Microbialites in a modern lagoonal environment: nature and distribution, Tikehau atoll (French Polynesia)

Microbialites, including stromatolitic structures, gelatinous masses and mats develop in Tikehau lagoon (Tuamotu, French Polynesia), on the flanks and slopes of numerous pinnacles and ‘motu’ islets between 0 and 25 m deep, on coral colonies, algal tufts or on lagoonal sediments. Individual stromatolitic structures consist of hemispherical domes and are produced by monospecific populations of filamentous cyanobacteria belonging to three distinctive Phormidium species, Symploca, and possibly to one new species of Schizothrix. Gelatinous masses hanging from the ceiling of caverns are produced by Phormidium laysanense. Microbial mats are well developed at all depths on pinnacle slopes and are produced by either monospecific or mixed cyanobacterial populations. The study of micro- and ultrastructure of stromatolites reveals that they result both from trapping and binding of sediments and from carbonate precipitation. In addition to phototrophic biomineralization, pervasive carbonate precipitation occurred in association with decaying organic matter. The basic constructional elements of the precipitated carbonate consist of 0.1–0.2-μm rounded bodies that form very fine anhedral to subhedral micrite; their surfaces comprise extremely small carbonate granules (20–50 nm). Gradual changes (ageing) in the ultrastructure of carbonate precipitates include the precipitation of successive euhedral calcite layers around these grains. Widespread development of microbialites in this pristine lagoonal environment results from rapid blooms of microbial benthic communities, possibly induced by temporary and local nutrient pulses. Nutrients seem to undergo a restricted circulation between the sediment and pinnacle reef frameworks. Pinnacles and reef islets are repeatedly affected by catastrophic storm events which cause redistribution of sediments and nutrients.