Sedimentation of photosynthetic pigments during the bloom of the green sulfur bacterium Chlorobium phaeobacteroides in Lake Kinneret: spatial patterns

Abstract

Sediment fluxes were studied in the subtropical Lake Kinneret (Israel) in 2006, 2007, and 2008 from mid-June to October, when lake was chemically stratified and the green sulfur bacterium Chlorobium phaeobacteroides formed a dense population in the anoxic metalimnion. The rate of seston accumulation in traps was measured with sedimentation traps positioned along an offshore transect connecting the littoral zone and lake center. The sediment fluxes increased from the lake center towards the littoral, while the percentage of organic material (OM) decreased correspondingly. High fluxes of bacteriochlorophyll e (BChl e—a signature pigment of Chl. phaeobacteroides) and chlorophyll a (Chl a—a marker for eukaryotic algae and cyanobacteria) were detected in all locations. The relative contributions of Chl a and BChl e to the bulk of the accumulated OM were higher in traps positioned below the thermocline in the pelagic zone than in traps located near the shore line. The presence of BChl e in traps exposed to oxic conditions in the littoral, where Chl. phaeobacteroides does not develop, implies horizontal translocation of cells from the lake center towards its periphery. We assume that seiche-mediated movement of particles embedded in the metalimnetic waters is the most probable explanation for the existence of Chl. phaeobacteroides tracer in an oxic environment, but do not exclude the possibility of resuspension of settled particles as source for BChl e in littoral traps. The green sulfur bacteria are potentially important component of the sediment flux of photosynthesizing organisms in a thermally stratified lake and should be taken into account when carbon budget are constructed.