Vertical stratification and microbial assemblage of a microbial mat in the Ebro Delta (Spain)

Vertically stratified communities in a temporarily inundated sand flat at the Ebro Delta were studied. Three different microbial mats were compared using light microscopy, scanning electron microscopy and pigment analysis. The youngest mat had less layers than wellestablished mats and contained mostly filamentous cyanobaeteria belonging to the genus Spiruliaa . Diatoms, as well as coccoid cyanobacteria, could be found on the surface of this mat. The only pigmented layer present was green in color and the main pigment found, belonging to these oxygenic phototrophs, was chlorophyll a . Small amounts of bacteriochiorophyll a were present. Well-developed mats were vertically stratified into three distinct pigmented layers. In the up-permost yellow-brown layer, Nitzschia sp., Navicula sp. and Amphora sp. (diatoms) and coccoid cyanobacteria were dominant. The second layer was composed mostly of Microcoleus sp. and a few filaments of Lyngbya sp. and coccoid cyanobacteria. The principal pigment in both layers was chlorophyll a . Bacteriochlorophyll a , predominantly found in anoxTgenic phototrophic bacteria, increased with depth. The deepest layer, purple or orange in color depending on sampling site, was composed basically of anoxygenic phototrophic bacteria, i.e. Chromatium sp. and Thiocapsa sp. Several species of non phototrophic bacteria, i.e. spirochetes and other unidentified small rods and cocci, could be observed in these communities. Although it is not common to find eukaryotes, with the exception of diatoms, a eopepod and an unidentified nematode were present in these communities. The different mat populations occurred within a few millimeters where close contacts could be established among them. The extracellular polymer production in several microorganisms allowed their attachment to sediment particles.