Population study of the filamentous sulfur bacteria Thioploca spp. off theBay of Concepcion, Chile

A population of filamentous sulfur bacteria Thioploca spp. living in the Ba y of Concepcion, Chile, and the adjoining shelf area was sampled for 14 mo at 4 to 6 wk intervals to investigate the influence of seasonal variations in upwelling intensity and oxygen concentrations on the population dynamics . The Thioploca population was described by its biomass, total number and d iameter of sheaths, number of trichomes and species per sheath, and abundan ce and depth distribution of different morphological forms, e.g. trichome d iameters and ratios of cell-length to diameter. Throughout the summer of 19 96, oxygen concentrations in the bottom water were near zero, nitrate was 1 0 to 20 mu M and the biomass was high, up to 160 g m(-2) wet weight without sheaths. During winter, the biomass declined due to higher oxygen concentr ations under reduced upwelling intensity. The depth distribution of Thioplo ca spp, changed strongly with seasonal variations, but the population struc ture remained mainly unchanged. During the 'El Nino' event in 1998, with hi gh oxygen and low primary production the biomass was very low. In the Bay o f Concepcion 2 populations of filamentous sulfur bacteria were observed, fi laments with short cells in sheaths, populating the upper 7 cm of the sedim ent, and filaments without sheaths living at the sediment surface.