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. Determining how foreseeable changes in temperature and UVR affect mixotrophic trade‐offs in favor of one or the other nutritional strategy, along the mixotrophic gradient, is key to understanding the fate of carbon and mineral nutrients in the aquatic ecosystem. Our two main hypotheses were: (i) that increased warming and UVR will divert metabolism toward phagotrophy, and (ii) 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 temperature and of UVR and their interaction. Our results showed that the joint action of these two factors increased the primary production:bacterivory ratio and stoichiometric values (N:P ratio) close to Redfield's ratio. Therefore, temperature and UVR shifted the metabolism of both organisms toward greater phototrophy regardless of the 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.

Disentangling the role of light and nutrient limitation on bacterivory by mixotrophic nanoflagellates

“…Due to the wide range of strategies amongst mixotrophic protists, especially in response to environmental conditions, the understanding of activity of mixotrophs under climate change scenarios is a critical challenge (Gonzalez‐Olalla et al, 2019). It is necessary to conduct in situ experiments that explore the responses of different metabolic processes under multiple stressors that are emerging in aquatic systems.…”

Eco-Evolutionary Perspectives on Mixoplankton

“…This mixotrophic spectrum is further affected by the degree of investment into nutrient uptake (Andersen et al, 2015), to which previously discussed potential trade-offs apply. How mixotrophy and mixotrophic communities change in response to environmental changes is a current scientific challenge (González-Olalla et al, 2019). Environmental conditions discussed here include light irradiance, temperature, and nutrients in the perspective of feeding and growth.…”

“…Nephroselmis pyriformis, for example, can achieve normal growth even when nutrient-limited by compensation via bacterivory (Anderson et al, 2018). Other seemingly non-phagotrophic species can be induced to perform phagotrophy by nutrient-limited treatments, e.g., in dinoflagellates (Smalley et al, 2003;Johnson, 2015), haptophytes (Chan et al, 2019), and cryptophytes (González-Olalla et al, 2019). The use of diluted media to provoke starvation indicates the complexity in understanding in detail the principal factors that could induce phagotrophy and identify mixotrophy.…”

Modeling the metabolic evolution of mixotrophic phytoplankton in response to rising ocean surface temperatures

“…This prediction has mixed empirical support however: In short-term studies of phenotypic plasticity, freshwater mixotrophs from the genus Ochromonas , marine Isochrysis galbana , and freshwater Chromulina sp. have shown increased rates of bacterivory with temperature and overall shifts towards a more heterotrophic metabolism [ 19 – 21 ]. In contrast, the freshwater dinoflagellate Dinobryon sociale and marine dinoflagellate Karlodinium armiger show increased relative contributions of photosynthesis with temperature [ 22 , 23 ] indicating that mixotrophs’ underlying physiological constraints will shape their thermal response [ 22 ] Further, it is unclear how mixotrophs will respond over evolutionary timescales due to the costs they experience from maintaining two fundamentally different forms of metabolism [ 24 – 26 ].…”