Persistent nitrogen depletion in sunlit open ocean waters provides a favorable ecological niche for nitrogen-fixing (diazotrophic) cyanobacteria, some of which associate symbiotically with eukaryotic algae. All known marine examples of these symbioses have involved either centric diatom or haptophyte hosts. We report here the discovery and characterization of two distinct marine pennate diatom-diazotroph symbioses, which until now had only been observed in freshwater environments. Rhopalodiaceae diatomsEpithemia pelagicasp. nov. andEpithemia catenatasp. nov. were isolated repeatedly from the subtropical North Pacific Ocean, and analysis of sequence libraries reveals a global distribution. These symbioses likely escaped attention because the endosymbionts lack fluorescent photopigments, havenifHgene sequences similar to those of free-living unicellular cyanobacteria, and are lost in nitrogen-replete medium. Marine Rhopalodiaceae-diazotroph symbioses are a previously overlooked but widespread source of bioavailable nitrogen in marine habitats and provide new, easily cultured model organisms for the study of organelle evolution.

Small eukaryotic phytoplankton are major contributors to global primary production and marine biogeochemical cycles. Many taxa are thought to be mixotrophic, but quantitative studies of phagotrophy exist for very few. In addition, little is known about consumers of Prochlorococcus, the abundant cyanobacterium at the base of oligotrophic ocean food webs. Here we describe thirty-nine new phytoplankton isolates from the North Pacific Subtropical Gyre (Station ALOHA), all flagellates ~2–5 µm diameter, and we quantify their ability to graze Prochlorococcus. The mixotrophs are from diverse classes (dictyochophytes, haptophytes, chrysophytes, bolidophytes, a dinoflagellate, and a chlorarachniophyte), many from previously uncultured clades. Grazing ability varied substantially, with specific clearance rate (volume cleared per body volume) varying over ten-fold across isolates and six-fold across genera. Slower grazers tended to create more biovolume per prey biovolume consumed. Using qPCR we found that the haptophyte Chrysochromulina was most abundant among the isolated mixotrophs at Station ALOHA, with 76–250 cells mL⁻¹ across depths in the upper euphotic zone (5–100 m). Our results show that within a single ecosystem the phototrophs that ingest bacteria come from many branches of the eukaryotic tree, and are functionally diverse, indicating a broad range of strategies along the spectrum from phototrophy to phagotrophy.

Nitrogen and iron largely determine phytoplankton production in the ocean, yet there are few direct incubation studies on nutrient limitation of phytoplankton in the western tropical North Pacific Ocean. Here, we assessed the effects of N and Fe amendments on both photosynthesis and biomass of phytoplankton (including final chlorophyll a biomass and taxonomic composition) in that region. Overall, the carrying capacity of phytoplankton biomass was primarily constrained by N, and Fe was a limiting resource only near the equator where phytoplankton was co-limited by N and Fe. Positive growth responses in nutrient-receiving treatments were predominantly caused by diatoms, sometimes together with Synechococcus. The often strong stimulation of large phytoplankton in these treatments substantially modified the phytoplankton assemblage from picoplankton- to diatom-dominated, which was often accompanied by a significant increase in photosynthetic competence. This is the first report showing that phytoplankton in the tropical western North Pacific are primarily limited by N, and co-limited by Fe in some areas. We combined our results with those from comparable studies in the tropical and subtropical Pacific Ocean to propose a general typology that describes and predicts the long-term effects of rate-limiting N and Fe on phytoplankton final biomass and changes in community structure.

In sunlit waters, significant predation is performed by unicellular, phagotrophic mixotrophs, that is, predators that also possess plastids. The success of a mixotrophic lifestyle will depend in part on how well mixotrophs acquire prey relative to specialized heterotrophs. Likewise, consequences of mixotrophy for productivity and element cycling will depend on the rate and efficiency at which mixotrophs consume prey biomass relative to heterotrophs. However, trait differences between mixotrophs and heterotrophs are not well characterized. In addition, cell size of mixotrophs varies widely, and constitutive mixotrophs include small flagellates deriving from diverse taxa, while larger species are primarily dinoflagellates. To determine whether similar constraints apply to phagotrophs across this broad range of size and taxa, we compiled 83 measurements of flagellate functional responses and compared maximum clearance rates (C_max) and maximum ingestion rates (I_max) between trophic modes. We found that the average mixotroph has a 3.7‐fold lower C_max and 7.8‐fold lower I_max than the average heterotroph, after controlling for cell size. The smaller penalty for C_max suggests that relative fitness of mixotrophs will be enhanced under dilute prey concentrations that are common in pelagic ecosystems. We also find that growth efficiency is greater for mixotrophs and for flagellates with lower C_max, indicating a spectrum of trophic strategies that may be driven by phototrophy vs. phagotrophy allocation as well as fast vs. slow metabolic variation. Allometric scaling shows that I_max is constrained by a common relationship among dinoflagellates and other taxa, but dinoflagellates achieve a greater volume‐specific C_max. These results should aid in interpreting protistan communities and modeling mixotrophy.