Mixotrophs combine photosynthesis with phagotrophy to cover their demands in energy and essential nutrients. This gives them a competitive advantage under oligotropihc conditions, where nutrients and bacteria concentrations are low. As the advantage for the mixotroph depends on light, the competition between mixo- and heterotrophic bacterivores should be regulated by light. To test this hypothesis, we incubated natural plankton from the ultra-oligotrophic Eastern Mediterranean in a set of mesocosms maintained at 4 light levels spanning a 10-fold light gradient. Picoplankton (heterotrophic bacteria (HB), pico-sized cyanobacteria, and small-sized flagellates) showed the fastest and most marked response to light, with pronounced predator-prey cycles, in the high-light treatments. Albeit cell specific activity of heterotrophic bacteria was constant across the light gradient, bacterial abundances exhibited an inverse relationship with light. This pattern was explained by light-induced top-down control of HB by bacterivorous phototrophic eukaryotes (PE), which was evidenced by a significant inverse relationship between HB net growth rate and PE abundances. Our results show that light mediates the impact of mixotrophic bacterivores. As mixo- and heterotrophs differ in the way they remineralize nutrients, these results have far-reaching implications for how nutrient cycling is affected by light.
KAUST Repository Item: Exported on 2020-10-01
Acknowledgements: The experiment was financed by the European Union Seventh Framework Program MESOAQUA (FP7/2009–2013) coordinated by JCN under grant agreement no. 228224. MESOAQUA granted Transnational Access to R. & R.P., S.D.M., M.S., S.S., A.G., K.A., P.L. (LightDynaMix), A.C., R.A.M., A.S., A.G., S.J.R., J.M.G. (LiMic) and S.I., S.Z. (LightCopFed). R.P. further acknowledges financial support by the German Research Council (DFG Pt 5/3-1), JMG by Spanish project REMEI (CTM2015-70340) and AC by project PROTOS (CTM2009-08783) from the Spanish Ministry of Economy and Competitiveness. T. Tanaka is highly acknowledged for contributing his data on phosphorus uptake and bacterial activity. The authors wish to thank Dr. P. Divanach for valuable advice on technical matters, G. Piperakis for his inspired technical assistance throughout the experiment, S. Zivanovic and E. Dafnomili for assistance with chemical analyses, D. Podaras for assistance during the experiment and N. Sekeris for his help with constructions and ideas. The captain and the crew of the R/V Philia are also thanked for their assistance during the transportation of water from the sea to the mesocosms. Comments by three anonymous reviewers have helped improving the manuscript.