The influence of mesoscale oceanographic features on coccolithophore diversity and life cycles in the Southern Indian Ocean

Coccolithophores are abundant microalgae that play a key role in the ocean carbon cycle through photosynthesis and their production of intricate calcium carbonate coccoliths. Coccolithophores are particularly abundant in the Great Calcite Belt within the Southern Ocean, although these communities remain largely understudied due to their remote location. Mesoscale oceanographic features can influence carbonate export in these regions, although their impact on coccolithophore communities and life cycles remains unclear. We have applied a combination of scanning electron microscopy (SEM) and molecular DNA metabarcoding approaches to characterize coccolithophore populations across a mesoscale meander system in the Southern Indian Ocean. DNA metabarcoding, using haptophyte‐specific 18S rRNA primers, revealed a rich diversity of coccolithophores in this region. This diversity was absent in previous studies using broad‐specificity 18S eukaryote primers, indicating that coccolithophores are likely to be massively underrepresented in many large‐scale sequencing studies. Scanning electron microscopy analysis revealed a similar community, dominated by Umbellosphaera tenuis, Gephyrocapsa huxleyi, and Syracosphaera halldallii, with broadly similar trends in community structure, richness, and diversity. Community composition and life cycle phases were strongly influenced by a mesoscale oceanographic frontal system associated with significant gradients in temperature, salinity, and nutrient availability. Our study identifies a high diversity of coccolithophores in the Great Calcite Belt and suggests that hydrographic gradients across mesoscale features can cause fine‐scale heterogeneity in coccolithophore communities that diverge from the broad‐scale patterns caused by latitudinal gradients.