• propagule—an embryonic plant that grows on

the parent plant• hypocotyl—long stem hanging below the

parent branch on which the propagule grows


Mangroves

• Ecological roles of mangroves– root systems stabilize sediments

• aerial roots aid deposition of particles in sediments

– epiphytes live on aerial roots– canopy is a home for insects and birds– mangals are a nursery and refuge– mangrove leaves, fruit and propagules are

consumed by animals– contribute to detrital food chains

© 2006 thomson-brooks cole chapter 7 multicellular primary producers

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