Distinct iron acquisition strategies in oceanic and coastal variants of the mixotrophic dinoflagellate <i>Karlodinium</i>

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.