COMMUNITY STRUCTURE OF FILAMENTOUS, SHEATH-BUILDING SULFUR BACTERIA, THIOPLOCA SPP, OFF THE COAST OF CHILE

The filamentous sulfur bacteria Thioploca spp, produce dense bacterial mats in the shelf area off the coast of Chile and Peru. The mat consi sts of common sheaths, shared by many filaments, that reach 5 to 10 cm dean into the sediment, The structure of the Thioploca communities of f the Bay of Concepcion was investigated,vith respect to biomass, spec ies distribution, and three-dimensional orientation of the sheaths, Th ioploca sheaths and filaments were found across the whole shelf area w ithin the oxygen minimum zone, The maximum wet weight of sheaths, 800 g m(-2), was found at a depth of 90 m, The bacterial filaments within the sheaths contributed about 10% of this weight, The highest density of filaments was found within the uppermost I cm of the mat, On the ba sis of diameter classes, it was possible to distinguish populations co ntaining only Thioploca spp, from mixed populations containing Beggiat oa spp, Three distinct size classes of Thioploca spp. were found, two of which have been described previously as Thioploca araucae and Thiop loca chileae. Many Thioploca filaments did not possess a visible sheat h, and about 20% of the sheaths contained more than one Thioploca spec ies. The three-dimensional sheath structure showed that Thioploca fila ments can move from the surface and deep into the sediment.