The Effects of Light-Dark Cycles on the Metabolism of Cyanothece sp. ATCC 51142 and Cyanothece sp. ATCC 7822

Sarah Ann Griffith, Purdue University

Abstract

Cyanothece spp. are unicellular, diazotrophic cyanobacteria that perform photosynthesis in the light and fix N2 during the dark. During N2 fixation, it produces H2, a promising candidate for bio-fuel usage. Cyanothece sp. ATCC 51142 (Cyanothece 51142) displays strong temporal regulation when grown under twelve hours of light and twelve hours of dark (12hL:12hD), but it is unknown how this strain responds when grown under 16hL:8hD, and 8hL:16hD. The effects of light-dark cycles on Cyanothece sp. ATCC 7822 (Cyanothece 7822) have not been detailed, and this study aims to identify what metabolic changes occur when both strains are grown under varying light-dark conditions. An initial hypothesis stated that, by altering the exposure to light-dark, the cultures would still maintain circadian patterns. A second hypothesis was that 16hL:8hD conditions would yield higher H2 production than 8hL:16hD and 12hL:12hD conditions. This hypothesis was based on the proposition that shorter days would produce less reducing power (NADPH) and energy (ATP) through photosynthesis, and that less glycogen would accumulate. As a result, there would be less substrate for respiration and not as much O2 would be consumed in the cytoplasm. In addition, less ATP would be available for nitrogenase activity, and less H2 would be produced. Similarly, long days would yield more reducing power and energy, more glycogen would accumulate, less O2 would be available to poison nitrogenase, and more H2 would be produced. Samples were collected every two to four hours for three days to evaluate the H2 production (using a GC), nitrogenase activity (using a GC), photosynthetic O2 evolution rates (using a Clark electrode), cellular respiration rates (using a Clark electrode), and cellular glycogen concentrations (through biochemical methods). We determined that, Cyanothece 7822 and 51142 could retain circadian patterns for nitrogenase and H2 production, except when grown in 8hL:16hD. Cyanothece 51142 produced the most H2 under 16hL:8hD, whereas Cyanothece 7822 produced the least amount of H2 under 16hL:8hD. We suggest that for Cyanothece 7822, under 16hL:8hD, the shortened time in the dark was not enough time for respiration to consume the excess O2 produced in the extended light period. As a result, nitrogenase was not as active and H2 production was lowered. Statistical analysis showed that the amount of glycogen stored in the light, the nitrogenase activity and the rate of H2 production were statistically different amongst the two strains, whereas the actual timings of the peaks were not significantly different from strain to strain. We conclude that when Cyanothece 7822 and 51142 were grown in short light periods, the regulation shifted from a circadian pattern. However, as shown previously, for 12hL:12hD, Cyanothece 51142 produced more H2 under all three periodicities than Cyanothece 7822.

Degree

M.S.

Advisors

Sherman, Purdue University.

Subject Area

Microbiology

COinS