Modern lagoonal microbialites of Tikehau and Moorea ( French Polynesia ) : nature, distribution and lithification processes

S. Sprachta

Microbialites are organosedimentary deposits that have accreted as a result of benthic (prokaryotic or eukaryotic) communities, trapping and binding detrital sediment and / or forming the locus of mineral precipitation (Burne & Moore, 1987). Stromatolites (in greek:  stroma, carpet and lithos, stone) represent the best known type of microbialite.

Microbialites were part of Earth’s History for more than 3 billion years with a variable importance. One supposed microbialites excluded from marine environment since Mesozoic. Recent discovery of their presence during some periods of Quaternary, characterized by environmental and climatic changes, led to examine their role in modern environments (Camoin & Montaggioni, 1994).

For about twenty years microbialites proliferation in modern reefal settings, subjected to an anthropic stress or not, was highlighted in many localities ( Bahamas , Great Barrier Reef , Antilles …). Rapid blooms of microbial benthic communities in tropical reefs seem to be linked with environmental stress periods  and/or climatic events that may have a harmful effect on the development of coralgal communities

Microbialites including stromatolitic structures, have been observed in French Polynesia, from the  fringing reef to the inner flat of the barrier reef of Moorea, a high island partially exposed to human impact, and in the whole lagoon of Tikehau atoll (pinnacle slopes and surrounding bottom) (Sprachta, et al., 1999),  in pristine conditions where no direct human impact evidenced (Sprachta et al, 2001). Similar specimens of cyanobacterial microbialites proliferate as well from the back reef zone to the fringing reef of Mayotte ( Comoros archipelago, Indian Ocean ) under heavy terrigenous inputs, (Sprachta, 2003).

The study of these modern lagoonal microbialites was supported by two national programs: “Programme National Récifs Coralliens”  (PNRCO) and “Programme National Environnement Côtier” (PNEC) in collaboration with CEREGE (Centre Européen de Recherche et d'Enseignement de Géosciences de l'Environnement, COM (Centre d’Océanologie de Marseille), Biological Science Center of the University of Boston (USA), Max Planck Institute of marine microbiology of Bremen (Germany) and Department of Microbiology of the University of Arizona (USA). Main objectives were the determination of nature and composition in micro-organisms implied in the formation of microbialites and evaluation of the importance of ‘microbialite' in biogeochemical cycles of the modern reef systems.

 A-Nature and distribution of lagoonal microbialites

Four morphological types, mainly composed of specific mono populations of filamentous cyanobacteria were observed (Abed et al, 2003) : 

1- decimeter sized hemispherical domes formed by Phormidium cf crosbyanum (Tilden),  P. sp. TK1, P. hendersonii (Howe), Schizothrix sp TK. domes of Phormidium crosbyanum
2- gelatinous masses produced by Phormidium laysanense (Lemmerman)
3- meter-sized microbial mats mainly made of Hydrocoleum chantharidosmum (Gomont), Hydrocoleum coccineum (Gomont), Phormidium sp. or Schizothrix sp. mats of Hydrocoleum coccineum
4- hairy microbialites produced by the cyanobacteria Symploca hydnoïdes (Gomont)

Lagoonal microbialites are able to colonize various sites characterized by different ecological conditions, such as Tikehau, a semi-closed atoll with no anthropic pressure; high islands lagoons largely open on the ocean and locally subjected to an anthropic pressure such as Moorea; or lagoon subjected to a strong human pressure like Mayotte (Sprachta, 2003).

Light, nutriments supplies, hydrodynamism and sedimentation are the specific factors of microbialitic development. Any modification of these factors led to a variability of density and diversity in the different types of microbialite along the slopes of pinnacles and motus. Their settlement and development at a given depth and on a particular substrate depends on cyanobacterial metabolic needs. Indeed, filamentous cyanobacteria, can accommodate to a wide range of ecological conditions thanks to specific physiological adaptations. Thus, lagoonal microbialites settle between the surface and 25 m deep, on various types of substrate (mud, sand, gravels, and coral colonies) and under extreme conditions of luminosity (Sprachta et al., 2001;  Fig.).

Zonation of lagoonal microbialites on the slopes of a pinnacle at Tikehau
 (Sprachta et al., 2001).

Proliferation of marine microbialites has significantly increased during the last fifteen to twenty years and is obviously facilitated by natural or human factors, injurious for the coralgal communities. Progressive disappearance of the coral colonies leaves a free field to replacement communities such as microbialites.

B-Lithification of lagoonal microbialites

Initially, trapping and binding mineral particles strengthen microbialites and play a significant role in their formation. These particles mainly bioclastic in origin, include skeletal fragments of corals, calcareous algae, forams or cocoliths.
Microphotograph of trapped particles among filamentous cyanobacteria	Close-up showing a cocolith glued to a decaying cyanobacterial sheath

Microbialite partial lithification results from biochemical reactions involving magnesian calcite precipitation mainly in the inner part of the domes where the decay of cyanobacterial sheaths produce an organic framework of very thin fibrils (fig.), (Sprachta et al., 2001).

Detailed view of a degraded cyanobacterial sheath encrusted by precipitated carbonates

Biochemical analyses of organic matter contained within microbialites highlighted the selection of some compounds towards intramineral organic matrices. These macromolecules are characterized by enrichment in aspartic and glutamic acids, responsible for the chelation of calcium.  The two antagonistic functions of crystal nucleation and crystal growth inhibition are allotted to these organic matrices. Specific electric charges of polypeptides and polysaccharides which compose intramineral organic matrices, are known to be implied in biomineralization. Acidic macromolecules, in relation to a b sheaths structure, initiate the nucleation of micro aggregates of magnesian calcite to their surface (Weiner & Addadi, 1991).

The nucleus growth continues in this alkaline media, saturated in carbonates (fig.). In the case of the most hardened microbialites, the precipitation of concentric layers of calcite around these nucleii leads to the formation of rhomboedral calcite crystal around the degraded sheaths (fig.) (Sprachta, 2003).
Cyanobacterial sheath heavily incrusted by precipitated carbonates  bbbbbb	Precipitation of rhomboedral calcite crystal around decaying cyanobacterial sheaths

References

Abed R., Golubic S., Garcia-Pichel F., Camoin G., Sprachta S. (sous presse) Characterization of microbialithe-forming cyanobacteria in a tropical lagoon: Tikehau atoll, Tuamotu, French Polynesia. Journal of Phycology

Burne R. V., Moore L. S. (1987) Microbialites : Organosedimentary deposits of benthic microbial communities. Palaios, v. 2, p. 241-254

Camoin G. F., Montaggioni L. F. (1994). High energy coralgal-stromatolite frameworks from Holocene reefs (Tahiti, French Polynesia). Sedimentology 41, 655-676

Sprachta S., Camoin G., Gautret P., Le Campion T., Golubic S. (1999) Preliminary biosedimentological and biochemical results on modern microbialites from a lagoonal environment (Tikehau atoll, French Polynesia). IAS-ISRS-SEPM Intern. Symp. " Paleoceanology of Reefs and carbonate platforms: Miocene to modern ", Aix-en-Provence, Abstracts with program. Publ. Sp. A.S.F., 32, 199-200.

Sprachta S., Camoin G., Golubic S., Le Campion T. (2001) Microbialites in a modern lagoonal environment : nature and distribution (Tikehau atoll, French Polynesia). Palaeogeogr., Palaeoclimatol., Palaeoecol. 175, 103-124

Sprachta S. (2003) Les microbialithes des systèmes récifaux actuels de Tikehau, Moorea (Polynésie Française) et de Mayotte (Comores, Océan Indien), nature, distribution et processus de lithification.  Thèse de doctorat, 220 p + annexes.

Weiner, S., Addadi, L., (1991) Acidic macromolecules of mineralized tissues : controllers of crystal formation. Trends in Biochemical Sciences v.16, p. 252-256.