Resumo

Microorganisms associated with corals regulate key processes in the holobiome, including energy acquisition, calcification, nutrient cycling and disease. Limestone-boring (endolithic) algae are commonly found living underneath stony corals living tissue, inside their calcium carbonate skeleton. The green algae genus Ostreobium (Chlorophyta, Ulvophyceae) is considered to be the most abundant one. These algae are among the main contributors of reef bioerosion, but have also been reported to facilitate coral survival during bleaching events. Despite their key role in reef ecosystems, limestone-boring communities are still poorly characterized in terms of taxonomic richness, phylogenetic diversity and ecological functioning. The goal of this study is to establish a baseline of the limestone-boring algae biodiversity in coral skeletons via high-throughput environmental sequencing. We use a combination of universal and protein-coding amplicons (16S, 18S, 23S and tufA) in order to obtain a wholesome microbial community characterization. We frame our analysis on the QIIME pipeline but use gene and taxon specific information to define taxonomic units and reconstruct their phylogenetic relationships. A pilot study using 10 coral skeletons samples allowed retrieving ca. 60 OTUs at genus level for each sample. With one of the universal markers (23S) we were able to recover representatives of many different algal divisions (including reds, greens and cyanobacteria) and several groups of algae not previously known to occur in boring communities. The protein-coding amplicon tufA allowed a more detailed characterization of the green algal diversity: a total of 230 OTUs at genus level were retrieved, 11 of which belonging to the Ostreobiaceae family. The results of this work will be compared to the habitat preferences of the main Ostreobium lineages in order to better understand the ecological diversity of the endolithic flora.