Lithochloron, a new genus of green algae residing in coral skeleton (Cladophorales, Chlorophyta)

Coral skeletons host diverse microbial communities, including endolithic green algae that contribute to holobiont functioning. The siphonous green alga Ostreobium (Bryopsidales) is arguably the best-known coral endolith, but recent environmental sequencing has indicated that other lineages of green algae may also inhabit the coral skeleton, including members of the Cladophorales. Here, we characterize a new endolithic green alga isolated from a Porites coral skeleton collected from Heron Island on the southern Great Barrier Reef. The alga forms uniseriate, sparsely branched grass-green filaments composed of elongated vegetative cells with infrequent cross walls. Phylogenetic analyses of 18S and 28S nrDNA gene sequences placed the strain as an early-diverging lineage within the Siphonocladus clade of the Cladophorales order of the Ulvophyceae. Based on morphological and molecular evidence, we describe the alga as the new genus and species Lithochloron capricornicum. Analysis of environmental 18S nrDNA gene amplicon data from Great Barrier Reef corals revealed extensive sequence diversity, suggesting the genus has additional yet undescribed species. Environmental sequencing further showed that it is most abundant at intermediate depths within the coral skeleton and suggested niche differentiation with Ostreobium along the vertical light gradient in the skeleton, with Lithochloron tending to peak at shallower depths than Ostreobium. The discovery of Lithochloron challenges the long-standing assumption that Ostreobium is the only dominant green algal endolith in coral skeletons and highlights the underestimated diversity of endolithic algae more broadly.