Stream biofilms: a prime site for priming

Priming occurs when the presence of labile organic carbon enhanced the metabolism of recalcitrant organic carbon. Mechanisms of priming and its implications for carbon cycle in increasingly studied and understood in soils but not in aquatic ecosystems. We propose to systematically and rigorously study priming in benthic biofilms in streams. We postulate that the close spatial proximity of algae, and their labile exudates, with microbial heterotrophy enables priming of putative recalcitrant terrigenous organic carbon in streams. Thus we postulate benthic biofilms as a prime site for priming and suggest a series of experiments to test this. Microcosms with biofilms and labeled (13C) recalcitrant organic carbon will serve to quantify priming and, and a combined genomic, transcriptomic and proteomic approach will unravel possible mechanisms, including shifts in community structure and enzymatic activities. Next, streamside flumes will serve to assess the implications of priming at the level of quasi-natural systems. Finally, the relevance for priming will be studied in selected stream reaches in Europe and the USA. The proposed research may contribute to better understand the net heterotrophy in numerous headwater streams and their contribution to global carbon cycling.

Streams and rivers play a hitherto poorly appreciated role in the global carbon cycle. They receive large terrestrial deliveries of organic carbon that microbial heterotrophs respire to CO2 and that is eventually emitted from streams and rivers into the atmosphere. Much of this organic carbon is old and putatively recalcitrant. It has been thought that fresh and labile organic carbon in streams, such as from algal exudates, primes these terrestrial deliveries. We postulated that the close proximity of algae and heterotrophic bacteria in biofilms may enhance priming of terrestrial organic carbon. To test this hypothesis we conducted several experiments combining stable isotope analysis with molecular approaches and enzymatic assays. We were not able to demonstrate that priming occurs in microbial biofilms. However, our results show that light intensity modulates critical processes related to carbon cycling and the microbial structure in biofilms.