Distinct iron acquisition strategies in oceanic and coastal variants of the mixotrophic dinoflagellate Karlodinium

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.

Keywords: dinoflagellates; iron acquisition; iron-limited regions; mixotrophy; oceanic and coastal variants; transcriptomics.

Figure 1

Growth parameters of representative oceanic (UNC1840) and coastal (CCMP1975) Karlodinium species. (A) Relative specific growth rates (GRs) measured based on microscopic cell counts, (B) relative maximum photochemical yield of PSII (F_v/F_m), and (C) relative cell volume (CV). The values above bars of (A), (B), and (C) are the absolute specific growth rate (μ, d⁻¹), maximum photochemical quantum yield, and cell volume (μm³) in each treatment, respectively. The y-axis represents the relative values (%) compared to the maximum values under four different treatment conditions for each strain. Treatments are Fe-replete, 20°C (+Fe, 20°C), Fe-limited, 20°C (−Fe, 20°C), Fe-replete, 12°C (+Fe, 12°C), and Fe-limited, 12°C (−Fe, 12°C). Error bars indicate the standard deviation of biological replicates.

Figure 2

Iron content parameters of representative oceanic (UNC1840) and coastal (CCMP1975) Karlodinium species. (A) Intracellular iron content (fmol cell⁻¹), (B) iron quota ratios under Fe-replete and Fe-limited conditions (Fe-Q_high: Fe-Q_low) at different temperatures, and (C) iron-use efficiency, defined as the rate of cellular carbon per unit of cellular iron per day (×10⁵ mol C Fe⁻¹ d⁻¹). Experimental conditions include Fe-replete, 20°C (+Fe, 20°C), Fe-limited, 20°C (−Fe, 20°C), Fe-replete, 12°C (+Fe, 12°C), and Fe-limited, 12°C (−Fe, 12°C). Error bars indicate the standard deviation of biological replicates.

Figure 3

Differential expression of class II (gene class) and class III level pathways (assigned by KEGG modules) between the UNC1840 (oceanic) and CCMP1975 (coastal) Karlodinium isolates. Columns denote the fold change of transcripts in iron-limited (low-Fe) versus iron-replete (high Fe) treatments at 12°C and 20°C. The heatmap indicates the Log₂ fold change in gene expression of combined genes belonging to each pathway. Pathways are categorized into gene classes according to KEGG.

Figure 4

Overview of transcriptional patterns and predicted subcellular localization of key proteins in oceanic (UNC1840) and coastal (CCMP1975) Karlodinium isolates under different iron and temperature conditions. Bubble heatmaps of the oceanic and coastal isolates depicting gene expression (Log₂ fold change and Log₂ normalized abundance) for genes involved in iron homeostasis, nitrogen assimilation, and photosynthesis. Columns denote the fold change of transcripts in iron-limited (low Fe) versus iron-replete (high Fe) incubations at 12°C and 20°C. The heatmap indicates the Log₂ fold change in gene expression, and the size of the circles indicates the Log₂ normalized transcript abundance. On the right is a cellular schematic of the putative localizations of the different proteins listed for iron homeostasis (orange proteins), nitrogen assimilation (purple proteins), and photosynthesis found in Karlodinium dinoflagellates. Created with BioRender.com. Gene/protein name for each abbreviation is provided in Supplementary Table S1.