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
• 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)
– brown algae (phylum Phaeophyta)
– green algae (phylum Chlorophyta)
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
• Consumed by an array of animals, e.g., sea
urchins, snails, fish
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 is governed primarily by light
and temperature
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
concentration, etc.
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
– intertidal algae can be killed if temperatures
become too hot or cold
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
• Lack vascular (conductive) tissue, roots,
stems, leaves and flowers
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
• e.g. carotenes, xanthophylls, phycobilins pass energy to
chlorophylls for photosynthesis
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
– Some have a protective cuticle—a multi-layered
protein covering
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
– 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, e.g., some sargassum
weeds
• Asexual reproduction through spore
formation
– haploid spores formed within an area of the
thallus (sporangium) through meiosis
– sporophyte (diploid): stage of the life cycle that
produces spores, which is diploid
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
(asexual sporophtye, sexual gametophyte) in
succession
HAPLOID
DIPLOID
Germinating
zygote
Zygote
(diploid)
Sporophyte
(diploid)
Gametes
fusing
Sporangium
Spores
Gametangium
(haploid)
+Spore
+Gametophyte Germinating
(haploid)
+spore
–Spore
+Gametophyte
+Gametes
(haploid)
–Gametes
(haploid)
Germinating
–spore
–Gametophyte
–Gametophyte
(haploid)
Stepped Art
Fig. 7-3, p. 164
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
– 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 herbivores with
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
• large, leafy 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 (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 greens, browns) and branched thalli that
carpet the seafloor over hard rock or loose sediment
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-tograze forms like algal turfs
– evolving complex life cycles which allow them to
rapidly replace grazed 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
HAPLOID
DIPLOID
Germinating
carpospore
Sporangia
Tetraspores
(haploid)
Diploid
carpospores
Tetrasporophyte
(diploid)
Egg
(haploid)
Growth
Filament
Female
gametophyte
(haploid)
Young
carposporophyte
Zygote
nucleus
(diploid)
Germinating
tetraspores
Zygote
nucleus
(diploid)
Male
gametophyte
(haploid)
Sperm and egg
Sperm
fuse
(haploid)
Stepped Art
Fig. 7-6, p. 168
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
• 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 animals
– red coralline algae precipitate calcium carbonate
from water and aid in consolidation of coral reefs
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
– cultivated for animal feed or fertilizer in parts of
Asia
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
carotenoid pigment fucoxanthin, masks
green pigment of chlorophylls a & c
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
– 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
conduct photosynthetic products (e.g. mannitol)
to deeper parts of the thallus
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
– rhizoids—root-like structures which attach the
fertilized egg and grow into a holdfast
HAPLOID
DIPLOID
Zygote
(diploid)
Sperm and
egg fuse
Gas bladders
Young sporophyte
(diploid)
Receptacle
Sperm
(haploid)
Eggs
(haploid)
Receptacles
Egg
Gametangium
containing eggs
(haploid)
Sporophyte
(diploid)
Cross-section
of a receptacle
Sperm
Gametangium
containing
sperm
(haploid)
Magnified
view of a
conceptacle
Stepped Art
Fig. 7-11, p. 172
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)
used as cattle feed in some coastal countries
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
surrounded by a protective outer layer
Marine Flowering Plants
– 2 types of seed bearing plants:
• conifers (bear seeds in cones)
• flowering plants (bear seeds in fruits)
– all conifers are terrestrial
– marine flowering plants are called halophytes,
meaning they are salt-tolerant
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
• Have few competitors and often form extensive
single-species stands on which other members of
the community depend
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,
and shoal grasses
Seagrasses
• 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 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
allow interaction with bacteria in sediments
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
– blade life cycles affect epiphytes on seagrasses
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
• tannins—antimicrobials produced as a chemical
defense against invasion of the aerenchyme by
pathogenic fungi or labyrinthulids
Seagrasses
• Reproduction in seagrasses
– some use fragmentation, drifting and rerooting 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)
– a few species produce seedlings on the
mother plant (viviparity)
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
•
•
•
•
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 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
• direct – extracted material used for food, medicine and industrial
application
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
• various shrubs and herbs, e.g., saltwort, glassworts
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
– seeds drop to sediment or are dispersed by
water currents
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
– shrubs and herbs have succulent parts
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
– are consumed by (at least in part) by crabs and
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
– associated with saline lagoons and
tropical/subtropical estuaries
– mangal: a mangrove swamp community
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)
• 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
• lenticels and aerenchyme of pneumatophores act as
ventilation system for black mangrove
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) or by concentrating salt in old leaves
that shed
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)
• propagule falls from parent tree and may drift in currents by the
buoyant hypocotyl for as long as 100 days
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