Karleskint
Karleskint
Small
Small
Turner
Turner
Chapter 7
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
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
Key Concepts
• Salt marsh plants and mangroves stabilize
bottom sediments, filter runoff from the
Multicellular Algae
• Most primary production in marine
ecosystems takes place by phytoplankton
but seaweed and flowering plants contribute
especially in coastal areas
• Seaweeds are multicellular algae that inhabit
the oceans
• Major groups of marine macroalgae:
– red algae (phylum Rhodophyta)
Multicellular Algae
• Scientists who study seaweeds and
phytoplankton are called phycologists or
algologists
• Seaweeds contribute to the economy of
coastal seas
• Produce 3 dimensional structural habitat for
other marine organisms
Distribution of Seaweeds
• Most species are benthic, growing on rock,
sand, mud, corals and other hard substrata in
the marine environment as part of the fouling
community
• 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 of Seaweeds
• 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
– distribution now believed to be more dependent on herbivory, competition, pigment
Distribution of Seaweeds
• Effects of temperature on seaweed distribution
– diversity of seaweeds is greatest in tropical waters, less in colder latitudes
– temperature not a limiting factor for algae in tropical/subtropical seas
– 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
• freezing and ice scouring can eliminate seaweeds in high latitudes
Structure of Seaweeds
• Thallus: the seaweed body, usually
composed of photosynthetic cells
– when flattened, 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
• Major distinctions among seaweed phyla is based on biochemistry
• Photosynthetic pigments
– Color of thallus due to 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 wavelengths of light, pass green light
– Accessory pigments absorb various colors
Biochemistry of Seaweeds
• 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 protective covering
• holds moisture, and may prevent desiccation • can be sloughed off to remove organisms
Biochemistry of Seaweeds
• Nature of food reserves
– Excess sugars are converted into polymers – Stored in cells as starches
– Chemistry of starches differs among groups of macroalgae
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, e.g., some sargassum weeds
• Asexual reproduction through spore formation
– haploid spores formed within an area of the thallus (sporangium) through meiosis
Reproduction in Seaweeds
• Sexual reproduction
– gametes fuse to form a diploid zygote
– Gametophyte (usually haploid): stage of the life cycle that produces gametes
– gametangia: structures in the gametophytes where gametes are typically produced
• Alteration of generations: the possession of 2
or more separate multicellular stages
+Gametes (haploid) Germinating zygote Sporophyte (diploid) Zygote (diploid) Gametes fusing +Spore +Gametophyte (haploid) Germinating +spore +Gametophyte Spores (haploid) Sporangium –Gametes (haploid) Gametangium HAPLOID DIPLOID –Gametophyte (haploid) Germinating –spore –Spore –Gametophyte Stepped Art
Green Algae (Phylum: Chlorophyta)
• Diverse group of microbes and multicellular
organisms that contain some pigments found in vasculaar plants, chlorphyll a & b and certain carotenoids
• Structure of green algae
– Most are unicellular or small multicellular filaments, tubes or sheets
– Some tropical green algae 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 but doesn’t divide, the nucleus divides
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 herbivores with strong jaws and fill stomachs with non-nutrient
minerals
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
• large, leafy sporophytes and gametophytes are nearly identical
• spores and gametes are similar, but spores have 4 flagella while gametes have 2
Red Algae (Phylum: Rhodophyta)
• Primarily marine and mostly benthic • Highest diversity among seaweeds • 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 or composed of branching filaments or heavily calcified
• algal turfs: low, dense groups of filamentous red (along with
Red Algae
• Annual red algae are seasonal food for sea
urchins, fish, molluscs and crustaceans
• 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 grazed biomass
Red Algae
• Reproduction in red algae
– 2 unique features of their variety of life cycles:
• absence of flagella
• occurrence of 3 multicellular stages:
Germinating carpospore Diploid carpospores Sporangia Tetrasporophyte (diploid) Growth
Sperm and egg fuse Young carposporophyte Zygote nucleus (diploid) Zygote nucleus (diploid) Egg (haploid) Filament Tetraspores (haploid) Germinating tetraspores Male gametophyte (haploid) Sperm (haploid) HAPLOID DIPLOID Female gametophyte (haploid) Stepped Art
Red Algae Life Cycle
• sperm from male gametophyte forms zygote on part of egg-containing female gametophyte, then divides while still attached to the gametophyte to form unique red algal stage called a
carposporophyte
• carposporophyte produces non-motile diploid spores called carpospores
• carpospores settle, germinate, and grow into an adult alga called a tetrasporophyte
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 animals
Red Algae
• Human uses of red algae
– phycocolloids (polysaccharides) from cell walls are valued for gelling or stiffening properties
• e.g. agar, carrageenan
– Irish moss is eaten in a pudding
– Porphyra are used in oriental cuisines
• e.g. sushi, soups, seasonings
Brown Algae (Phylum: Phaeophyta)
• Familiar examples:
– rockweeds – kelps
– sargassum weed
• 99.7% of species are marine, mostly
benthic (sargassum – not benthic)
• Olive-brown color comes form the
Brown Algae
• Distribution of brown algae
– more diverse and abundant along the coastlines of high latitudes
– most are temperate
Brown Algae
• Structure of brown algae
– most species have thalli that are well differentiated into holdfast, stipe and blade
– bladders—gas-filled structures found on larger
blades of brown algae, and used to help buoy the blade and maximize light
– cell walls are made up of cellulose and alginates (phycocolloids) that lend strength and flexibility – trumpet cells—specialized cells of kelps that
Brown Algae
• Reproduction in brown algae
– usual life cycle, i.e., alternation of generations between a sporophyte (often perennial) and a gametophyte (usually an annual)
– rockweed (Fucus) eliminates gametophyte stage; meiosis occurs on inflated tips
(recepticles) of the sporophyte in chambers called conceptacles, fertilization occurs in the water column
Magnified view of a conceptacle
Cross-section of a receptacle Sporophyte (diploid) Receptacle Young sporophyte (diploid) Zygote (diploid) Sperm and egg fuse Gametangium containing sperm (haploid) Sperm (haploid) Sperm Eggs (haploid) Gametangium containing eggs (haploid) Egg Gas bladders Receptacles HAPLOID DIPLOID Stepped Art
Brown Algae
• Brown algae as habitat
– kelp forests house many marine animals
– sargassum weeds of the Sragasso Sea form floating masses that provide a home for unique organisms
• Human uses of brown algae
– thickening agents are made from alginates – once used as an iodine source
– used as food (especially in Asia)
Marine Flowering Plants
• Seagrasses, Marsh Plants, Mangroves
• 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 dormant embryos and nutrients
Marine Flowering Plants
– 2 types of seed bearing plants:
• conifers (bear seeds in cones)
• flowering plants (bear seeds in fruits)
– all conifers are terrestrial
Invasion of the Sea by Plants
• Flowering plants evolved on land and then adapted to estuarine and marine environments
• Flowering plants compete with seaweeds for light and with other benthic organisms for space
• Their bodies are composed of polymers like
cellulose and lignin that are indigestible to most marine organisms
Seagrasses
• Seagrasses are hydrophytes (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,
Seagrasses
• Structure of seagrasses
– vegetative growth—growth by extension and branching of horizontal stems (rhizomes) from which vertical stems and leaves arise
Seagrasses (Structure)
– stems
• have cylindrical internodes (sections) 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 • grow slowly ensuring leaf production keeps up with sediment
accumulation
– roots
• arise from nodes of stems and anchor plants • usually bear root hairs—cellular extensions • Absorb mineral nutrients
Seagrasses (Structure)
– 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
– upper blade that accomplishes all photosynthesis of the plant using chloroplasts in its epidermis undergo periods of growth and senescence
Seagrasses (Structure)
– 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 provides buoyancy to the leaves so they can remain upright for sunlight exposure
Seagrasses
• Reproduction in seagrasses
– some use fragmentation, drifting and re-rooting and do not flower
– inconspicuous 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)
Seagrasses
• Ecological roles of seagrasses
– highly productive on local sale
– role of seagrasses as primary producers
• less available and less digestible than seaweeds
• contribute to food webs through fragmentation and loss of leaves – sources of detritus
– role of seagrasses in depositing and stabilizing sediments
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 also increases complexity in surrounding sediment
• the young of many commercial species of fish and shellfish live in seagrass beds
– human uses of seagrass
• indirect – fisheries depend on coastal seagrass meadows
Salt Marsh Plants
• Much less adapted to marine life than
seagrasses; must be exposed to air by ebbing
tide
• 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
Salt Marsh Plants
• Structure of salt marsh plants
– smooth cordgrass, initiates salt marsh formation, grows in tufts of vertical stems connected by rhizomes,
dominates lower marsh • culm: vertical stem
• tillers: additional stems produced by a culm at its base, gives a tufted appearance
– aerenchyme allows diffusion of oxygen from blades to rhizomes and roots
– flowers are pollinated by the wind
Salt Marsh Plants
• Adaptations of salt marsh plants to a saline
environment
– facultative halophytes—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, secrete salt to outside
Salt Marsh Plants
• Ecological roles of salt marsh plants
– contribute heavily to detrital food chains
– stabilize coastal sediments and prevent shoreline erosion
– serve as refuge, feeding ground and nursery for other marine organisms
– rhizomes of cordgrass help recycle phosphorus through transport from bottom sediments to leaves
– remove excess nutrients from runoff
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
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
– associated with saline lagoons and tropical/subtropical estuaries
Mangroves
• Structure of mangroves
– trees with simple leaves, complex root systems
– plant parts help tree conserve water, supply oxygen to roots and stabilize tree in shallow, soft sediment – 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)
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, never
submerged
• stomata: openings in the leaves for gas exchange and water loss
• salt is eliminated through salt glands (black
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, breaks through fruit wall and grows an elongated cigar-shaped stem (hypocotyl)
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
