Experimental assessment of a possible microbial priming effect in a humic boreal lake

Global change models normally do not include interaction effects between different pools of recalcitrant humic organic carbon which can alter carbon cycling via their influence on biological activities. This issue is especially important in northern regions where lakes receive high inputs of allochthonous dissolved organic carbon (DOC) from the extensive surrounding peatlands. We investigated the threshold of added labile DOC necessary to promote a priming effect (PE); i.e. stimulation of bacterial metabolism with a subsequent increase in the mineralization of recalcitrant DOC and the accompanying changes in microbial community structure and function. Our study was carried out in a small highly humic lake (Mekkojärvi, southern Finland), physically divided by a plastic curtain into two experimental basins, one where fish were present (+FISH) and one that was fishless (−FISH). In each basin, we performed a factorial mesocosm experiment in which different amounts of labile DOC were supplied as cane sugar (control +6, +9, +12 mg C L⁻¹). Our results showed no priming effect in any carbon treatment, either in +FISH or in −FISH basins, despite a decreasing trend in total DOC concentration. Bacterial abundance and production did not increase as a response to carbon additions, while mixotrophic algae increased their abundance over time. In our experiments, the organisms that benefitted most after addition of labile DOC were mixotrophic algae, which can transform carbon into biomass by obtaining inorganic nutrients through phagotrophy. This appears most likely due to strong bacterial N limitation and dependence on resource availability and stoichiometry.