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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 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.