2016-11, Hemp, James, Lücker, Sebastian, Schott, Joachim, Pace, Laura A., Johnson, Jena E., Schink, Bernhard, Daims, Holger, Fischer, Woodward W.

Oxygenic photosynthesis evolved from anoxygenic ancestors before the rise of oxygen ~2.32 billion years ago; however, little is known about this transition. A high redox potential reaction center is a prerequisite for the evolution of the water-oxidizing complex of photosystem II. Therefore, it is likely that high-potential phototrophy originally evolved to oxidize alternative electron donors that utilized simpler redox chemistry, such as nitrite or Mn. To determine whether nitrite could have had a role in the transition to high-potential phototrophy, we sequenced and analyzed the genome of Thiocapsa KS1, a Gammaproteobacteria capable of anoxygenic phototrophic nitrite oxidation. The genome revealed a high metabolic flexibility, which likely allows Thiocapsa KS1 to colonize a great variety of habitats and to persist under fluctuating environmental conditions. We demonstrate that Thiocapsa KS1 does not utilize a high-potential reaction center for phototrophic nitrite oxidation, which suggests that this type of phototrophic nitrite oxidation did not drive the evolution of high-potential phototrophy. In addition, phylogenetic and biochemical analyses of the nitrite oxidoreductase (NXR) from Thiocapsa KS1 illuminate a complex evolutionary history of nitrite oxidation. Our results indicate that the NXR in Thiocapsa originates from a different nitrate reductase clade than the NXRs in chemolithotrophic nitrite oxidizers, suggesting that multiple evolutionary trajectories led to modern nitrite-oxidizing bacteria.

2007, Griffin, Benjamin M., Schott, Joachim, Schink, Bernhard

We report a previously unknown process in which anoxygenic phototrophic bacteria use nitrite as an electron donor for photosynthesis. We isolated a purple sulfur bacterium 98% identical to Thiocapsa species that stoichiometrically oxidizes nitrite to nitrate in the light. Growth and nitrate production strictly depended on both light and nitrite. This is the first known microbial mechanism for the stoichiometric oxidation of nitrite to nitrate in the absence of oxygen and the only known photosynthetic oxidation in the nitrogen cycle. This work demonstrates nitrite as the highest-potential electron donor for anoxygenic photosynthesis known so far.

2011, Schott, Joachim

This thesis describes the novel process of anaerobic oxidation of nitrite to nitrate performed by phototrophic bacteria and its qualitative and, in parts, its quantitative distribution in the environment. Bicarbonate-buffered enrichment cultures which had 1 mM nitrite as sole electron donor, were obtaioned from many freshwater and some saltwater sites. In these cultures, nitrite was almost stoichiometrically oxidized to nitrate with concomitant increase in optical density in the light. Quantitative measurements of three sampling sites via the MPN-method revealed cell densities of 104 cells per ml in activated sewage sludge whereas sediments of Lake Constance and sediments of the slightly acidic lake Dingelsdorfer Ried contained substantially less cells per ml. Also in nitrite oxidation, enrichment cultures from activated sewage sludge were the most active ones, from which two morphological different bacterial strains could be isolated: strain KS1 and strain LQ17.

Both strains oxidized nitrite to nitrate anaerobically in the light with concomitant biomass formation. Without light, no growth or nitrite oxidation was detectable.
While strain LQ17 oxidized 1 mM nitrite incompletely to 0.6 mM nitrate within three months, strain KS1 oxidized nitrite stoichiometrically to nitrate within few days. If these strains were fed with nitrite at concentrations higher than 1.5 mM, the lag phase increased and growth was slowed down, and at concentrations above 4 mM no nitrite oxidation was observed and the OD of the cultured decreased permanently. Cultivation of strain KS1 in molybdenum-free medium with nitrite as sole electron donor revealed no nitrite oxidation or growth unless molybdenum (300 nM) was added. With organic electron donors in darkness, no anaerobic growth was observed with both strains, neither with nitrate nor with sulfate as alternative electron acceptor, whereas both strains were able to utilize organic substrates under air. When grown phototrophically, both strains utilized many organic and some inorganic substrates, and further physiological experiments such as, e.g., utilized nitrogen or sulfur sources or the in-vivo absorption spectra together with 16S rRNA gene analyses allowed to assign strain LQ17 to the genus Rhodopseudomonas and of strain KS1 to the genus Thiocapsa. Of already isolated strains, the two Thiocapsa roseopersicina strains DSM221 and DSM217 were also able to oxidize nitrite stoichiometrically to nitrate.

When grown with nitrite as sole electron donor, cell-free extracts of strain KS1 exhibited no nitrite oxidase but a specific nitrate reductase activity of more than 1 U per mg protein. Comparison with cell-free extracts of strain KS1 grown with fructose as e-donor and nitrate as N-source exhibited only few mU per mg protein. Subsequent SDS-PAGE analysis revealed two protein bands of 130-150 kDa and 55-60 kDa, which were strongly expressed specifically after growth with nitrite, and resembled the α- and β-subunit of the membrane-bound nitrate reductase.

2010, Schott, Joachim, Griffin, Benjamin M., Schink, Bernhard

In anaerobic enrichment cultures for phototrophic nitrite-oxidizing bacteria from different freshwater sites, two different cell types, i.e. non-motile cocci and motile, rod-shaped bacteria, always outnumbered all other bacteria. Most-probable-number (MPN) dilution series with samples from two freshwater sites yielded only low numbers (≤ 3x10 3 cm 3) of phototrophic nitrite oxidizers. Slightly higher numbers (about 10 4 cm 3) were found in activated sewage sludge. Anaerobic phototrophic oxidation of nitrite was studied with two different isolates, the phototrophic sulfur bacterium strain KS1 and the purple nonsulfur bacterium strain LQ17, both of which were isolated from activated sludge collected from the municipal sewage treatment plant in Konstanz, Germany. Strain KS1 converted 1 mM nitrite stoichiometrically to nitrate with concomitant formation of cell matter within 2 3 days, whereas strain LQ17 oxidized only up to 60% of the given nitrite to nitrate within several months with the concomitant formation of cell biomass. Nitrite oxidation to nitrate was strictly light-dependent and required the presence of molybdenum in the medium. Nitrite was oxidized in both the presence and absence of oxygen. Nitrite inhibited growth at concentrations higher than 2 mM. Hydroxylamine and hydrazine were found to be toxic to the phototrophs in the range 5 50 μM and did not stimulate phototrophic growth. Based on morphology, substrate-utilization pattern, in vivo absorption spectra, and 16S rRNA gene sequence similarity, strain KS1 was assigned to the genus Thiocapsa and strain LQ17 to the genus Rhodopseudomonas. Also, Thiocapsa roseopersicina strains DSM 217 and DSM 221 were found to oxidize nitrite to nitrate with concomitant growth. We conclude that the ability to use nitrite phototrophically as electron donor is widespread in nature, but low MPN counts indicate that its contribution to nitrite oxidation in the studied habitats is rather limited.