Is the Gelatinous Matrix of Nassellaria (Radiolaria) a Strategy for Coping With Oligotrophy?

Radiolaria are heterotrophic protists abundant in the world's oceans, playing important roles in biogeochemical cycles. Some host photosynthetic algae, contributing to primary production. Such mixotrophic behaviour is believed to explain their success in oligotrophic waters, notably Collodaria, exclusively mixotrophic radiolarians within a gelatinous matrix. Yet, understanding of Radiolaria ecology is limited to direct observations, as they have so far withstood reproduction in culture and lack genome data. Sampling oligotrophic California Current revealed abundant, rarely observed Nassellaria of the genus Phlebarachnium, characterised to live within a gelatinous matrix. Phylogenetic reconstruction of ribosomal DNA suggests that distantly related Nassellaria lineages independently developed the ability to produce a gelatinous matrix ~150 million years ago. By matching physical samples with genetic data, we identified these rarely observed organisms in global datasets, revealing their affinity for oligotrophic conditions. Co-occurrence networks showed distinct biogeography patterns for gelatinous matrix-forming Radiolaria compared to those without. Results suggest the matrix might be an adaptation to oligotrophic waters, increasing the effective volume, favouring prey capture, and creating a larger microenvironment for symbionts, thus promoting ecological success in nutrient-depleted waters. This study advances our understanding of the adaptation of poorly known eukaryotic groups, specifically when evolution occurs independently across lineages.