Microbialites are organosedimentary deposits that have accreted as a result of growth and binding detrital sediment by a benthic microbial community. Microbialite structures have been reported bearing surface, abundantly in the fossil record: Quaternary times. the earliest microfossils at the and throughout the in the Precambrian Earth's till Phanerozoic Modern microbialites are widespread in tropical lagoonal environments where they have been reported from the surface down to 20- 25m water depths, which implies that they can cope with a wide range of Olivier Pringault et al. light irradiance conditions. This essay focuses on the functioning of these microbial structures with a special attention to their photo- adaptation. This includes an efficient light utilization and a phototactic migration when exposed to high light intensities. Microbialites productivity is comparable to the values measured in cyanobacterial stromatolites (microbial mats) that grow in shallow water environments. However, microbialites are characterized by a lower respiration production ratio indicating a higher growth efficiency. The productivity of microbialites seems not to be limited by nutrients although they thrive in oligotrophic waters characterized by low nutrient concentrations. For their development, microbialites require a substratum, such as sand or green algae thalli but also dead or live corals. In coral reef systems, development of microbialites on live corals may be damaging for the latter due to shade effects. By combining an efficient light utilization and a high growth efficiency, cyanobacterial microbialites seems to be very competitive for light with respect to living corals.
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