Artificial cyanobacterial mats: structure and composition of the biota

The structure of cyanobacterial mars growing in and on foam rubber (initiated with inoculates of defaunated sediments) is described by Light and transmission electron microscopy. Different horizontal layers can be recognised which largely correspond to what has previously been described from stromatolitic mats in hyperhaline habitats, but phototroph diversity was lower than recorded from natural mats. The maximum volume fraction of filamentous cyanobacteria was found within the upper 100 to 200 mu m of the mat (about 10 or 25% when mucous sheaths are included); deeper in the mat (down to 3-4 mm where purple sulphur bacteria appear) their volume fraction was reduced to about about 1/3 of that of the surface layer. The total length of cyanobacterial filaments beneath 1 cm(2) mat is about 14 km. Various types of cyanobacteria (chroococcoids, Phormidium, Microcoleus, and Nostoc) were present, but 2 morphospecies of Pseudoanabaena were quantitatively dominant in the studied mats. Vertical migration of the organism in response to changing light conditions probably played a minor role in these mats.