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Mangroves are highly productive unique ecosystems harboring diverse unexplored microbial communities that play crucial roles in nutrient cycling as well as in maintaining ecosystem services. The mangrove-associated microbial communities transform the dead vegetation into nutrient sources of nitrogen, phosphorus, potash, etc. To understand the genetic and functional diversity of the bacterial communities involved in nitrogen cycling of this ecosystem, this study explored the diversity and distribution of both the nitrogen fixers and denitrifiers associated with the rhizospheres of Avicennia marina, Rhizophora mucronata, Suaeda maritima, and Salicornia brachiata of the Pichavaram mangroves. A combination of both culturable and unculturable (PCR-DGGE) approaches was adopted to explore the bacterial communities involved in nitrogen fixation by targeting the nifH genes, and the denitrifiers were explored by targeting the nirS and nosZ genes. Across the rhizospheres, Gammaproteobacteria was found to be predominant representing both nitrogen fixers and denitrifiers as revealed by culturable and unculturable analyses. Sequence analysis of soil nifH, nirS and nosZ genes clustered to unculturable, with few groups clustering with culturable groups, viz., Pseudomonas sp. and Halomonas sp. A total of 16 different culturable genera were isolated and characterized in this study. Other phyla like Firmicutes and Actinobacteria were also observed. The PCR-DGGE analysis also revealed the presence of 29 novel nifH sequences that were not reported earlier. Thus, the mangrove ecosystems serve as potential source for identifying unexplored novel microbial communities that contribute to nutrient cycling., (© 2022. The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature.)

The taxonomic position of strain MSSRFBL1(T), isolated from chickpea rhizosphere soil from Kannivadi, India, was determined. Strain MSSRFBL1(T) formed bluish black colonies, stained Gram-negative and was motile, aerobic, capable of fixing dinitrogen, oxidase-negative and catalase-positive. Q-10 was the major respiratory quinone. Major fatty acids of strain MSSRFBL1(T) were C18 : 1ω7c and C19 : 0cycloω8c. Minor amounts of C18 : 0, C12 : 0, C14 : 0 3-OH, C18 : 0 3-OH, C16 : 0, C16 : 1ω6c/C16 : 1ω7c, C17 : 0 3-OH and C20 : 1ω7c were also present. Polar lipids included diphosphatidylglycerol, phosphatidylethanolamine, phosphatidylmethylethanolamine, phosphatidylcholine and two unidentified glycolipids. Bacteriohopane derivatives (BHD1 and 2), diplopterol, diploptene, bishomohopanediol, adenosylhopane and 2β-methyl bacteriohopanetetrol were the major hopanoids of strain MSSRFBL1(T). The genomic DNA G+C content was 71 mol%. EzTaxon-e-based blast analysis of the 16S rRNA gene indicated the highest similarity of strain MSSRFBL1(T) to Ensifer adhaerens LMG 20216(T) (97.3 %) and other members of the genus Ensifer (<96.9 %) in the family Rhizobiaceae of the class Alphaproteobacteria. However, phylogenetic analysis based on 16S rRNA, recA, thrC and dnaK gene sequences showed distinct out-grouping from the recognized genera of the family Rhizobiaceae. Based on phenotypic, genotypic and chemotaxonomic characters, strain MSSRFBL1(T) represents a novel species in a new genus in the family Rhizobiaceae for which the name Ciceribacter lividus gen. nov., sp. nov. is proposed. The type strain of Ciceribacter lividus is MSSRFBL1(T) ( = DSM 25528(T) = KCTC 32403(T)).