Light versus Dark gene expression in SAR11. Candidatus Pelagibacter ubique HTCC1062

We investigated the transcriptional response of cand. Pelagibacter ubique cultures to light and dark when grown under carbon replete and deplete conditions. Results: During exponential phase, few genes were differentially transcribed between light:dark growth conditions. In stationary phase, a high proportion (9.7%) of coding sequences were found differentially expressed between treatments; 4.7% being up-regulated in the light (n=64) and 4.9% being up-regulated in the dark (n=67). Several components of the oxidative phosphorylation pathway were up-regulated in the dark, supporting physiological data showing higher respiration rates in darkness than in the light under starvation conditions. We also observed up-regulation of a proton translocating pyrophosphate synthase (SAR11_1040), which may be an additional energy production mechanism utilized by SAR11 cells in the dark. Finally, we noted the up-regulation of pili formation genes in the array data, consistent with the observation of pili in dark grown cells via electron microscopy. Overall design: Triplicate biological replicates were collected from five growth conditions (n=15 total): 1) exponential phase, 24 hour light (n=3) 2) exponential phase, 24 hour dark (n=3) 3) stationary phase, 12:12 hour light:dark cycles (n=3) 4) stationary phase, 24 hour dark (n=3) 5) stationary phase, 6 hours after transfer to darkness (n=3)