Light-activated, -inhibited and -independent denitrification by a denitrifying phototrophic bacterium

Connectivity of the intracytoplasmic membrane of Rhodobacter sphaeroides: a functional approach

The photosynthetic apparatus in the bacterium Rhodobacter sphaeroides is mostly present in intracytoplasmic membrane invaginations. It has long been debated whether these invaginations remain in topological continuity with the cytoplasmic membrane, or form isolated chromatophore vesicles. This issue is revisited here by functional approaches. The ionophore gramicidin was used as a probe of the relative size of the electro-osmotic units in isolated chromatophores, spheroplasts, or intact cells. The decay of the membrane potential was monitored from the electrochromic shift of carotenoids. The half-time of the decay induced by a single channel in intact cells was about 6 ms, thus three orders of magnitude slower than in isolated chromatophores. In spheroplasts obtained by lysis of the cell wall, the single channel decay was still slower (~23 ms) and the sensitivity toward the gramicidin concentration was enhanced 1,000-fold with respect to isolated chromatophores. These results indicate that the area of the functional membrane in cells or spheroplasts is about three orders of magnitude larger than that of isolated chromatophores. Intracytoplasmic vesicles, if present, could contribute to at most 10% of the photosynthetic apparatus in intact cells of Rba. sphaeroides. Similar conclusions were obtained from the effect of a ∆pH-induced diffusion potential in intact cells. This caused a large electrochromic response of carotenoids, of similar amplitude as the light-induced change, indicating that most of the system is sensitive to a pH change of the external medium. A single internal membrane and periplasmic space may offer significant advantages concerning renewal of the photosynthetic apparatus and reallocation of the components shared with other bioenergetic pathways.

Removal of phosphorus from oyster farm mud sediment using a photosynthetic bacterium, Rhodobacter sphaeroides IL106

Removal of phosphorous compounds from the mud sediment of an oyster farm was carried out by a series of bio-processes under anaerobic conditions. Anaerobic acidogenic fermentation of a mud sediment suspension (200 g wet wt/l artificial sea water) was initially carried out. With the addition of vitamins such as thiamine, nicotinic acid and biotin, acidogenic fermentation was enhanced to yield acetic acid of approximately 2 g/l. Furthermore, approximately 20 mg/l of PO4(3-) (10% of total phosphorus on mud weight) and 5300 mg/l of COD(Cr) (82% of organic matter on mud weight) were released into the culture broth after fermentation for 7 d. The supernatant of this culture broth was used to cultivate Rhodobacter sphaeroides IL106, a denitrifying photosynthetic bacterium. After 4 d, 3.32 g/l of biomass containing carotenoid and ubiquinone was obtained, and COD(Cr) and acetic acid were reduced by 58% and 72%, respectively. In addition, PO4(3-) was reduced by 97%, suggesting that the removal of PO4(3-) from the mud sediment might be possible by combining anaerobic acidogenic fermentation with R. sphaeroides cultivation.

Light-Mediated Nitrite Accumulation during Denitrification by Pseudomonas sp. Strain JR12

The effect of light on the denitrifying characteristics of a nonphotosynthetic denitrifier, Pseudomonas sp. strain JR12, was examined. Already at low light intensities, nitrite accumulated as a result of light inhibition of nitrite but not of nitrate reduction rates. Exposure of this bacterium to light caused a photooxidation of cytochrome c , an intermediate electron carrier in its respiratory pathway. Photoinhibition of nitrite reduction was reversible, as nitrite reduction rates returned to preillumination levels when light-exposed cells were returned to dark conditions. Antimycin A reversed the inhibitory effect of light on nitrite reduction by preventing a reversed electron flow. Aerobic respiration by this bacterium was not affected by light.

Denitrification and its control

Denitrification in bacteria comprises a series of four reduction reactions; for nitrate, nitrite, nitric oxide and nitrous oxide. Nitrogen gas is the final product. The nature of the enzymes catalysing these reactions is described along with the the properties of the underlying electron transport systems. The factors influencing the expression of the reductases for the four reactions are reviewed along with the effect of oxygen on the activities of the enzymes of denitrification. The main emphasis is on observations made with Paracoccus denitrificans and Pseudomonas stutzeri.

Inhibition of bacteriochlorophyll synthesis in Rhodobacter sphaeroides subsp. denitrificans grown in light under denitrifying conditions

The inclusion of nitrate or nitrite in cultures of Rhodobacter spaeroides subsp. denitrificans grown heterotrophically in light depressed the formation of bacteriochlorophyll a. The pigment biosynthesis was inhibited at the stage of the reduction of chlorophyllide (chlorin) to bacteriochlorophyllide (tetrahydroporphyrin) since 3-hydroxyethylchlorophyllide a accumulated in the culture medium. The addition of exogenous 5-aminolevulinic acid to these cultures resulted in a complete restoration of bacteriochlorophyll synthesis accompanied by the accumulation of 3-vinylbacteriopheophorbide. This indicates that under these conditions bacteriochlorophyll was formed via an alternative route, in which the reduction of chlorins to tetrahydroporphyrins precedes modifications of the C-3 side chain. The multiple forms of 5-aminolevulinic acid synthase were purified from cells grown with and without nitrate. Antibodies against these proteins were raised in rabbits and used in enzyme-linked immunosorbent assays for various forms of 5-aminolevulinic acid synthase. In denitrifying cells, the amount and activity of fraction I of the enzyme was reduced by approximately 40 and 30%, respectively. Partly active enzymes from both types of cells were activated by cystine trisulfide.