Poribacteria

Poribacteria are a candidate phylum of bacteria originally identified in the microbiome of sea sponges (Porifera). Poribacteria were distinguished from other microorganisms associated with sea sponges by their distinctive morphology featuring a large membrane-bound cellular compartment that frequently contains DNA, a highly unusual feature for a prokaryote.^[1] Poribacteria are Gram-negative mixotrophs.^[2]

Genome

Single-cell genomics analysis of poribacteria reveals a genome with a lower size bound of 1.88 megabases and 1585 protein-coding genes, of which an unusually high 24% have no homology to known genes. Among the genes of identifiable homology, genetic infrastructure can be identified for aerobic metabolism, denitrification and urea uptake, and carbon fixation through the Wood-Ljungdahl pathway.^[2]

The poribacterial genome is also reported to contain an unusually high number of phyH-domain proteins, which are enzymes involved in oxidative reactions. The functional significance of this observation is not known.^[3]

Cell compartmentalization

Cell compartmentalization into distinct membrane-bound organelles is a universal and defining property of eukaryotes, but had not been not observed in prokaryotes other than the Planctomycetes before the identification of Poribacteria.^[1] The distinctive poribacterial compartments were originally identified using fluorescence in situ hybridization and electron microscopy and were found to frequently, but not always, contain DNA.^[1] Genomic evidence reveals the presence of proteins associated with compartmentalization, but not of membrane coat proteins.^[3]

Eukaryote-like proteins

Genomic analyses of poribacteria reveal several families of cell-surface repeat proteins that resemble those found in eukaryotes, and are infrequently found in prokaryotes. Examples include ankyrin and leucine-rich repeat domains,^[2] as well as tetratricopeptides.^[3] Unusual low-density lipoprotein receptor repeat proteins are also found, of unknown function. Most of these protein families are thought to be involved in surface interactions with the sponge host.^[3]

In addition, genetic infrastructure for sterol biosynthesis is observed in poribacterial genomes, otherwise found almost exclusively in eukaryotes and the planctomycete Gemmata obscuriglobus.^[2]

Ecological niche

Poribacteria are symbionts of sea sponges, among the most abundant microorganisms in the highly diverse microbiome of the sponge mesohyl.^[2] They have been found in a large variety of sponge species from diverse geographic origins.^[4] The distribution of microorganisms in the sponge microbiome can be vertically inherited, with adult sponges transmitting their distinctive microbial communities to offspring.^[5]

References