Publications

4439

AbstractAquatic environments with high levels of dissolved ferrous iron and low levels of sulfate serve as an important systems for exploring biogeochemical processes relevant to the early Earth. Boreal Shield lakes, which number in the tens of millions globally, commonly develop seasonally anoxic waters that become iron rich and sulfate poor, yet the iron–sulfur microbiology of these systems has been poorly examined. Here we use genome-resolved metagenomics and enrichment cultivation to explore the metabolic diversity and ecology of anoxygenic photosynthesis and iron/sulfur cycling in the anoxic water columns of three Boreal Shield lakes. We recovered four high-completeness and low-contamination draft genome bins assigned to the class Chlorobia (formerly phylum Chlorobi) from environmental metagenome data and enriched two novel sulfide-oxidizing species, also from the Chlorobia. The sequenced genomes of both enriched species, including the novel “Candidatus Chlorobium canadense”, encoded the cyc2 gene that is associated with photoferrotrophy among cultured Chlorobia members, along with genes for phototrophic sulfide oxidation. One environmental genome bin also encoded cyc2. Despite the presence of cyc2 in the corresponding draft genome, we were unable to induce photoferrotrophy in “Ca. Chlorobium canadense”. Genomic potential for phototrophic sulfide oxidation was more commonly detected than cyc2 among environmental genome bins of Chlorobia, and metagenome and cultivation data suggested the potential for cryptic sulfur cycling to fuel sulfide-based growth. Overall, our results provide an important basis for further probing the functional role of cyc2 and indicate that anoxygenic photoautotrophs in Boreal Shield lakes could have underexplored photophysiology pertinent to understanding Earth’s early microbial communities.

Summary Cyanobacteria of the genus Synechococcus are major contributors to global primary productivity and are found in a wide range of aquatic ecosystems. This Synechococcus collective (SC) is metabolically diverse, with some lineages thriving in polar and nutrient‐rich locations and others in tropical or riverine waters. Although many studies have discussed the ecology and evolution of the SC, there is a paucity of knowledge on its taxonomic structure. Thus, we present a new taxonomic classification framework for the SC based on recent advances in microbial genomic taxonomy. Phylogenomic analyses of 1085 cyanobacterial genomes demonstrate that organisms classified as Synechococcus are polyphyletic at the order rank. The SC is classified into 15 genera, which are placed into five distinct orders within the phylum Cyanobacteria: (i) Synechococcales ( Cyanobium , Inmanicoccus , Lacustricoccus gen. Nov., Parasynechococcus , Pseudosynechococcus , Regnicoccus , Synechospongium gen. nov., Synechococcus and Vulcanococcus ); (ii) Cyanobacteriales ( Limnothrix ); (iii) Leptococcales ( Brevicoccus and Leptococcus ); (iv) Thermosynechococcales ( Stenotopis and Thermosynechococcus ) and (v) Neosynechococcales ( Neosynechococcus ). The newly proposed classification is consistent with habitat distribution patterns (seawater, freshwater, brackish and thermal environments) and reflects the ecological and evolutionary relationships of the SC.

A novel Gram-stain-negative, non-endospore-forming, motile, and aerobic bacterial strain, M105T, was isolated from coral Porites lutea, and was subjected to a polyphasic taxonomic study. Global alignment based on 16S rRNA gene sequences indicated that M105T shares the highest sequence identity of 94.5 % with Aliikangiella marina GYP-15T. The average nucleotide identity (ANI) and average amino acid identity (AAI) between M105T and A. marina GYP-15T was 69.8 and 71.6 %, respectively. On the basis of the results of phenotypic, chemotaxonomic, phylogenetic, phylogenomic, and comparative genomic analyses, it is concluded that M105T should represent a novel species in the genus Aliikangiella , for which the name Aliikangiella coralliicola sp. nov. is proposed. The type strain is M105T (=MCCC 1K03773T= KCTC 72442T). Furthermore, the family Kangiellaceae was classified into two families on the basis of phylogenetic, phylogenomic, polar lipid profile and motility variations. The novel family Pleioneaceae fam. nov. is proposed to accommodate the genera Aliikangiella and Pleionea .