Poribacteria are a candidate phylum of bacteria originally discovered
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
. 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
Auxotrophy 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 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
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.
Genomic evidence suggests the presence of protein-bound organelles, but not for membrane-bound organelles.
More recently, correlative light-electron microscopy, confirmed two elements of poribacterial subcellular compartmentation:
Firstly, Bacterial microcompartments, atypically localized at the cell membrane. Secondly, spherical bipolar compartments which are discussed to be most likely carbon rich storage polymers such as Polyhydroxybutyrate.
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,
as well as tetratricopeptides.
Unusual
LDL 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.
In addition, genetic infrastructure for
sterol biosynthesis is observed in poribacterial genomes, otherwise found almost exclusively in eukaryotes and the planctomycete
Gemmata obscuriglobus.
Ecological niche
Poribacteria are
symbiosis of marine
, among the most abundant microorganisms in the highly diverse microbiome of the sponge
mesohyl.
They have been found in a large variety of sponge species from diverse geographic origins.
The composition of microorganisms in the sponge microbiome can be vertically inherited, with adult sponges transmitting their distinctive microbial communities to offspring.
