Genome Sequence of the Photoarsenotrophic Bacterium Ectothiorhodospira sp. Strain BSL-9, Isolated from a Hypersaline Alkaline Arsenic-Rich Extreme Environment.

The full genome sequence of Ectothiorhodospira sp. strain BSL-9 is reported here. This purple sulfur bacterium encodes an arxA-type arsenite oxidase within the arxB2AB1CD gene island and is capable of carrying out "photoarsenotrophy" anoxygenic photosynthetic arsenite oxidation. Its genome is composed of 3.5 Mb and has approximately 63% G+C content.

GENOME ANNOUNCEMENT

Arsenic-rich soda lakes are ideal environments for culturing microorganisms with unique metabolic capabilities for coupling cellular energy production to arsenic oxidation and reduction (1–6). Here, we report the assembled genome of an anoxygenic photosynthetic arsenite-oxidizing (“photoarsenotrophic”) bacterium, Ectothiorhodospira sp. strain BSL-9. This microbe was isolated from Big Soda Lake, an arsenic-rich (~25 µM), hypersaline (26 to 88 g/liter total dissolved solids), alkaline (pH 9.7) lake located in Nevada (39°31ʹN 118°52ʹW) (7–9). Moreover, this crater lake has a well-defined seasonal bloom of purple sulfur bacteria (Chromatium and Ectothiorhodospira species) (10) that are proposed to contribute to the arsenic geochemical cycle.

Assessment of the BSL-9 genome revealed an arsenic gene island, arxB2AB1CD (11), which is predicted to encode the arsenite oxidase gene arxA. Moreover, arxB2AB1CD encodes a [4Fe-4S]-containing protein (arxB2), a second [4Fe-4S]-containing protein (arxB1), a membrane protein (arxC), and a TorD-like protein involved in molybdenum enzyme biogenesis (arxD). In the chemoautotrophic bacterium Alkalilimnicola ehrlichii sp. strain MLHE-1, arxA is required for anaerobic arsenite oxidation coupled to nitrate (12). The BSL-9 genome lacks the AioA-type arsenite oxidase. A BSL-9 arxA mutant strain shows that arxA is the sole arsenite oxidase for photoarsenotrophy (13).

Ectothiorhodospiraceae are common anoxygenic phototrophs with versatile abilities to metabolize inorganic and organic electron donors (14–16), which enables them to occupy distinct euphotic hypersaline alkaline environments. In addition to arsenite, BSL-9 can grow photoautotrophically with sulfide or thiosulfate. This is consistent with the presence of sox and dsr genes, which are involved in sulfur oxidation (17, 18). Moreover, BSL-9 can also grow as a photoheterotroph with various organic acids (e.g., acetate, malate, propionate, lactate, fumarate, succinate, and pyruvate). BSL-9 is sensitive to chloramphenicol, resistant to kanamycin, carbenicillin, gentamicin, and tetracycline, and grows optimally at 35°C at pH 8, 2% NaCl; these growth patterns are consistent with other Ectothiorhodospira species (14–16). Although BSL-9 is an anaerobe, the presence of cytochrome c oxidase genes (e.g. cbb3) found in BSL-9 may explain its tolerance to atmospheric oxygen. For example, cytochrome c oxidases (cbb3) are known for having high oxygen affinity, and cytochrome c peroxidases protect cells from reactive oxygen species. The BSL-9 genome also encodes photosynthetic complex genes, such as bacteriochlorophyll a synthase, the light-harvesting complex pucAB, and two copies of the carbon fixation-related gene rbcL (type III RuBisCO). Having the full genome sequence of BSL-9 opens numerous possibilities for studying the metabolic abilities, physiology, and the ecological environmental impact of photoarsenotrophy to the arsenic biogeochemical cycle.

The genome was done at the UC Davis Genome Sequencing Center using PacBio technology. The Hierarchical Genome Assembly Process (HGAP_v2) assembly pipeline (19) was used with ~300× sequence coverage. For annotation, the NCBI Public Genome Annotation Pipeline service was used. The resulting assembly was 3.5 Mb, with 63% G+C content.

Accession number(s).

The genome sequence of Ectothiorhodospira sp. strain BSL-9 was deposited in the GenBank database under the accession no. CP011994, NCBI BioProject accession no. PRJNA232800, and BioSample accession no. SAMN03795182.