Poribacteria

Poribacteria are a candidate phylum of bacteria originally discovered in the microbiome of marine sponges (Porifera). Poribacteria are Gram-negative primarily aerobic mixotrophs with the ability for oxidative phosphorylation, glycolysis, and autotrophic carbon fixation via the Wood – Ljungdahl pathway. Poribacterial heterotrophy is characterised by an enriched set of glycoside hydrolases, uronic acid degradation, as well as several specific sulfatases. This heterotrophic repertoire of poribacteria was suggested to be involved in the degradation of the extracellular sponge host matrix.

Genome

Single-cell genomics and metagenomic shotgun sequencing approaches reveal a poribacterial genome size range between about 4.2 and 6.5 megabases encoding 4,254 protein-coding genes, of which an unusually high 24% have no homology to known genes. Among the genes of identifiable homology, reconstructed pathways suggest that the poribacterial central metabolism is capable of glycolysis, tricarboxylic acid cycle, pentose phosphate pathways, oxidative phosphorylation, the Entner-Doudoroff pathway, and autotrophic carbon fixation via Wood–Ljungdahl pathway. Further, Poribacteria seem to engage in assimilatory denitrification and ammonia scavenging with potential relevance in nitrogen re-cycling within the sponge holobiont. The poribacterial genome is also reported to contain an unusually high number of phage defence systems including CRISPR-CAS and restriction modification systems.

Cell compartmentalization

Cell compartmentalization into distinct membrane-bound organelles is a universal and defining property of eukaryotes, but had not been observed in prokaryotes other than the Planctomycetota. Poribacteria were previously thought to be distinguished from other microorganisms associated with marine sponges by such a distinctive morphology featuring a large membrane-bound cellular compartment that was suggested to contain DNA. The distinctive poribacterial compartments were originally identified using fluorescence in situ hybridization and electron microscopy.