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Abstract The availability of the micronutrient iron is important in regulating phytoplankton growth across much of the world’s oceans, particularly in the high-nutrient, low-chlorophyll regions. Compared to known mechanisms of iron acquisition and conservation in autotrophic protists (e.g. diatoms), those of dinoflagellates remain unclear, despite their frequent presence in offshore iron-limited waters. Here, we investigate the strategies of an ecologically important mixotrophic dinoflagellate to coping with low iron conditions. Coupled gene expression and physiological responses as a function of iron availability were examined in oceanic and coastal strains of the dinoflagellate Karlodinium. Under iron-replete conditions, grazing was only detected in coastal variants, resulting in faster growth rates compared to when grown autotrophically. Under iron-limited conditions, all isolates exhibited slower growth rates, reduced photosynthetic efficiencies, and lower cellular iron quotas than in iron-replete conditions. However, oceanic isolates exhibited higher relative growth rates compared to coastal isolates under similar low iron concentrations, suggesting they are better adapted to coping under iron limitation. Yet the oceanic isolates did not exhibit the ability to appreciably reduce cell volume or increase iron-use efficiencies compared to the coastal isolates to cope with iron limitation, as often observed in oceanic diatoms. Rather, molecular pathway analysis and corresponding gene expression patterns suggest that oceanic Karlodinium utilizes a high-affinity iron uptake system when iron is low. Our findings reveal cellular mechanisms by which dinoflagellates have adapted to low iron conditions, further shedding light on how they potentially survive in variable iron regions of the world’s oceans. 

ABSTRACT Eastern boundary upwelling currents are some of the most biologically productive and diverse regions in the world's oceans. Driven by equatorward winds and Ekman transport, surface waters are transported offshore and replaced by cold, nutrient‐rich deep waters that seed extensive phytoplankton blooms. Studying phytoplankton community succession and physiological acclimation during the initial stages of upwelling is critical to building a comprehensive understanding of phytoplankton responses to upwelling in these important regions. Additionally, factors like lateral transport, seed population dynamics and physiological and molecular shifts are conducive to shaping the community assemblage and primary productivity. This study examines how phytoplankton gene expression and resulting physiology change between early and later phases of upwelling. By incorporating metatranscriptomic analyses and stable isotope incubations to measure nutrient uptake kinetics into our assessment of early and later upwelling stages, we observed variability in phytoplankton assemblages and differential gene expression of phytoplankton that were de‐coupled from their physiology. We show that the gene expression response to a fresh upwelling event precedes their physiological response. Ultimately, understanding how phytoplankton change through the course of an upwelling event is critical to assessing their importance to regional biological rate processes, trophic systems and resulting biogeochemistry. 

ABSTRACT The Galápagos Islands are a biodiversity hotspot, largely due to the Equatorial Undercurrent (EUC) which supplies nutrient‐rich waters to the euphotic zone and supports enhanced levels of primary productivity performed by phytoplankton. Understanding phytoplankton responses to changing environmental conditions is crucial for regional conservation and management efforts. Research cruises conducted between 2014 and 2022, spanning a major El Niño event in 2015 and a La Niña event in 2022, observed varying oceanic conditions and diverse phytoplankton community composition. At most EUC‐influenced stations, larger‐sized phytoplankton groups (≥ 5 μm) were dominant while warmer, oligotrophic sites favoured smaller‐sized phytoplankton groups (< 5 μm). Predictably, nutrient supply was suppressed during the El Niño event associated with the weakening of the EUC and deepening of the thermocline. Counterintuitively, nutrient levels were not significantly enhanced during the La Niña event likely because increased stratification between the mixed and deep water layers reduced entrainment, particularly at Eastern stations. Protist community composition was evaluated using 18S rRNA gene metabarcoding; the majority of detected OTUs were associated with upwelling conditions prevalent around the archipelago. Taxonomic variability reflected heterogeneous environmental conditions generated by the convergence of multiple ocean currents. These results highlight the dynamic interplay of physical and biological factors shaping primary productivity in the Galápagos marine ecosystem. 

Abstract The oceanic biogeochemical cycling of iron is globally important yet difficult to fully understand due to the many chemical processes involved. There is potential to use scandium, which has a similar ionic size and charge density to trivalent iron but lacks redox cycling, as a simpler analog for specific parts of the iron cycle, if we can sufficiently develop our understanding of scandium's reactivity. Here we move closer to this understanding. We look at particle reactivity and solubility through a 24‐hr incubation experiment: 5 nmol/kg of dissolved scandium and/or iron were added to filtered and unfiltered California Current System water. Particulate scandium formed only in the unfiltered treatments, at a quantity unlikely to have been taken up biologically. This is the first direct observation of scavenging of scandium, an attribute shared with iron. Our results also serve as the first test of scandium solubility in seawater: 1.9 nmol/kg of dissolved scandium was stable in the filtered treatment, 50 times more than the highest natural concentrations so far observed. This indicates that, in contrast to iron, scandium's oceanic cycling is unlikely to be influenced by solubility limits. We also compare particulate depth profiles: labile particulate iron was disproportionally higher than that of scandium in shelf‐influenced samples, likely due to iron reductively dissolving in the sediments, which scandium cannot do, and then precipitating in oxic seawater. Due to this combination of behaviors, our results suggest that paired observations of scandium and iron may help distinguish between iron sourced from sediment resuspension and reductive dissolution. 

Abstract Coastal upwelling currents such as the California Current System (CCS) comprise some of the most productive biological systems on the planet. Diatoms dominate these upwelling events in part due to their rapid response to nutrient entrainment. In this region, they may also be limited by the micronutrient iron (Fe), an important trace element primarily involved in photosynthesis and nitrogen assimilation. The mechanisms behind how diatoms physiologically acclimate to the different stages of the upwelling conveyor belt cycle remain largely uncharacterized. Here, we explore their physiological and metatranscriptomic response to the upwelling cycle with respect to the Fe limitation mosaic that exists in the CCS. Subsurface, natural plankton assemblages that would potentially seed surface blooms were examined over wide and narrow shelf regions. The initial biomass and physiological state of the phytoplankton community had a large impact on the overall response to simulated upwelling. Following on‐deck incubations under varying Fe physiological states, our results suggest that diatoms quickly dominated the blooms by “frontloading” nitrogen assimilation genes prior to upwelling. However, diatoms subjected to induced Fe limitation exhibited reductions in carbon and nitrogen uptake and decreasing biomass accumulation. Simultaneously, they exhibited a distinct gene expression response which included increased expression of Fe‐starvation induced proteins and decreased expression of nitrogen assimilation and photosynthesis genes. These findings may have significant implications for upwelling events in future oceans, where changes in ocean conditions are projected to amplify the gradient of Fe limitation in coastal upwelling regions. 

Abstract Diatom community composition has a critical influence on global ocean health and ecological processes. Developing accurate and efficient methods for diatom identification under dynamic environmental conditions is essential to understanding the implications of diatom community changes. Two developing methods for identifying and enumerating phytoplankton, cell imaging and molecular sequencing, are experiencing rapid advancements. This study aims to compare diatom taxonomic composition results within natural assemblages derived from rapidly advancing methods, FlowCam imaging and metabarcoding of the V4 region of the 18S rRNA gene, with traditional light microscopy cell counting techniques. All three methods were implemented in tandem to analyze changes in dynamic diatom assemblages within simulated upwelling experiments conducted in the California upwelling zone. The results of this study indicate that, summed across all samples, DNA sequencing detected four times as many genera as morphology‐based methods, thus supporting previous findings that DNA sequencing is the most powerful method for analyzing species richness. Results indicate that all three methods returned comparable relative abundance for the most abundant genera. However, the three methods did not return comparable absolute abundance, primarily due to barriers in deriving quantities in equal units. Overall, this study indicates that at the semi‐quantitative level of relative abundance measurements, FlowCam imaging and metabarcoding of the V4 region of the 18S rRNA gene yield comparable results with light microscopy but at the qualitative and quantitative levels, enumeration metrics diverge, and thus method selection and cross‐method comparison should be performed with caution. 

Abstract Despite generally low primary productivity and diatom abundances in oligotrophic subtropical gyres, the North Atlantic Subtropical Gyre (NASG) exhibits significant diatom-driven carbon export on an annual basis. Subsurface pulses of nutrients likely fuel brief episodes of diatom growth, but the exact mechanisms utilized by diatoms in response to these nutrient injections remain understudied within near-natural settings. Here we simulated delivery of subsurface nutrients and compare the response among eukaryotic phytoplankton using a combination of physiological techniques and metatranscriptomics. We show that eukaryotic phytoplankton groups exhibit differing levels of transcriptional responsiveness and expression of orthologous genes in response to release from nutrient limitation. In particular, strategies for use of newly delivered nutrients are distinct among phytoplankton groups. Diatoms channel new nitrate to growth-related strategies while physiological measurements and gene expression patterns of other groups suggest alternative strategies. The gene expression patterns displayed here provide insights into the cellular mechanisms that underlie diatom subsistence during chronic nitrogen-depleted conditions and growth upon nutrient delivery that can enhance carbon export from the surface ocean. 

Abstract The Tara Oceans program has delivered major advances in our knowledge of ocean plankton diversity and complexity, shedding light on key interactions that explain their success on a planetary scale. In this issue, Caputi et al. (2019,https://doi.org/10.1029/2018GB006022) further contribute to this knowledge through combining comprehensive bio‐oceanographic genomic and transcriptomic Tara Oceans data sets with iron distributions derived from two global‐scale biogeochemical models. Their findings reveal the prevalence of iron as a limiting nutrient in pelagic ecosystems at both local and global scales, exerting a considerable force that drives plankton evolution and shapes community structure. Integration of omics data (i.e., genomics, transcriptomics, proteomics, and metabolomics) with oceanographic properties and biogeochemical models will transform our view of the ocean ecosystem and its role on a changing planet. 

Abstract. We compare primary productivity estimates based on different photosynthetic ‘currencies’ (electrons, O2 and carbon) collected from the dynamic coastal upwelling waters of the California Current. Fast Repetition Rate Fluorometry and O2/N2 measurements were used to collect high-resolution underway estimates of photosynthetic electron transport rates and net community productivity, respectively, alongside on-station 14C uptake experiments to measure gross carbon fixation rates. Our survey captured two upwelling filaments at Cape Blanco and Cape Mendocino with distinct biogeochemical signatures and iron availabilities, enabling us to examine photosynthetic processes along a natural iron gradient. Significant differences in photo-physiology, cell sizes, Si:NO3- draw-down ratios, and molecular markers of Fe-stress indicated that phytoplankton assemblages near Cape Mendocino were Fe-stressed, while those near Cape Blanco were Fe-replete. Upwelling of O2-poor deep water to the surface complicated O2-based net community productivity estimates, but we were able to correct for these vertical mixing effects using continuous [N2O] surface measurements and depth-profiles of ∂[O2]∂[N2O]. Vertical mixing corrections were strongly correlated to sea surface temperature, which serves as an N2O-independent proxy for upwelling. Following vertical mixing corrections, all three productivity estimates reflected trends in Fe-stress physiology, indicating greater productivity near Cape Blanco compared to Cape Mendocino. For all assemblages, carbon fixation varied as a hyperbolic function of electron transport rates, but the derived parameters of this relationship were highly variable and significantly correlated with physiological indicators of Fe-stress (σPSII, FV/FM, Si:NO3- and diatom-specific PSI gene expression), suggesting that iron availability influenced the coupling between photosynthetic electron transport and subsequent carbon fixation. Net community productivity showed strong coherence with daily-integrated photosynthetic electron transport rates across the entire cruise track, with no apparent relationship with Fe-stress. This result suggests that fluorescence-based estimates of gross photochemistry are still a good indicator for bulk primary productivity, even if Fe-limitation influences the stoichiometric relationship between productivity currencies. 

The Galápagos Archipelago is a globally significant biodiversity hotspot. However, compared to the relatively well-known megafauna, the distribution and ecological significance of marine protists in this system are poorly understood. To gain an understanding of the protistan assemblages across trophic modes, an intensive oceanographic survey was conducted in the Galápagos Marine Reserve (GMR) in October of 2018. The Equatorial Undercurrent (EUC)-influenced region had higher chlorophyll- a (Chl- a ) concentrations than those of the eastern regions of the archipelago, along with higher abundances of protistan grazers. Specifically, proportions of autotrophic and potentially mixotrophic dinoflagellates were higher in the EUC, whereas in the eastern regions, heterotrophic dinoflagellates and chlorophytes dominated. Taxonomic composition and biochemical indicators suggested proportions of micrograzers and their associated heterotrophic biomass was higher in the oligotrophic, low Chl- a regions in the east. We also report observations from a dinoflagellate bloom in the western archipelago, which was heavily influenced by upwelling of the EUC. The red tide-forming dinoflagellate Scrippsiella lachrymosa was highly detected through light microscopy and DNA amplicon sequencing. In addition, the heterotrophic dinoflagellate Polykrikos kofoidii was detected and, based on cell densities observed in this study and grazing rates obtained from the literature, estimated to potentially graze up to 62% of S. lachrymosa bloom population. Our findings thus provide new insights into the composition of micrograzers and their potential roles in structuring protistan communities in the Galápagos Archipelago.