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Mixotrophic protists combine photosynthesis with the ingestion of prey to thrive in resource-limited conditions in the ocean. Yet, how they fine-tune resource investments between their two different metabolic strategies remains unclear. Here, we present a modeling framework (Mixotroph Optimal Contributions to Heterotrophy and Autotrophy) that predicts the optimal (growth-maximizing) investments of carbon and nitrogen as a function of environmental conditions. Our model captures a full spectrum of trophic modes, in which the optimal investments reflect zero-waste solutions (i.e., growth is colimited by carbon and nitrogen) and accurately reproduces experimental results. By fitting the model to data forOchromonas, we were able to predict metabolic strategies at a global scale. We find that high phagotrophic investment is the dominant strategy across different oceanic biomes, used primarily for nitrogen acquisition. Our results therefore support empirical observations of the importance of mixotrophic grazers to upper ocean bacterivory. 

Abstract Rising ocean temperatures affect marine microbial ecosystems directly, since metabolic rates (e.g., photosynthesis, respiration) are temperature‐dependent, but temperature also has indirect effects mediated through changes to the physical environment. Empirical observations of the long‐term trends in biomass and productivity measure the integrated response of these two kinds of effects, making the independent components difficult to disentangle. We used a combination of modeling approaches to isolate the direct effects of rising temperatures on microbial metabolism and explored the consequences for food web dynamics and global biogeochemistry. We evaluated the effects of temperature sensitivity in two cases: first, assuming that all metabolic processes have the same temperature sensitivity, or, alternatively, that heterotrophic processes have higher temperature sensitivity than autotrophic processes. Microbial ecosystems at higher temperatures are characterized by increased productivity but decreased biomass stocks as a result of transient, high export events that reduce nutrient availability in the surface ocean. Trophic dynamics also mediate community structure shifts resulting in increased heterotroph to autotroph ratios at higher temperatures. These ecosystem thermal responses are magnified when the temperature sensitivity of heterotrophs is higher than that of autotrophs. These results provide important context for understanding the combined food web response to direct and indirect temperature effects and inform the construction and interpretation of Earth systems models used in climate projections. 

Abstract The Mid‐Atlantic Bight (MAB) hosts a large and productive marine ecosystem supported by high phytoplankton concentrations. Enhanced surface chlorophyll concentrations at the MAB shelf‐break front have been detected in synoptic measurements, yet this feature is not present in seasonal means. To understand why, we assess the conditions associated with enhanced surface chlorophyll at the shelf break. We employ in‐situ and remote sensing data, and a 2‐dimensional model to show that Ekman restratification driven by upfront winds drives ephemerally enhanced chlorophyll concentrations at the shelf‐break front in spring. Using 8‐day composite satellite‐measured surface chlorophyll concentration data from 2003–2020, we constructed a daily running mean (DRM) climatology of the cross‐shelf chlorophyll distribution for the northern MAB region. While the frontal enhancement of chlorophyll is apparent in the DRM climatology, it is not captured in the seasonal climatology due to its short duration of less than a week. In‐situ measurements of the frontal chlorophyll enhancement reveal that chlorophyll is highest in spring when the shelf‐break front slumps offshore from its steep wintertime position causing restratification in the upper part of the water column. Several restratification mechanisms are possible, but the first day of enhanced chlorophyll at the shelf break corresponds to increasing upfront winds, suggesting that the frontal restratification is driven by offshore Ekman transport of the shelf water over the denser slope water. The 2‐dimensional model shows that upfront winds can indeed drive Ekman restratification and alleviate light limitation of phytoplankton growth at the shelf‐break front. 

One pathway of the biological pump that remains largely unquantified in many export models is the active transport of carbon from the surface ocean to the mesopelagic by zooplankton diel vertical migration (DVM). Here, we develop a simple representation of zooplankton DVM and implement it in a global export model as a thought experiment to illustrate the effects of DVM on carbon export and mesopelagic biogeochemistry. The model is driven by diagnostic satellite measurements of net primary production, algal biomass, and phytoplankton size structure. Due to constraints on available satellite data, the results are restricted to the latitude range from 60°N to 60°S. The modeled global export flux from the base of the euphotic zone was 6.5 PgC/year, which represents a 14% increase over the export flux in model runs without DVM. The mean (± standard deviation, SD) proportional contribution of the DVM‐mediated export flux to total carbon export, averaged over the global domain and the climatological seasonal cycle, was 0.16 ± 0.04 and the proportional contribution of DVM activity to total respiration within the twilight zone was 0.16 ± 0.06. Adding DVM activity to the model also resulted in a deep local maximum in the oxygen utilization profile. The model results were most sensitive to the assumptions for the fraction of individuals participating in DVM, the fraction of fecal pellets produced in the euphotic zone, and the fraction of grazed carbon that is metabolized.