| Poribacteria | |
|---|---|
 | |
| Poribacteria localisation in the sponge Aplysina aerophoba | |
Scientific classification | |
| Domain: | Bacteria |
| Kingdom: | Pseudomonadati |
| Phylum: | Poribacteria |
Poribacteria are acandidate phylum ofbacteria originally discovered[1] in themicrobiome of marinesponges (Porifera). Poribacteria areGram-negative primarilyaerobicmixotrophs with the ability foroxidative phosphorylation,glycolysis, and autotrophic carbon fixation via theWood – Ljungdahl pathway.[2][3] 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.[3]
Single-cellgenomics andmetagenomic shotgun sequencing approaches reveal a poribacterialgenome size range between about 4.2 and 6.5megabases[2][3][4][5] encoding 4,254protein-coding genes, of which an unusually high 24% have nohomology to knowngenes. Among the genes of identifiable homology, reconstructed pathways suggest that the poribacterial central metabolism is capable ofglycolysis,tricarboxylic acid cycle,pentose phosphate pathways,oxidative phosphorylation, theEntner-Doudoroff pathway, andautotrophiccarbon fixation viaWood–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 includingCRISPR-CAS andrestriction modification systems.[6]
Cell compartmentalization into distinctmembrane-bound organelles is a universal and defining property of eukaryotes, but had not been observed inprokaryotes other than thePlanctomycetota. Poribacteria were previously thought to be distinguished from other microorganisms associated with marinesponges by such a distinctive morphology featuring a large membrane-bound cellular compartment that was suggested to contain DNA.[1] The distinctive poribacterial compartments were originally identified usingfluorescence in situ hybridization andelectron microscopy.[1] Genomic evidence suggests the presence of protein-bound organelles, but not for membrane-bound organelles.[6] More recently,correlative light-electron microscopy, confirmed two elements of poribacterial subcellular compartmentation:[7] 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 asPolyhydroxybutyrate.
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 includeankyrin andleucine-rich repeat domains,[2] as well astetratricopeptides.[6] Unusuallow-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.[6] In addition, genetic infrastructure forsterol biosynthesis is observed in poribacterial genomes, otherwise found almost exclusively in eukaryotes and the planctomyceteGemmata obscuriglobus.[2]
Poribacteria aresymbionts of marinesponges, among the most abundant microorganisms in the highly diverse microbiome of the spongemesohyl.[2] They have been found in a large variety of sponge species from diverse geographic origins.[8] The composition of microorganisms in the sponge microbiome can be vertically inherited, with adult sponges transmitting their distinctive microbial communities to offspring.[9]