Combined Effects of Thermal and Diet-Induced Stressors on the Physiology of Marine Heterotrophic and Mixotrophic Protists

Abstract Although temperature and inorganic nutrient availability are closely linked in marine ecosystems, their combined effects on key functional traits of plankton remain poorly studied. We investigated the responses of three protists, Oxyrrhis marina (heterotrophic dinoflagellate), Karlodinium armiger (mixotrophic dinoflagellate), and Strombidium arenicola (heterotrophic ciliate), to chronic warming and acute heat stress (both +3°C), under nitrogen- or phosphorus-limited conditions. Protists were fed prey with either balanced (nutrient-replete) or imbalanced (nutrient-limited) cellular stoichiometry. When prey were nutrient-replete, physiological rates increased under both warming treatments in all species; gross-growth efficiencies remained insensitive to temperature in heterotrophs but declined in the mixotroph. Under N limitation, both warming treatments generally increased growth rates in all protists while reducing ingestion rates relative to nutrient-replete conditions; gross-growth efficiencies remained similar to or higher than those under nutrient-replete diets. In contrast, P limitation reduced growth rates in heterotrophs under both warming scenarios, particularly in S. arenicola , whereas K. armiger showed overall similar performance to nutrient-balanced conditions. Cell volume decreased under both warming treatments in nutrient-replete conditions in all the species. Cell size was greatest under P limitation in all the protists, although temperature responses varied across protists. Under P limitation, C:P and N:P ratios significantly increased in all the species; however, the overall effects of temperature and nutrient stress on stoichiometric ratios differed among protists. Our results suggest that, among the species examined, the two dinoflagellates may be more tolerant of combined thermal and nutrient stress than the ciliate S. arenicola . This greater tolerance may reflect their weaker elemental regulation and, in the case of K. armiger , its mixotrophic capacity, particularly under P limitation and acute heat stress.