Dark bacterial-protozoan culture: acidification, nitrite accumulation, and protozoan inhibition in absence of phytoplankton

Abstract

A 50-1 batch culture of sea water has been performed in a dark, aerated, and thermostated tank. A daily organic enrichment promoted the growth of bacteria and protozoans. Degradation of organic matter and biomass production were studied through transfers between organic and inorganic nitrogen compartments. Quantitative and qualitative development of bacteria and protozoans compared with dissolved free amino-acid (DFAA) concentrations interactions and limiting factors in a bacterial-protozoan food web have been studied. A steady state in microbial production appeared after 2 wk. It coincided with the development of a heterotrophic bacterial community utilizing DFAAs more as growth factors than an energy source. The limiting factor seems to be nitrifying activity: autotrophic bacteria as ammonia-oxidisers utilized inorganic nitrogen more for their energy requirement than for biomass production. Furthermore, the small amount of biomass produced by nitrifiers was coupled with nitrite accumulation and acidification of the culture. This paper illustrates the role of autotrophic activity when phytoplankton growth is avoided and the difficulty of obtaining a large biomass production under these conditions.