Red Algae
or Rhodophyta

Galaxaura fasciculata

Brown Algae
or Fucuphyceae

Green Algae
or Chlorophyta

Caulerpa bikinensis

Blue Algae
or Cyanobacteria

Phormidium sp

Algae are autotrophic organisms, which are able to manufacture their own organic molecules from elements containing carbon and nitrogen. Their energy is obtained from sunlight, which is trapped by the chlorophyll pigment. Furthermore, certain algae such as Ulva, are capable of directly incorporating organic susbtances, while the unicellular algae euglenoids and dinoflagellates capture, phagocytose and digest their prey. They are  basically aquatic organisms, even if some (like the green algae Rhizoclonium) temporarily colonise exposed habitats.

Chlorophyta are algae whose thallus is typically green in colour due to chlorophyll a and b pigments that are dominant in the chloroplasts. However, prolonged exposure to strong light leads to the synthesis of photoprotecting pigments (carotenoids) that turns the thalli orange to yellow. This group of algae which is poorly diversified in temperate waters is in fact rich in species and forms in tropical waters. Green algae are present in all aquatic systems, from marine to freshwater habitats. 

The most diversified Chlorophyta in French Polynesia are Caulerpa and Halimeda.

Caulerpa urvilliana The presence of long creeping stolons (up to a metre in length) contributes locally to the retainment of sediments at the bottom of atoll lagoons. Caulerpa bikinensis  Veritable meadows on hard surfaces, such as is the case on the atoll of Takapoto, on the external reef slope, beyond a depth of 30 metres.  Caulerpa racemosa These algae which look like grapes are named "rimu" in Australes islands, where they are still consumed by the inhabitants to their diet in salad form. Halimeda taenicola Halimeda, with calcified articulated segments are particularly important because of their considerable contribution to the formation of sediments. Their abundance varies from one area of the reef to another, or from island to island, and some beaches in the Gambier Islands are almost exclusively composed of the eroded and bleached segments of Halimeda.

We find the cosmopolitan Ulva and Enteromorpha species abundant in calm waters with variable salinity that are occasionnally over-charged in nutrients. However, Ulva blooms remain relatively infrequent on our reefs compared to the " green tides " that they create in several areas of the world. In French Polynesia and in the Cook Islands, the Boodlea kaeneana alga can, on certain reefs and in the lagoons, bloom in spectacular fashion during the southern summer. An overcharge in nutrients linked to growing urbanisation, in addition to strong sunlight, is the most probable hypothesis to explain this proliferation.

Within the Chromophyta, brown algae are grouped in the Fucophyceae class, formerly called Phaeophyceae. These are almost exclusively marine algae. Their colour is due to the abundance of  brown fucoxanthin  pigments that mask a and c chlorophyll. 

Fucophyceae display great morphological diversity, from relatively simple filamentous forms to the large brown algae (Turbinaria, Sargassum) whose complex morphology approaches the leafy stems of higher plants. 

Brown algae are mainly diversified in cold and temperate seas where they form large underwater forests (kelp forests). In tropical waters they represent fewer species, but have the largest thalli and form the densest populations.

We could not find big brown algae in the Tuamotu until the 80's.

Since 10 years, Turbinaria ornata appears in several atoll lagoons. Their origin is not yet known, but the thalli rafts adrift floating on the surface of the ocean have certainly contributed to the appearance of this new species in the Tuamotu regions. A recent study has reinforced this hypothesis showing that the thallus segments can be scattered on a long distance and keep their fertility (Stiger et Payri, 1999).

Rhodophita are by far the most numerous in tropical regions.

They show a particular originality with their dominant red (phycoerythrins) and blue (phycocyanins) pigments that mask chlorophyll. The relative proportions of the different pigments, in conjunction with the shape of the thallus, result in a range of all imaginable colours from dark brown to light pink, purple reds and orange tints. Furthermore, within a single species, colour varies according to the exposure to light and often individuals that grow in strong light display faded colours where orange-yellows  dominate due to the strong concentration of photoprotectant carotenoid pigments.

Blue-green algae, or Cyanobacteria, fundamentally differ from other groups of algae, since they are categorised as bacteria. They belong to the most ancient forms of life on Earth. During the Precambrian era (about 1.5 billion years ago), they built-up rocky formations called stromatoliths, either by the precipitation of calcium or by the trapping of sediments. Despite their very ancient origin, we should not consider them as a relic group, on the contrary, though discrete, they occupy all types of habitats, even those inacessible to other organisms.

They are generally microscopic filamentous forms that bore into calcareous substrata, or adhere to each other to create colonies of strongly variable sizes, shapes and colours. Like red algae, they possess more blue (phycocyanin) and red (phycoerythrin) pigments that mask chlorophyll a. Despite their former name of blue-green algae, they are rarely blue but more often red, green with blue hues, violet, brown, yellow or orange. Most of them have a gelatinous or even sticky texture, owing to mucilaginous secretions, but this is not generally the rule. In addition to their photosynthetic ability, Cyanobacteria play an important role in the biosphere by transforming atmospheric nitrogen into nitrates which are directly used by other organisms. This ability is of fundamental importance on coral reefs, where nutrients are scarce.

Furthermore, Cyanobacteria are for humans choice microorganisms in several branches of biotechnology, owing to the numerous families of chemical molecules that they manufacture, which have a potential value.

Despite an apparent simplicity in the organisation of forms, the taxonomic identification of Cyanobacteria currently remains difficult and complex. 

Summary of the French Polynesian algal communities which appears in those pages

References

Payri, C., N'Yeurt A.R. & Orempüller, J. - 2001 - Algae of french Polynesia -Algues de Polynésie Française. Edition Au Vent des îles - tahiti, 320pp.

Payri, C.E. & N'Yeurt, A.D.R. 1997. A revised Checklist of Polynesian benthic Marine Algae, Australian Systematic Botany, 10: 867-910.

Stiger, V. & Payri, C.E. – 1999 – Spatial and temporal patterns of settlement of the brown macroalgae Turbinaria ornata and Sargassum mangarevense in a coral reef on Tahiti. MEPS, 191: 91-100.