gen. nov., sp. nov., isolated from a deep subsurface aquifer and proposal of phyl. nov., formerly called Marine Group A, SAR406 or Marinimicrobia.

A Gram-negative, obligatory anaerobic, chemoheterotrophic bacterium, designated strain IA91, was isolated from sediments and formation water from deep aquifers in Japan. IA91 derives its peptidoglycan, energy and carbon from exogenous cell wall fragments, namely muropeptides, released from actively reproducing bacteria, and is dependent on other bacteria for cell wall formation, growth and even cell shape: IA91 is irregular rod-shaped but coccoids when muropeptide is absent. IA91 grew in a temperature range of 25-45 °C with optimum growth at 40 °C. IA91 utilized limited substrates, yeast extract, muropeptides and d-lactate. The major end products from yeast extract degradation were acetate, hydrogen and carbon dioxide. Co-cultivation with a hydrogen-scavenging methanogenic archaeon promoted IA91 growth. No anaerobic respiration with nitrate, nitrite, sulphate or Fe(III) was observed. The major cellular fatty acids are C16 : 0, C18 : 1 9, C18 : 0 and C17 : 0. The G+C content of the genomic DNA was 45.6 mol%. Phylogenetic analysis based on 16S rRNA gene and conserved protein sequences involved in replication, transcription and translation indicated that IA91 belonged to the candidate phylum Marine Group A (MG-A, SAR406 or . Marinimicrobia) with no cultivated representatives. Based on the phenotypic and phylogenomic characteristics, a new genus and species, gen. nov., sp. nov., is proposed for IA91 (= JCM 39387 = KCTC 25736). In addition, a new bacterial phylum named phyl. nov. is proposed for the candidate phylum MG-A represented by and fam. nov., ord. nov. and classis nov.