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15. Macroalgae
G. Diaz-Pulido
N OVERVIEW
Generalities
Macroalgae are often referred to as seaweeds, yet they are
not actually ‘weeds’. Rather, macroalgae is a collective
term used for large algae that are macroscopic and generally grow in the sea. They differ from other plants such as
seagrasses and mangroves, in that algae lack roots, leafy
shoots, flowers, and vascular tissues. In fact, algae are a
polyphyletic group with diverse evolutionary histories.
Whilst still used, the term ‘algae’ is no longer recognised
as a formal taxon although it remains a useful name when
referring to those protists that are photosynthetic.
The macroalgae of the Great Barrier Reef (GBR) are
a very diverse and complex group of species and forms.
Forms include crusts, foliose, and filamentous thalli
(thallus refers to the body of an alga), ranging from simple branching structures to complex forms with highly
specialised structures. The specialised structures are
adaptations for light capture, reproduction, support,
flotation, and/or substrate attachment. The size of coral
reef macroalgae ranges from a few millimetres to plants
of up to 3–4 m high (such as the brown alga Sargassum).
However, in the more nutrient rich temperate regions of
the oceans, brown algal kelps may grow to over 50 m in
length (e.g. giant bull kelp in California). These organisms are not found on coral reefs.
Tropical macroalgae occupy a variety of habitats,
including shallow and deep sections of coral reefs,
inter-reefal areas, sandy bottoms, seagrass beds, mangrove roots, rocky intertidal areas, or even within the
skeletons of healthy and dead corals, shells and limestone material (endolithic algae). Macroalgae are the
major food source for a variety of herbivores, are major
reef formers and create habitat for invertebrates and
vertebrates of economic interest. They also play critical
roles in reef degradation, when coral dominated reefs
are replaced by rocky reefs covered in macroalgae.
N DIVERSITY OF GREAT BARRIER REEF
MACROALGAE
Taxonomic diversity and classification
Macroalgae are taxonomically classified into four different Phyla: Rhodophyta (from the Greek ‘rhodon’
meaning ‘red rose’ and ‘phyton’ meaning ‘plant’: red
algae), Ochrophyta (Class Phaeophyceae, from the
Greek ‘phaios’ meaning ‘brown’: brown algae), Chlorophyta (from the Greek ‘chloros’ meaning ‘green’: green
algae) and Cyanobacteria (from the Greek ‘cyanos’
meaning ‘dark blue’: blue-green algae) (Table 15.1).
This systematic classification is based on the composition of pigments involved in photosynthesis.
There are c. 630 species and varieties of macroalgae
recorded for the GBR according to the Australian Marine
Algal Name Index. However, this list is preliminary and
it is likely that the number will increase with future field
exploration. In fact, the macroalgal flora from the GBR,
15. Macroalgae
G. Diaz-Pulido
N OVERVIEW
Generalities
Macroalgae are often referred to as seaweeds, yet they are
not actually ‘weeds’. Rather, macroalgae is a collective
term used for large algae that are macroscopic and generally grow in the sea. They differ from other plants such as
seagrasses and mangroves, in that algae lack roots, leafy
shoots, flowers, and vascular tissues. In fact, algae are a
polyphyletic group with diverse evolutionary histories.
Whilst still used, the term ‘algae’ is no longer recognised
as a formal taxon although it remains a useful name when
referring to those protists that are photosynthetic.
The macroalgae of the Great Barrier Reef (GBR) are
a very diverse and complex group of species and forms.
Forms include crusts, foliose, and filamentous thalli
(thallus refers to the body of an alga), ranging from simple branching structures to complex forms with highly
specialised structures. The specialised structures are
adaptations for light capture, reproduction, support,
flotation, and/or substrate attachment. The size of coral
reef macroalgae ranges from a few millimetres to plants
of up to 3–4 m high (such as the brown alga Sargassum).
However, in the more nutrient rich temperate regions of
the oceans, brown algal kelps may grow to over 50 m in
length (e.g. giant bull kelp in California). These organisms are not found on coral reefs.
Tropical macroalgae occupy a variety of habitats,
including shallow and deep sections of coral reefs,
inter-reefal areas, sandy bottoms, seagrass beds, mangrove roots, rocky intertidal areas, or even within the
skeletons of healthy and dead corals, shells and limestone material (endolithic algae). Macroalgae are the
major food source for a variety of herbivores, are major
reef formers and create habitat for invertebrates and
vertebrates of economic interest. They also play critical
roles in reef degradation, when coral dominated reefs
are replaced by rocky reefs covered in macroalgae.
N DIVERSITY OF GREAT BARRIER REEF
MACROALGAE
Taxonomic diversity and classification
Macroalgae are taxonomically classified into four different Phyla: Rhodophyta (from the Greek ‘rhodon’
meaning ‘red rose’ and ‘phyton’ meaning ‘plant’: red
algae), Ochrophyta (Class Phaeophyceae, from the
Greek ‘phaios’ meaning ‘brown’: brown algae), Chlorophyta (from the Greek ‘chloros’ meaning ‘green’: green
algae) and Cyanobacteria (from the Greek ‘cyanos’
meaning ‘dark blue’: blue-green algae) (Table 15.1).
This systematic classification is based on the composition of pigments involved in photosynthesis.
There are c. 630 species and varieties of macroalgae
recorded for the GBR according to the Australian Marine
Algal Name Index. However, this list is preliminary and
it is likely that the number will increase with future field
exploration. In fact, the macroalgal flora from the GBR,