Abstract
BACKGROUND: Chromerid algae are the closest photosynthetic relatives of apicomplexan parasites. While chromerids have been central to understanding the evolutionary transition from free-living algae to parasitism within Apicomplexa, their ecology remains poorly understood. Although often considered coral-associated symbionts, emerging evidence suggests this link is incidental and that chromerids may be more broadly associated with calcium carbonate environments, including microbialites. These microbial structures represent modern analogues of ancient reef-like ecosystems but are difficult to study due to their rarity and protected status as world heritage sites. Prokaryotic members of the microbialite microbiome have been studied at length, while the microeukaryotes associated with these environments have gone mostly ignored. To further investigate the link between microbialites and chromerid algae, we re-analyzed previously published microbialite sequencing data with the aim of investigating chromerid diversity and distribution. RESULTS: Through a novel plastid-focused metagenomic binning workflow combined with re-analysis of rRNA metabarcoding data, we reveal that chromerid algae are consistent associates of microbialites across diverse marine and freshwater environments worldwide. Most notably, we report the first recovery of plastid genomes from microbialite-associated chromerids: a complete Vitrella brassicaformis plastid genome and a second, partial plastid genome from a previously undescribed Chromera-related lineage in Highborne Cay thrombolites. This partial plastid genome contained photosystem genes, confirming this novel Chromera-related lineage as a photosynthetic chromerid. These findings not only expand the known ecological and biogeographic range of chromerids but also provide evidence for their overlooked diversity. CONCLUSIONS: Our analyses prove that this overlooked algal lineage is not found exclusively associated with corals, but instead occurs across a wide range of microbialite habitats, including those found in freshwater. By extending their known distribution beyond coral hosts and the marine environment, our results not only highlight the diversity and ecological range of the most recently discovered algal lineage but also broaden our understanding of the ancestral lifestyles that may have preceded apicomplexan evolution. This research underscores the value of targeted mining of public sequencing datasets to address specific ecological questions, particularly in rare or hard-to-access environments such as microbialites.