Complex microbial communities composed of auto- and heterotrophic bacteria and protists are found both in common and in unusual environments such as anoxic basins. Stratified water masses with an anoxic bottom layer offer a great diversity of ecological niches, causing a complex interplay between chemical stratification and vertical zonation of the microbial assemblage, including both prokaryotes and eukaryotes. The role of free-living phagotrophic protists in the carbon flux of oxygenated aquatic systems is well established, but less is known on their impact on assemblages of anaerobic primary producers.
On the other hand, although the development of phototrophic sulfur bacteria blooms in stratified basins has been widely studied, the fate of their primary production is poorly understood. I then tried to quantify the impact of predation by protists on a summer bloom of brown-colored green sulfur bacteria (GSBs) in Faro Lake, particularly within the GSB bacterial plate, which is characterized by extremely high picoplankton biomass and also by an elevated biomass of anaerobic or microaerophilic protozoa.
My aim was to elucidate the function of phagotrophic protists in channeling the prokaryotic primary production to higher trophic levels in environments, where photosynthetic sulfur bacteria play a major role as primary producers. Unfortunately, gaining insight into the structure and functioning of anaerobic communities is hampered by practical difficulties, because even trace amounts of oxygen inhibit or kill many microorganisms. Thus, modifications of known in situ grazing experiment procedures, designed with the aim of circumventing this and other methodological drawbacks, have been necessary.
My study demonstrated significant grazing activity by nanosized protozoa upon picoplankton in the GSB bloom layer. Furthermore, although the stable meromixis of Faro Lake could be considered as a filter barrier that confines secondary producers to the anoxic hypolimnion, it seems that this natural barrier can be broken by motile predators - i.e. copepods - which are able to swim downwards into anoxic waters to feed (unpublished data), and thus export the anaerobic primary production to oxygenated waters when they accomplish the inverse migration .