Chapter 7: Multicellular

Primary Producers

• Most primary productivity done by phytoplankton

Multicellular Algae

• Seaweed (macroalgae) inhabit the oceans

• Red, brown, and green in color

• Phycologists/algologists study seaweeds and phytoplankton

Distribution of Seaweeds

• Most are benthic• Grow on rocks, sand,

mud, coral, and other organisms

• Are parts of fouling communities– Plants and animals that

live on pilings, bulkheads, boat hulls, moorings, and other artificial surfaces

• Inhabit 2% of seafloor

• Effects of light:– Numbers vary by depth,

latitude, sea conditions, and season

– Red algae is at greatest depth

– Green algae at shallowest depth

– Brown algae is at intermediate depths • Effects of temperature:

– Greatest diversity is at tropical waters• Decreases as you

get farther north or south of the equator

Structure of Seaweeds• Body is called the thallus– All but a few cells are

photosynthetic– Lacks vascular tissue– No stems, roots, or leaves– Can occur in complex

shapes

• Holdfast attaches thallus to a surface

• Stipe: stemlike region between holdfast and blade

Biochemistry of Seaweeds• Photosynthetic pigments– Due to wavelengths of light

not absorbed by seaweed pigments

– Chlorophyll absorbs blue and red wavelengths of light• Pass green light

– Accessory pigments absorb different wavelengths of light• Pass energy to chlorophyll for

photosynthesis• Can protect chlorophyll from

damage by light

• Cell wall composition– Primarily cellulose–Many secrete a slimy gelatinous mucilage• Made of sugar polymers• Covers their cells• Can hold a great deal of water• Acts as protective covering to prevent desiccation

– Can have a cuticle• Multilayered covering of protein

• Food reserves

–Excess sugars converted into polymers

• Stored as starches

–Unique sugars and alcohols can be used as antifreeze at high latitudes

Reproduction in Seaweeds• Asexual– Fragmentation: thallus

breaks into pieces• Each new piece grows into

new alga– Spore formation

• Sexual– Fertilization of

gametes to form a zygote (fertilized egg cell)

Green Algae (Chlorophyta)

• Contain pigments like in vascular plants• Most are freshwater species• Seasonal sources of food for marine animals• Contribute to formation of coral reefs• Structure: unicellular or small multicellular

filaments, tubes, or sheets

• Response to herbivory:

–Rapid growth and release of huge numbers of spores and zygotes prevent elimination of their populations by herbivores

–Small size allows them to occupy crevices on rocky shores and reefs

Red Algae (Rhodophyta)• Primarily marine (98%)

• Highest diversity among seaweeds

• Mostly benthic

• Are not always red

• Structure: – Almost all are multicellular– Less than 1m long– Thallus varies widely in shape

and organization

• Response to herbivory:– Food source for sea urchins, fish,

mollusks, and crustaceans– Do not produce many toxins to deter

herbivores– Can make thallus less edible– Can change growth patterns to make

it more difficult to graze

• Commercial uses:

–Agar

–Thickening agent in ice cream, pudding, and salad dressings

–Source of food

–Animal feed and fertilizer

Brown Algae (Phaeophyta)

• Ex: rockweeds, kelp, sargassum weed• 1500 species marine• Range in size from microscopic and filamentous to

giant• Distribution:–More diverse and abundant along coastlines of

high latitudes– Less diverse in the tropics

• Structure:–Well-defined thallus– Develop large, flat, leaflike blades– Gas-filled bladders• Help buoy the blade to get maximum sunlight

exposure– Cell walls composed of cellulose

• Habitat: found from low tide line to a depth of 10m

• Commercial products:– Thickening agents in textile,

dental, cosmetic, and food industries

– Source of iodine– Used as food– Cattle feed

Marine Flowering Plants

• General characteristics:

–Presence of xylem (carries water) and phloem (carries nutrients)

–Reproduce by seeds (protected by a fruit)

–Roots, stems, and leaves

Invasion of the Sea by Plants

• New source of habitat and food

• Compete with seaweeds for light and other benthos for space

• Bodies composed of polymers (cellulose, lignin) that most marine organisms cannot digest

• Have few competitors

Seagrasses (Hydrophytes)• Live beneath the water• Only ones that are truly marine• 60 species (0.02%)

• Classification and distribution:

–Not true grasses

–Related to lilies and other freshwater plants

– Inhabit temperate zones and higher latitudes

–Others are tropical and subtropical

• Structure:

–Form from vegetative growth

–Roots, stems, leaves

–Aerenchyme

•Gas-filled tissue

• Provide buoyancy to leaves

• Potential for invasion by pathogenic fungi

• Reproduction:

– Fragmentation, drifting, and plant re-rooting

–Some do not flower

– If they flower, they lack showy petals

• Small and inconspicuous

• Male or female

–Hydrophilous pollination

• Ecological roles:– Primary producers

food for herbivores– Depositing and

stabilizing coastal sediments reduces turbidity

– Beds provide habitats for many marine species

Salt Marsh Plants

• Less adapted to marine life

• Must be exposed to air to flourish– Restricted to intertidal zone

• Biological filter for terrestrial runoff on its way to the sea

• Must tolerate higher sediment salinities

• Classification and distribution:

–Well developed along slopes of river deltas and shores of lagoons and bays

–Distribution is limited at high and low latitudes

• Structure:– Grows in tufts of vertical

stems (culms) connected by rhizomes

– Each vertical stem produces additional stems (tillers) at its base

– Roots connected at culms– Plants stand as high as 3m• Well-drained nutrient-rich

soils– Flowers pollinated by wind• Seeds drop to the

sediment or are carried by water currents

• Adaptations to a saline environment:

–Facultative halophytes

•Can tolerate salty and freshwater conditions

–Tend to lose water to their environment by osmosis

–Thick, water-retaining succulent parts

• Ecological roles:– Detritus supplies nutrient needs for fish and

shellfish– Shallow roots and rhizomes• Helps stabilize coastal sediments• Prevents shoreline erosion

– Accumulate and stabilize sediments• Recycles phosphorous• Filter runoff from coastal areas

– Removes toxic pollutants

• Maintains water quality– Removes excess nutrients

Mangroves

• Trees and shrubs of tropical waters

• Little is submerged by the tide

• Classification and distribution:– 54 species– Specialized roots that descend to or rise from the

sediment– Thrive along protected tropical shores with limited wave

action, low slope, high rates of sedimentation, and soils that are waterlogged, anoxic, and high in salts

– Occur at low latitudes– Associated with saline lagoons– Found in tropical estuaries, islands, and atolls– Dominant vegetation in mangrove swamps or mangals

• Structure:– Roots• Adapted to grow in loose,

shallow, anoxic, saline sediments• Many above ground (aerial)• Ability to prevent salts from

entering the system from the sediment

• Leaves– Simple, oval, leathery,

and thick– Never submerged by

flood tide– Salt glands can secrete

a concentrated salt solution

• Reproduction:– Simple flowers pollinated

by wind or bees

• Ecological roles:– Root systems stabilize

sediments– Aerial roots aid deposition

of particles– Canopy is host to many

insects and birds– Nursery and refuge to many

animals