A phylogenetically novel cyanobacterium most closely related to Gloeobacter

Abstract

Clues to the evolutionary steps producing innovations in oxygenic photosynthesis may be preserved in the genomes of organisms phylogenetically placed between non-photosynthetic Vampirovibrionia (formerly Melainabacteria) and the thylakoid-containing Cyanobacteria. However, only two species with published genomes are known to occupy this phylogenetic space, both within the genus Gloeobacter. Here, we describe nearly complete, metagenome-assembled genomes (MAGs) of an uncultured organism phylogenetically placed near Gloeobacter, for which we propose the name Candidatus Aurora vandensis {Au'ro.ra. L. fem. n. aurora, the goddess of the dawn in Roman mythology; van.de'nsis. N.L. fem. adj. vandensis of Lake Vanda, Antarctica}. The MAG of A. vandensis contains homologs of most genes necessary for oxygenic photosynthesis including key reaction center proteins. Many accessory subunits associated with the photosystems in other species either are missing from the MAG or are poorly conserved. The MAG also lacks homologs of genes associated with the pigments phycocyanoerethrin, phycoeretherin and several structural parts of the phycobilisome. Additional characterization of this organism is expected to inform models of the evolution of oxygenic photosynthesis.

Fig. 1. Phylogeny of A. vandensis.

a Genome phylogeny based on the concatenation of 37 single-copy marker genes showing A. vandensis as a sister group to the Gloeobacter; b 16S rRNA gene phylogeny showing the genus Aurora as a sister group to all other Cyanobacteria; c Ribosomal protein L2 phylogeny with A. vandensis sister to the Gloeobacter; and d IF3 C terminal phylogeny showing A. vandensis as sister to the Gloeobacter.

Fig. 2. Phylogenetic position of D1 protein located in the green mat assembly.

a D1 and D2 phylogeny showing D1 groups as defined by [18] Location of putative A. vandensis D1 highlighted by black box. b Region of tree highlighted by black box in A showing the putative A. vandensis sequence and its closest relatives.

Fig. 3

Alignment of D1 protein sequences from Synechococcus elongatus and Gloeobacter kilaueensis and the translated D1 sequence from the green mat assembly.