Fine structure of the stratified microbial community at Laguna Figueroa, Baja California, Mexico. I. Methods of in situ study of the laminated sediments

The microbial community of the flat laminated sediments in the hypersaline lagoon at Laguna Figueroa has undergone dramatic changes due to prolonged periods of submergence under fresh water. This paper describes the techniques used in the first comprehensive in situ ultrastructural study of a stratified microbial community formerly dominated by Microcoleus sp. and introduces the changes caused by flooding and burial. Flat mat desiccation polygons fixed in 2.5% buffered glutaraldehyde were studied by light and transmission electron microscopy. Typical pre-flood Microcoleus-dominated laminated mat from 1977 was compared to post-flood laminated mat from 1979. The technique faithfully preserves the various laminae with their associated organisms and sediment, Morphological identification of several genera of photosynthetic prokaryotes including cyanobacteria, purple, and green bacteria was made by analysis of cell membrane and wall structure. Amoebic cysts could also be recognized in situ. Bacteria of unknown generic affinity, but with distinctive ultrastructure were discovered. A filamentous purple photosynthetic bacterium was found to be a common component of the 1979 mat and two bacteria within a bacterium associations were seen in sections from the 1977 mat.