Extensive diversity of Symbiochlorum-related algae from environmental sequences and culture strains supports the description of the new family Symbiochloraceae (Ulvophyceae, Chlorophyta).

The genus Symbiochlorum, initially described from a single strain isolated from a coral in the South China Sea, was shown to be a sister lineage of Ignatius within the green algal order Ignatiales. Its significant phylogenetic divergence from Ignatius raises the possibility of its classification as a new family. To further investigate this hypothesis, we conducted a more elaborate analysis of sequence diversity within the Symbiochlorum clade. We aligned the 18S nuclear ribosomal DNA gene sequences of newly isolated Symbiochlorum culture strains from coral in the South China Sea and environmental sequences from the Great Barrier Reef. Strains isolated from Porites lutea coral colonies exhibited morphological similarities to typical S. hainanense (CCTCC M2018096). Analysis of the 18S nuclear ribosomal DNA gene revealed substantial diversity in both the V4 and V9 regions of the gene, with sequences clustering into two distinct lineages. Lineage 1 (L1), represented solely by environmental sequences from Great Barrier Reef sediment samples, displayed high levels of sequence divergence (2.2%-5.8%), suggesting it consists of multiple species. Lineage 2 (L2) included coral-derived strains and environmental sequences from the South China Sea and the Great Barrier Reef, as well as an ascidian-associated strain from Palau. The significant divergence between L1 and L2 (3.1%-9.1%) suggests they represent different genera. Based on these results, we propose the recognition of the new family Symbiochloraceae within the Ignatiales order.