COLORLESS SULFUR BACTERIA, BEGGIATOA SPP AND THIOVULUM SPP IN O-2 AND H2S MICROGRADIENTS

Abstract

The interactions between colorless sulfur bacteria and the chemical microgradients at the O2-H2S interface were studied in Beggiatoa mats from marine sediments and in Thiovulum veils developing above the sediments. The gradients of O2, H2S and pH were measured by microelectrodes at depth increments of 50 .mu.m. An unstirred boundary layer in the water surrounding the mats and veils prevented microturbulent or convective mixing of O2 and H2S. The 2 substrates reached the bacteria only by molecular diffusion through the boundary layer. The bacteria lived as microaerophiles or anaerobes even under stirred, oxic water. O2 and H2S zones overlapped by 50 .mu.m in the bacterial layers. Both compounds had concentrations in the range of 0-10 .mu.mol l-1 and residence times of 0.1-0.6 s in the overlapping zone. H2S oxidation was purely biological. Diffusion calculations showed that formation of mats on solid substrates or of veils in the water represented optimal strategies for the bacteria to achieve a stable microenvironment, a high substrate supply and an efficient competition with chemical H2S oxidation. The continuous gliding movement of Beggiatoa cells in mats or the flickering motion of Thiovulum cells in veils were important for the availability of both O2 and H2S for the individual bacteria.