Genomic insights into bacterial communities of coenocytic algae using metagenome-assembled genomes

Coenocytic algae are organisms that undergo karyokinesis without cytokinesis, resulting in multinucleated cells. Most research on bacterial communities in coenocytic algae has used 16S rRNA sequencing, primarily focusing on the order Bryopsidales of green coenocytic algae. Recent studies have analyzed metagenome-assembled genomes (MAGs) from algal hosts across multiple taxa, such as Chlorophyta, Phaeophyta, and Rhodophyta, revealing further insights into bacterial biosynthetic machinery and potential symbiotic relationships. Given the cosmopolitan distribution of coenocytic algae, such as Bryopsis, Caulerpa, Codium, and the yellow-green alga Vaucheria, and their unique morphology, there is a need to better understand their associated bacterial communities. To address this, filaments of Vaucheria bursata LB2067 were sequenced using the Illumina NovaSeq instrument, and all publicly available short- and long-read datasets from coenocytic algae were screened for MAG recovery. All recovered MAGs from both Bryopsidales and Vaucheriales showed a high dominance of Pseudomonadota at the phylum level, but no consistent patterns at the order or family levels. Core metabolic pathways, including glycolysis, the TCA cycle, and amino acid biosynthesis, were present across MAGs from both coenocytic and non-coenocytic algae. Pathways with significantly higher completeness in coenocytic-associated MAGs included protein secretion (Sec-SRP), which showed the strongest effect, motility (flagellar assembly), thiamin biosynthesis, and polyhydroxybutyrate (PHB) synthesis (Mann-Whitney U, padj < 0.05). Notably, N-acetylglutaminylglutamine amide (NAGGN) biosynthetic gene clusters (BGCs) were found only in MAGs from coenocytic orders, whereas polysaccharide utilization loci (PULs) were present in all MAGs analyzed. These results indicate that bacterial communities associated with coenocytic algae are complex, with diverse survival strategies adapted to challenging and variable environments.