Difficulties in interpretation of microbial heterotrophy from sulfate uptake data: Laboratory studies 1

Inorganic carbon uptake (primary production) and sulfate uptake were studied in axenic cultures of a Chlorella sp., a Nannochloris sp., and a Scenedesmus sp. All of the cultures took up significant amounts of sulfate in the dark, and it is concluded that the direct use of dark sulfate uptake as a measure of in situ bacterial heterotrophic production is probably not valid. The C:S uptake ratios of the algal cultures in the light also varied widely so that the indirect use of a single C:S value for calculating microbial heterotrophic production is also not valid. Carbon:sulfur uptake ratios appear to have some regular but complex relationship to primary production.