Mixotrophic trade-off under warming and UVR in a marine and a freshwater alga

Abstract

Mixotrophic protists combine phagotrophy and phototrophy within a single cell. Greater phagotrophic activity could reinforce the bypass of carbon (C) flux through the bacteria-mixotroph link and thus lead to a more efficient transfer of C and other nutrients to the top of the trophic web. To determine whether foreseeable changes in temperature and UVR upset the mixotrophic trade-off in favor of one or the other nutritional strategy along the mixotrophic gradient is key to understanding the fate of C and mineral nutrients in the aquatic ecosystem. Two main hypotheses arise: 1) that increased warming and UVR will divert metabolism toward phagotrophy, and 2) that the magnitude of this shift will vary according to the organism’s position along the mixotrophic gradient. To test these hypotheses, we used two protists (Isochrysis galbana and Chromulina sp.) located in different positions on the mixotrophic gradient, subjecting them to the action of T and of UVR and their interaction. Our results show that the joint action of the two factors increased the primary production (PP):bacterivory (BV) ratio and stoichiometric values (N:P ratio) close to Redfield’s ratio. Therefore, T and UVR shifted the metabolism of both organisms toward greater phototrophy regardless of original position of the organism on the mixotrophic gradient. Weaker phagotrophic activity could cause a less efficient transfer of C to the top of trophic webs.