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It is a contribution of the Marine Zooplankton Ecology Group (2017 SGR 87), Proper thermal adaptation is key to understanding how species respond to temperature. However, this is seldom considered in protozooplankton and mixoplankton experiments. In this work, we studied how two heterotrophic dinoflagellates (Gyrodinium dominans and Oxyrrhis marina), one heterotrophic ciliate (Strombidium arenicola), and one mixotrophic dinoflagellate (Karlodinium armiger) responded to warming, comparing strains adapted at 16, 19 and 22 °C and those adapted at 16 °C and exposed for 3 days at 19 and 22 °C (acclimated treatments). Neither CNP contents nor the corresponding elemental ratios showed straightforward changes with temperature, except for a modest increase in P contents with temperature in some grazers. In general, the performance of both acclimated and adapted grazers increased from 16 to 19 °C and then dropped at 22 °C, with a few exceptions. Therefore, our organisms followed the “hotter is better” hypothesis from 16 to 19 °C; above 19 °C, however, the results were variable. Despite the disparity in the responses between species and physiological rates, in general, it seems that 19 °C-adapted organisms performed better than acclimated-only organisms. However, at 22 °C, most species were at the limit of their metabolisms and were unable to fully adapt. Nevertheless, adaptation to higher temperatures conferred some advantages prior to sudden increases in temperature (up to 25 °C) that simulated a heatwave episode. In summary, adaptation to temperature seems to confer a selective advantage to protistan grazers within a narrow range (i.e., ca. 3 °C). Adaptation to much higher temperatures (i.e., 6 °C) does not confer any clear physiological advantage (with few exceptions; e.g., the mixotroph K. armiger), at least within the time frame of our experiments, This research was funded by Grant CTM2017-84288-R by Fondo Europeo de Desarrollo Regional (FEDER)/ Ministerio de Ciencia, Innovación y Universidades—Agencia Estatal de Investigación (AEI), and by Grant PID2020-118645RB-I00 by Ministerio de Ciencia e innovación (MCIN)/AEI/ 10.13039/501100011033 and by “ERDF A way of making Europe”. With the institutional support of the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000928-S), Para 4 especies: Volumen (µm3/depredador), Tasa crecimiento (µ 1/d), Tasa de ingestion (cells/ind/d), Eficiencia bruta de crecimiento (GGE, %), Peer reviewed

We aimed at studying the mechanisms underneath the ascending and descending sections of the thermal performance curves in marine protozoans. To do so, we exposed Gyrodinium dominans and two strains of Oxyrrhis marina from different origins to three temperatures representative of each section of the thermal response curve (12ºC, ascending section; 18ºC, top; 25ºC, descending section). As variables, we measured growth, ingestion, and respiration rates (this latter with and without food). The growth rates of O. marina strains plotted as a function of temperature showed a triangular response with maximum values at the intermediate temperature. However, G. dominans showed similar growth rates at 12 and 18ºC, and even if showed a marked decrease in growth rates at 25ºC, this was not significant. Ingestion rates were higher at 18ºC for all the strains. The respiration rates of G. dominans were unaffected by temperature, but the respiration rates of both O. marina strains increased with temperature. The specific dynamic action produced by feeding activity ranged from 2 to 20% of the daily carbon ingestion for all organisms investigated. The calculated energetic budget indicated that the responses to temperature were diverse, even within strains of the same species. G. dominans maintained similar growth at all temperatures by balancing anabolism and catabolism functions. In O. marina strains, on the other hand, the decrease in growth rates at the lowest temperature was driven mainly by reduced ingestion rates. However, increased respiration seemed the primary factor affecting the decrease in growth rates at the highest temperature. These results are discussed in the light of previous studies and on its suitability to understand the response of wild organisms to fluctuations in temperature, This research was funded by Grant PID2020-118645RB-I00 by Ministerio de Ciencia e innovación (MCIN)/AEI/ 10.13039/501100011033 and by “ERDF A way of making Europe”. R.A.M. was funded by a PhD fellowship from the National Commission of Science (CONICYT), Ministry of Education, Chile. It is a contribution of the Marine Zooplankton Ecology Group (2017 SGR 87). With the institutional support of the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000928-S), Para 3 especies de protozoos marino: Tasa de respiración con y sin comida (µmol O2/ind/h), Tasa crecimiento (µ 1/d), Tasa de ingestion (cells/ind/d), Eficiencia bruta de crecimiento (GGE, %), Peer reviewed

13 pages, 6 figures, 3 tables.-- Data Availability Statement: The raw data supporting the conclusions of this article can be found at the open repository (Digital.CSIC, http://hdl.handle.net/10261/262244, https://doi.org/10.20350/digitalCSIC/14517).-- It is a contribution of the Marine Zooplankton Ecology Group (2017 SGR 87), Proper thermal adaptation is key to understanding how species respond to long-term changes in temperature. However, this is seldom considered in protozooplankton and mixoplankton experiments. In this work, we studied how two heterotrophic dinoflagellates (Gyrodinium dominans and Oxyrrhis marina), one heterotrophic ciliate (Strombidium arenicola), and one mixotrophic dinoflagellate (Karlodinium armiger) responded to warming. To do so, we compared strains adapted at 16, 19, and 22°C and those adapted at 16°C and exposed for 3 days to temperature increases of 3 and 6°C (acclimated treatments). Neither their carbon, nitrogen or phosphorus (CNP) contents nor their corresponding elemental ratios showed straightforward changes with temperature, except for a modest increase in P contents with temperature in some grazers. In general, the performance of both acclimated and adapted grazers increased from 16 to 19°C and then dropped at 22°C, with a few exceptions. Therefore, our organisms followed the “hotter is better” hypothesis for a temperature rise of 3°C; an increase of >6°C, however, resulted in variable outcomes. Despite the disparity in responses among species and physiological rates, 19°C-adapted organisms, in general, performed better than acclimated-only (16°C-adapted organisms incubated at +3°C). However, at 22°C, most species were at the limit of their metabolic equilibrium and were unable to fully adapt. Nevertheless, adaptation to higher temperatures allowed strains to maintain physiological activities when exposed to sudden increases in temperature (up to 25°C). In summary, adaptation to temperature seems to confer a selective advantage to protistan grazers within a narrow range (i.e., ca. 3°C). Adaptation to much higher increases of temperatures (i.e., +6°C) does not confer any clear physiological advantage (with few exceptions; e.g., the mixotroph K. armiger), at least within the time frame of our experiments, This research was funded by Grant CTM2017-84288-R by Fondo Europeo de Desarrollo Regional (FEDER)/Ministerio de Ciencia, Innovación y Universidades-Agencia Estatal de Investigacioìn (AEI), and by Grant PID2020-118645RB-I00 by Ministerio de Ciencia e innovación (MCIN)/AEI/10.13039/501100011033 and by ERDF A way of making Europe. [...] With the institutional support of the Severo Ochoa Centre of Excellence accreditation (CEX2019-000928-S). The open-access publication fee was partially covered by the CSIC Open Access Publication Support Initiative through its Unit of Information Resources for Research (URICI)

9 pages, 3 figures, 2 tables.-- Data Availability Statement: Publicly available datasets were analyzed in this study. This data can be found here: https://doi.org/10.20350/digitalCSIC/14561..-- It is a contribution of the Marine Zooplankton Ecology Group (2017 SGR 87)., We exposed Gyrodinium dominans and two strains of Oxyrrhis marina to temperatures well above (25°C) and below (12°C) their maintenance temperature (18°C) to study the mechanisms controlling the overall physiological response to thermal stress. As variables, we measured growth, ingestion, and respiration rates (this latter with and without food). The growth rates of O. marina strains plotted as a function of temperature showed maximum values at the maintenance temperature, as expected in a typical unimodal functional response. However, G. dominans showed similar growth rates at 12 and 18°C, and even a marked decrease in growth rates at 25°C, happened to be not significant. G. dominans ingestion rates were not significantly different at all temperatures (although apparently decreased at 25°C), whereas both O. marina strains showed higher ingestion rates at 18°C. The respiration rates of G. dominans were unaffected by temperature, but those of O. marina strains increased with temperature. The specific dynamic action (respiration increase produced by feeding activity) ranged from 2 to 20% of the daily carbon ingestion for all organisms investigated. The calculated energetic budget indicated that the responses to temperature were diverse, even between in strains of the same species. G. dominans maintained similar growth at all temperatures by balancing metabolic gains and losses. In O. marina strains, on the other hand, the decrease in growth rates at the lowest temperature was driven mainly by reduced ingestion rates. However, increased respiration seemed the primary factor affecting the decrease in growth rates at the highest temperature. These results are discussed in the light of previous studies and on its suitability to understand the response of wild organisms to fluctuations in temperature, This research was funded by Grant PID2020-118645RB-I00 by Ministerio de Ciencia e innovación (MCIN)/AEI/10.13039/501100011033 and by “ERDF A way of making Europe”. The open access publication fee was partially covered by the CSIC Open Access Publication Support Initiative through its Unit of Information Resources for Research (URICI). [...] With the institutional support of the ’Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000928-S), Peer reviewed

10 pages, 6 figures, 3 tables, supplementary data https://doi.org/10.1016/j.marenvres.2022.105693, Sudden environmental changes like marine heatwaves will become more intense and frequent in the future. Understanding the physiological responses of mixoplankton and protozooplankton, key members of marine food webs, to temperature is crucial. Here, we studied two dinoflagellates (one protozoo- and one mixoplanktonic), two ciliates (one protozoo- and one mixoplanktonic), and two cryptophytes. We report the acute (24 h) responses on growth and grazing to a range of temperatures (5–34 °C). We also determined respiration and photosynthetic rates for the four grazers within 6 °C of warming. The thermal performance curves showed that, in general, ciliates have higher optimal temperatures than dinoflagellates and that protozooplankton is better adapted to warming than mixoplankton. Our results confirmed that warmer temperatures decrease the cellular volumes of all species. Q10 coefficients suggest that grazing is the rate that increases the most in response to temperature in protozooplankton. Yet, in mixoplankton, grazing decreased in warmer temperatures, whereas photosynthesis increased. Therefore, we suggest that the Metabolic Theory of Ecology should reassess mixoplankton's position for the correct parameterisation of future climate change models. Future studies should also address the multigenerational response to temperature changes, to confirm whether mixoplankton become more phototrophic than phagotrophic in a warming scenario after adaptation, This project has received funding from the European Union's Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No. 766327. [...] Thanks for financial support are also due to the ERASMUS + traineeship program and to the Grants CTM2017-84288-R and PID2020-118645RB-I00 funded by MCIN/AEI Spain/10.13039/501100011033 and by “FEDER Una manera de hacer Europa”. This work is a contribution of the Marine Zooplankton Ecology Group (2017 SGR 87) with the institutional support of the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX 2019-000928-S), Peer reviewed

The temperature-size rule hypothesized that there is a negative relationship between the size (volume) of an organism and the temperature. This applies to both unicellular and pluricellular organisms. Here, we question this hypothesis for the particular case of protozoans, because in these organisms the volume is directly related to the consumption of prey, and on most of the occasions the true volume of the cell is unknown. To prove our arguments, we designed a series of experiments with the heterotrophic dinoflagellate O. marina, including functional and numerical responses, time-dependent acclimation responses, and estimation of the protozoan volume during long periods of starvation. Our data showed that, in fact, the observed temperature-size rule in unicellular grazers results from anabolic and catabolic imbalances, and that the relationship between size and temperature weakens after proper thermal adaptation. We also showed that once prey are fully digested, the protozoan’ size is the same irrespectively of the temperature. Finally, we set the basis for proper acclimation during short-term temperature experiments, which specifies that at least 3 days should be allowed for proper temperature acclimation. We also suggest that, for trustable experiments, the grazer should be incubated at the target prey concentration for at least 24h before conducting the experiments. The ecological implications of a lack of correlation between microzooplankton size and temperature are also discussed, This research was funded by Grant PID2020-118645RB-I00 by Ministerio de Ciencia e innovación (MCIN)/AEI/ 10.13039/501100011033 and by “ERDF A way of making Europe”. It is a contribution of the Marine Zooplankton Ecology Group (2017 SGR 87). With the institutional support of the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000928-S), Para Oxyrrhis marina: Tasas de ingestion a diferentes concentraciones de alimento (presa/ind/d), Tasa crecimiento (µ 1/d), Volumen (µm3); para la presa (Rhodomonas salina): volume (µm3), Peer reviewed

We exposed the calanoid copepod Paracartia grani, reared in the laboratory at 19°C, to warmer conditions (22°C and 25°C) for 11 generations and examined the direct effects of temperature on feeding, fecundity, development, population sex ratio and somatic traits (adult and egg sizes and adult carbon content), This research was funded by Grants [CTM2017-84288-R and PID2020-118645RB-I00] by MCIN/AEI/10.13039/501100011033 and by “ERDF A way of making Europe”. CJ was supported by Grant [PRE2018-084738] funded by MCIN/AEI/10.13039/501100011033 and by “ESF Investing in your future”, With the institutional support of the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000928-S), Rearing temperature, generation, prosome length, carbon content, ingestion rate, egg production rate, egg diameter, gross-growth efficiency, hatching success, Peer reviewed