A new article is about to be published in the Journal of Bacteriology, describing a whole genome analysis of the response of Prochlorococcus marinus MED4 to different light conditions (light quantities and wavelengths). The microarray analysis reveals that this phototroph probably uses the same light sensing mechanism to respond to high-intensity white and blue light. This enables it to thrive over a wide range of depths, up to 200 meters where only blue part of the light spectrum can be utilised for photosynthesis. The response involves differential gene expression of genes possibly encoding bacterial chryptochromes and alterations of the redox state of the electron transport chain.
Microarrays is a really great technique to investigate environmental response of cyanobacteria. It's huge importance emerges when investigating organisms like Prochlorococcus, which is a dominating photosynthetic organism in huge areas of the open ocean, contributing up to 50% of the primary production in the oligotrophic gyres of the open ocean between 40..N and 40..S). In addition, MED4 has the smallest genome of a free-living photoautotroph (1716 genes), a third of which have not been functionally characterized, which makes it easier and more interesting to analyse, assuming that during the billions of years of evolution, the redundant genetic material was shed, turning MED4 into a perfectly adapted organism to low-nutrient environments, such as oligotrophic ocean.
Of course, my interest is elicited by the fact that MED4 is also a model organism in my research ;).
Prochlorococcus MED4 has, with a total of only 1716 annotated protein-coding genes, the most compact genome of a free-living photoautotroph. Although light quality and quantity play an important role in regulating the growth rate of this organism in its natural habitat, the majority of known light-sensing proteins are absent from its genome. To explore the potential for light-sensing in this phototroph, we measured its global gene expression pattern in response to different light qualities and quantities using high density Affymetrix microarrays. Though seven different conditions were tested, only blue light elicited a strong response. In addition, hierarchical clustering revealed that the response to high white light and blue light was very similar, and different from that of the lower intensity of white light, suggesting that the actual sensing of high light is mediated via a blue light receptor. Bacterial cryptochromes seem to be good candidates for the blue light sensors. The existence of a signaling pathway for the redox state of the photosynthetic electron transport chain was suggested by the presence of genes that responded similarly to red and blue light, as well as genes that responded to the addition of DCMU, a specific inhibitor of photosystem II-mediated electron transport.Link (subscription access)