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Summary: Frequent blooms of phytoplankton occur in coastal upwelling zones creating hotspots of biological productivity in the ocean. As cold, nutrient‐rich water is brought up to sunlit layers from depth, phytoplankton are also transported upwards to seed surface blooms that are often dominated by diatoms. The physiological response of phytoplankton to this process, commonly referred to as shift‐up, is characterized by increases in nitrate assimilation and rapid growth rates. To examine the molecular underpinnings behind this phenomenon, metatranscriptomics was applied to a simulated upwelling experiment using natural phytoplankton communities from the California Upwelling Zone. An increase in diatom growth following 5 days of incubation was attributed to the genera Chaetoceros and Pseudo‐nitzschia. Here, we show that certain bloom‐forming diatoms exhibit a distinct transcriptional response that coordinates shift‐up where diatoms exhibited the greatest transcriptional change following upwelling; however, comparison of co‐expressed genes exposed overrepresentation of distinct sets within each of the dominant phytoplankton groups. The analysis revealed that diatoms frontload genes involved in nitrogen assimilation likely in order to outcompete other groups for available nitrogen during upwelling events. We speculate that the evolutionary success of diatoms may be due, in part, to this proactive response to frequently encountered changes in their environment. [ABSTRACT FROM AUTHOR]

We compare the relative contribution of large phytoplankton (>5 and >10 µm) to uptake rates of carbon (C), nitrate (NO3-), and dissolved silicon (Si) and uptake ratios of Si:NO3- and Si:C in Monterey Bay, California (a high-iron region) with three low-iron regions in the eastern tropical Pacific: the Costa Rica upwelling dome, Humboldt current, and Peru upwelling. We also demonstrate the effect of iron enrichment on the above parameters in the latter three regions. In Monterey Bay waters, the µ5-µm size fraction accounted on average for the majority of particulate organic C and was responsible for 92% of total C uptake, 81% of total NO3- uptake, and 98% of total Si uptake, In contrast, in low-iron eastern tropical Pacific waters, the >5-µm size fraction accounted for less than half of the particulate organic C and was responsible for a substantially smaller proportion of total C and NO3- uptake: 27-43% for C and 34-59% for NO3-. Iron enrichment experiments in the eastern tropical Pacific resulted in much higher NO3-, C, and Si uptake rates, although increases were restricted to cells in the >5-µm size fraction. Si:NO3- and Si:C uptake ratios decreased after iron addition at most locations, and decreases were a direct result of lower Si : NO3- and Si : C uptake ratios in the >5-µm size fraction. Our results suggest that iron availability is a major factor regulating primary production, new production, Si uptake, and Si:NO3- and Si:C uptake ratios in the larger phytoplankton size classes in high-nitrate, low-chlorophyll (HNLC) regions in the eastern tropical Pacific. [ABSTRACT FROM AUTHOR]

The physicochemical speciation of dissolved iron (Fe) across natural dissolved Fe gradients in the oceanic and shelf domains of the southeastern Bering Sea was examined in surface and subsurface samples using competitive ligand exchange-adsorptive cathodic stripping voltammetry with the added ligand salicylaldoxime. Two ligand classes were measured in all samples, a stronger L1 ligand class and a weaker L2 ligand class. Conditional stability constants for both ligand classes were comparable between surface and subsurface samples, with mean log Kcond FeL1,Fe0 = 11.5 ± 0.3 and mean log KcondFeL2,Fe0 = 10.3 ± 0.3 in surface samples, and mean log KcondFeL1,Fe0 = 11.4 ± 0.2 with a weaker ligand and mean log KcondFeL2,Fe0 of 10.2 ± 0.2 in subsurface samples. The concentrations of dissolved Fe were strongly correlated with ambient stronger L1 ligand concentrations for all samples with dissolved Fe concentrations greater than 0.2 nmol L-1. In samples with dissolved Fe concentrations less than 0.2 nmol L-1, large and variable excesses of L1 ligand concentrations were measured, coincident with observed Fe stress or limitation on the ambient phytoplankton. These observations suggest that the phytoplankton community is readily able to access dissolved Fe from the FeL1 complex, resulting in excess L1 in these waters. The available speciation data from other sources indicate that a significant correlation exists between dissolved Fe and L1 ligand concentrations in samples with intermediate dissolved Fe, and this is a seemingly ubiquitous feature of dissolved Fe cycling in the marine environment. [ABSTRACT FROM AUTHOR]