Scientists describe variation in oceanic bacterial photopigments that convert light into biochemical energy

MOSS LANDING, California—Monterey Bay Aquarium Research Institute (MBARI) microbiologists report in the 14 June 2001 issue of the journal Nature the discovery of the widespread occurrence and depth-specific adaptation of a new energy-generating, light-absorbing pigment, proteorhodopsin. Last fall in the journal Science, MBARI researchers described the discovery of the first marine bacterium with this photopigment that can generate cellular energy using light; however, the function of those microbes in the ocean environment remained a mystery.

“Advances in technology are letting us view the marine microbial world in new ways," said Ed DeLong, leader of the research group. First author of the Nature paper, Oded Béjà, adds, "We were lucky to find these different proteorhodopsins out there in the vast ocean. The diversity in the field is probably much greater."

In the more recent study, samples of oceanic bacteria collected from Monterey Bay, Antarctica, and Hawaii were analyzed for the presence of active photopigment. In collaboration with John and Elena Spudich from the University of Texas Medical School, the group used laser flash spectroscopic techniques on naturally occurring marine microbes to search for the new photochemical activity in oceanic waters. The scientists observed chemical activity stimulated by light flashes in native marine microbes, similar to the activity seen in earlier laboratory studies of proteorhodopsin and bacteriorhodopsin. These observations showed that the microbes and active photopigment were present in abundance at the ocean’s surface.

The researchers also showed that genetic variants of the photoactive microbes contain different proteorhodopsins in different ocean habitats. The protein pigments appear to be tuned to absorb light of different wavelengths that match the quality of light available in different environments. Specific adaptations in the photopigment structure have optimized different variants functioning best at different depths in the water column.

DeLong and his colleagues are excited by the implications of this research for two main reasons. First, the study takes the initial laboratory observations out to the ocean, showing how common and widespread this photopigment is throughout the world’s oceans. In addition, the concentration of the photopigment suggests that it has the capacity to generate a significant amount of energy for oceanic microbes. DeLong also notes the variations observed in the photopigment suggest its importance not only at the ocean’s surface, but throughout the photic zone. This new study further supports the notion that the microbes containing the rhodopsin-like photopigments significantly impact carbon and energy cycles in the world’s oceans.

Note to media: Images available on request.