Distribution of phototrophic microbes in the flat laminated microbial mat at Laguna Figueroa, Baja California, Mexico.

The microbial mat community in the saltmarsh/evaporate flat interface at Laguna Figueroa involved in the deposition of laminated sediments was investigated. Pigment analysis, light microscopy and transmission electron microscopy were used to determine the relative abundance and distribution of phototrophic species. The community is vertically stratified into four distinct phototrophic populations. The layering could be distinguished by pigment and species composition. The two layers closest to the surface contained mostly oxygenic phototrophs and chlorophyll a as the primary photosynthetic pigment. Anoxic phototrophs predominated in the bottom two layers with bacteriochlorophylls a and c in the third layer and bacteriochlorophyll a and b in the bottom layer. The surface yellow layer was composed primarily of Navicula, Rhopalodia and other diatom species as well as the cyanobacteria Aphanothece sp. and Phormidium sp. Microcoleus chthonoplasces and Chroococcidiopsis sp. were the major cyanobacteria in the green colored second layer. In the third layer, pinkish-purple in color, purple photographs (Chromatium sp., Thiocapsa roseoparsicina) and filamentous green phototrophs (Chloroflexus sp., Oscillochloris sp.) were abundant. The fourth and deepest photosynthetic layer was salmon colored and composed primarily of Thiocapsa pfennigii, and other purple sulfur phototrophs. The pattern of alternating light (oxygenic community) and dark (anoxygenic community) layering preserved in older laminae is a consequence of this community structure. Study of the flat laminated mat over the 10-year period (1978-1988) including and after its destruction by catastrophic flooding events in 1978 and 1980, showed a succession of stratified communities culminating in the return of Microcoleus and the full compliment of layers by the fall of 1984.