Evidence for the Priming Effect in a Planktonic Estuarine Microbial Community

The ‘priming effect’, in which addition of labile carbon and/or nutrients changes remineralization rates of recalcitrant organic matter, has been intensively studied in soils, but is less well-documented in aquatic systems. We investigated the extent to which additions of nutrients or labile organic carbon could influence remineralization rates of microbiallydegraded, recalcitrant C-labeled phytoplankton necromass in microcosms inoculated with microbial communities drawn from Groves Creek Estuary in coastal Georgia, USA. We found that amendment with labile protein plus phosphorus increased recalcitrant organic carbon mineralization rates by up to 100%, whereas acetate slightly decreased mineralization rates relative to an unamended control. Addition of ammonium and phosphate induced a smaller effect, whereas addition of ammonium alone had no effect. Counterintuitively, alkaline phosphatase activities increased in response to the addition of protein under P-replete conditions, indicating that production of enzymes unrelated to the labile priming compound may be a mechanism for the priming effect. The observed priming effect was transient: after 36 days of incubation roughly the same quantity of organic carbon had been mineralized in all treatments including no-addition controls. This timescale suggests that priming in coastal systems does not influence the long-term preservation of organic carbon, but may influence the exchange of organic carbon between fresh waters, estuaries, and the coastal ocean. ∗corresponding author: [email protected] 1 . CC-BY 4.0 International license peer-reviewed) is the author/funder. It is made available under a The copyright holder for this preprint (which was not . http://dx.doi.org/10.1101/030916 doi: bioRxiv preprint first posted online Nov. 7, 2015;