Nitrifying microorganisms, including ammonia-oxidizing archaea, ammonia-oxidizing bacteria, and nitriteoxidizing bacteria, are the most abundant chemoautotrophs in the ocean and play an important role in the global carbon cycle by fixing dissolved inorganic carbon (DIC) into biomass. The release of organic compounds by these microbes is not well quantified, but may represent an as-yet unaccounted source of dissolved organic carbon (DOC) available to marine food webs. Here, we provide measurements of cellular carbon and nitrogen quotas, DIC fixation yields and DOC release of 10 phylogenetically diverse marine nitrifiers. All investigated strains released DOC during growth, representing on average 5-15% of the fixed DIC. Changes in substrate concentration and temperature did not affect the proportion of fixed DIC released as DOC, but release rates varied between closely related species. Our results also indicate previous studies may have underestimated DIC fixation yields of marine nitrite oxidizers due to partial decoupling of nitrite oxidation from CO 2 fixation, and due to lower observed yields in artificial compared to natural seawater medium. The results of this study provide critical values for biogeochemical models of the global carbon cycle, and help to further constrain the implications of nitrification-fueled chemoautotrophy for marine food-web functioning and the biological sequestration of carbon in the ocean.

A large fraction of marine primary production is performed by diverse small protists, and many of these phytoplankton are phagotrophic mixotrophs that vary widely in their capacity to consume bacterial prey. Prior analyses suggest that mixotrophic protists as a group vary in importance across ocean environments, but the mechanisms leading to broad functional diversity among mixotrophs, and the biogeochemical consequences of this, are less clear. Here we use isolates from seven major taxa to demonstrate a tradeoff between phototrophic performance (growth in the absence of prey) and phagotrophic performance (clearance rate when consuming Prochlorococcus ). We then show that trophic strategy along the autotrophy-mixotrophy spectrum correlates strongly with global niche differences, across depths and across gradients of stratification and chlorophyll a . A model of competition shows that community shifts can be explained by greater fitness of faster-grazing mixotrophs when nutrients are scarce and light is plentiful. Our results illustrate how basic physiological constraints and principles of resource competition can organize complexity in the surface ocean ecosystem.

Nitrifying microorganisms, including ammonia-oxidizing archaea, ammonia-oxidizing bacteria and nitrite-oxidizing bacteria, are the most abundant chemoautotrophs in the ocean and play an important role in the global carbon cycle by fixing dissolved inorganic carbon (DIC) into biomass. The release of organic compounds by these microbes is less well known but may represent an as-yet unaccounted source of dissolved organic carbon (DOC) available to heterotrophic marine food webs. Here, we provide measurements of cellular carbon and nitrogen quotas, DIC fixation yields and DOC release of ten phylogenetically diverse marine nitrifiers grown in multiple culture conditions. All investigated strains released DOC during growth, making up on average 5-15% of the fixed DIC. Neither substrate concentration nor temperature affected the proportion of fixed DIC released as DOC, but release rates varied between closely related species. Our results also indicate previous studies may have underestimated DIC fixation yields of marine nitrite oxidizers due to partial decoupling of nitrite oxidation from CO2 fixation, and due to lower observed yields in artificial compared to natural seawater medium. The results of this study provide values for biogeochemical models of the global carbon cycle, and help to further constrain the implications of nitrification-fueled chemoautotrophy for marine food-web functioning and the biological sequestration of carbon in the ocean.

Digital imaging technologies are increasingly used to study life in the ocean. To deal with the large volume of image data collected over space and time, scientists employ various machine learning and deep learning algorithms to perform automated image classification. Training of classifiers requires a large number of expertly curated sets of images, a time‐consuming process that requires taxonomic knowledge and understanding of the local ecosystem. The creation of these labeled training sets is the critical bottleneck for building skillful automated classifiers. Here, we discuss how we overcame this barrier by leveraging taxonomic knowledge from a group of specialists in a workshop setting and suggest best practices for effectively organizing image annotation efforts. In our experience, this 2 day workshop proved very insightful and facilitated classification of over 4 years of plankton images obtained at Scripps Pier (La Jolla, CA), focusing on diatoms and dinoflagellates. We highlight the importance of facilitating a dialog between taxonomists and engineers to better integrate ecological goals with computational constraints, and encourage continuous involvement of taxonomic experts for successful implementation of automated classifiers.