15 – Macroalgae
153
layer of angular or polygonal cells, resembling a
honeycomb. Hollow to solid inside depending on
the species. Common (Fig. 15.2I).
L Halimeda: plants erect, lightly to heavily calcified,
pale to dark green. Branches formed by calcified
segments separated by deep constrictions. Segments
can be flattened (triangular to discoid or kidneyshaped) to cylindrical (Fig. 15.2J, K).
L Ostreobium: microscopic green filaments, cylindrical
to inflated, usually within skeletons of healthy and
dead corals and other carbonate substrates. Widespread on deep and shallow reefs (Fig. 15.2L).
L Udotea: upright calcified, stalked and fan-shaped
plant, anchored to the substratum by a rhizoidal
mass. Grey-green (Fig. 15.2M).
L Ulva: bright green, sheet-like or membranous blades.
Uncommon on reefs. Species of Enteromorpha (now
belong to the genus Ulva) are small (a few mm) and
have the form of a hollow tube; common on algal
turfs.
L Ventricaria: globose plants up to several cm in diameter. Glossy dark green with bright reflective glare.
Usually epiphytised by pink crustose calcareous algae. Common throughout the reef.
Reproduction
Macroalgae reproduce either asexually or sexually.
Asexual reproduction involves the release of spores
(propagules) or by fragmentation (pieces of plant braking off to produce new individuals). In sexual reproduction, male and female gametes are released into the
water, however, there are some examples where female
gametes are retained by the parent and the resulting
embryo develops (at least temporarily) on the parent
gametophyte.
Macroalgae have complex life cycles, of which there
are three types in coral reef algae. (1) haplontic life cycle
(meiosis of a zygote occurs immediately after karyogamy); (2) diplontic life cycle (the zygote divides mitotically to produce a multicellular diploid individual),
and (3) diplobiontic life cycle or alternation of generations
(the haploid and diploid phases are alternated, each
phase consisting of one of two separate, free living organisms: a gametophyte, which is genetically haploid,
and a sporophyte, which is genetically diploid).
N ECOLOGICAL ROLES
Contribution to primary production
A large proportion of the primary production (the creation of organic matter by plants from inorganic material like CO 2 and sunlight during photosynthesis) in a
coral reef comes from the contribution of benthic algae.
Net primary production is variable ranging from
148–500 (g C m
2 yr
1 ) for algal turfs, 146–1095 (g C m
2
yr
1
) for fleshy macroalgae, and 73–475 (g C m
2 yr
1 )
for crustose coralline algae. Planktonic microalgae and
algal symbionts of scleractinian corals also contribute
to reef productivity but to a much lesser degree. The
organic matter (carbon) produced by planktonic microalgae enters the reef food chain either by: (1) consumption by herbivorous fishes, crabs, sea urchins and
zooplankton; (2) release of Dissolved Organic Matter
(DOM) by the algae into the water column where it is
consumed by bacteria that in turn may be consumed by
a variety of filter feeders, or (3) export to adjacent ecosystems such as seagrass meadows, mangroves or to
the sea floor by currents and tides. See further details
on the energy flow through coral reefs in Chapter 7.
Nitrogen fixation and nutrient cycling
Filamentous blue-green algae like Calothrix living in algal turf communities (Fig. 15.2O) fix significant amounts
of atmospheric (inorganic) nitrogen into ammonia,
which is then used by the blue-green algae themselves
to build organic matter. Because of the rapid growth
rates of blue-green algae and intense grazing on turf
communities, the organic nitrogen is then distributed
throughout the reef, contributing to reef nutrition
(Chapter 7). Macroalgae take up, store, and release
nutrients, thereby contributing to nutrient cycling in
coral reef ecosystems.
Construction
Many macroalgae make important contributions to
the construction of the reef framework by depositing
153
layer of angular or polygonal cells, resembling a
honeycomb. Hollow to solid inside depending on
the species. Common (Fig. 15.2I).
L Halimeda: plants erect, lightly to heavily calcified,
pale to dark green. Branches formed by calcified
segments separated by deep constrictions. Segments
can be flattened (triangular to discoid or kidneyshaped) to cylindrical (Fig. 15.2J, K).
L Ostreobium: microscopic green filaments, cylindrical
to inflated, usually within skeletons of healthy and
dead corals and other carbonate substrates. Widespread on deep and shallow reefs (Fig. 15.2L).
L Udotea: upright calcified, stalked and fan-shaped
plant, anchored to the substratum by a rhizoidal
mass. Grey-green (Fig. 15.2M).
L Ulva: bright green, sheet-like or membranous blades.
Uncommon on reefs. Species of Enteromorpha (now
belong to the genus Ulva) are small (a few mm) and
have the form of a hollow tube; common on algal
turfs.
L Ventricaria: globose plants up to several cm in diameter. Glossy dark green with bright reflective glare.
Usually epiphytised by pink crustose calcareous algae. Common throughout the reef.
Reproduction
Macroalgae reproduce either asexually or sexually.
Asexual reproduction involves the release of spores
(propagules) or by fragmentation (pieces of plant braking off to produce new individuals). In sexual reproduction, male and female gametes are released into the
water, however, there are some examples where female
gametes are retained by the parent and the resulting
embryo develops (at least temporarily) on the parent
gametophyte.
Macroalgae have complex life cycles, of which there
are three types in coral reef algae. (1) haplontic life cycle
(meiosis of a zygote occurs immediately after karyogamy); (2) diplontic life cycle (the zygote divides mitotically to produce a multicellular diploid individual),
and (3) diplobiontic life cycle or alternation of generations
(the haploid and diploid phases are alternated, each
phase consisting of one of two separate, free living organisms: a gametophyte, which is genetically haploid,
and a sporophyte, which is genetically diploid).
N ECOLOGICAL ROLES
Contribution to primary production
A large proportion of the primary production (the creation of organic matter by plants from inorganic material like CO 2 and sunlight during photosynthesis) in a
coral reef comes from the contribution of benthic algae.
Net primary production is variable ranging from
148–500 (g C m
2 yr
1 ) for algal turfs, 146–1095 (g C m
2
yr
1
) for fleshy macroalgae, and 73–475 (g C m
2 yr
1 )
for crustose coralline algae. Planktonic microalgae and
algal symbionts of scleractinian corals also contribute
to reef productivity but to a much lesser degree. The
organic matter (carbon) produced by planktonic microalgae enters the reef food chain either by: (1) consumption by herbivorous fishes, crabs, sea urchins and
zooplankton; (2) release of Dissolved Organic Matter
(DOM) by the algae into the water column where it is
consumed by bacteria that in turn may be consumed by
a variety of filter feeders, or (3) export to adjacent ecosystems such as seagrass meadows, mangroves or to
the sea floor by currents and tides. See further details
on the energy flow through coral reefs in Chapter 7.
Nitrogen fixation and nutrient cycling
Filamentous blue-green algae like Calothrix living in algal turf communities (Fig. 15.2O) fix significant amounts
of atmospheric (inorganic) nitrogen into ammonia,
which is then used by the blue-green algae themselves
to build organic matter. Because of the rapid growth
rates of blue-green algae and intense grazing on turf
communities, the organic nitrogen is then distributed
throughout the reef, contributing to reef nutrition
(Chapter 7). Macroalgae take up, store, and release
nutrients, thereby contributing to nutrient cycling in
coral reef ecosystems.
Construction
Many macroalgae make important contributions to
the construction of the reef framework by depositing