Abstract
The amino acid composition of organic matrices associated with calcium carbonate precipitates in microbialites built by different Phormidium species (cyanobacteria) has been compared for samples recovered in lagoonal settings from two regions of the Southern Tropical Pacific separated by more than 4000 km: New Caledonia (Nouméa lagoon) and French Polynesia (Tikehau atoll). Calcium carbonate precipitation in these microbial structures was observed mainly in the interior of the domes and clearly separated from the photosynthetically active surface layer. This study focuses on the hydrolysable amino acid composition of the associated organic matrices that are typically rich in cysteine, leucine, alanine and arginine in New Caledonia, whereas they are particularly rich in dicarboxylic amino acids in French Polynesia. This striking difference is seemingly related to different environmental conditions that characterize the two reef settings. The high cysteine content suggests an origin from metallothioneins produced by the cyanobacteria and/or by epiphytic diatoms that were observed on the top layer, as the result of the input of metals from terrestrial origin in the Nouméa lagoon. In addition, we analysed the bulk organic matter of the photosynthetically active surface layer and of the interior of the domes. The former showed remarkable variations of amino acid composition throughout the year 2001, which may potentially reflect the impact of climatological events (e.g. cyclones) and/or a much stronger seasonality in New Caledonia than in French Polynesia. Although the mechanisms behind the differences remain elusive, our study clearly shows that environmental conditions can be reflected by amino acid compositions, particularly for the organic matrices associated with carbonate precipitates.