Proteorhodopsin phototrophy in the ocean

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Proteorhodopsin is a membrane protein that contains retinal; during light absorption it goes through conformational changes that cause protons to be transported across a membrane. This results in an electrochemical gradient across the membrane, which is then utilized in ATP synthesis.

Previously it was thought that this type of “rhodopsin-mediated, light-driven proton pumping” was only present in species of halophilic archaea. The authors provide evidence of proteorhodopsin in the genome of an uncultivated bacterioplankton in Monterey Bay. This was achieved by the use of flash photolysis. The flash-photolysis data provide direct physical evidence for the existence of proteorhodopsin-like transporters and endogenous retinal molecules in the microbial fraction of these coastal surface waters.

Furthermore the paper examines the prevalence, expression and genetic variability of proteorhodopsin in bacterioplankton found in marine surface waters. Comparisons were made from samples collected from three sites; Monterey Bay, the Southern Ocean, Antarctica and waters of the central North Pacific Ocean (Hawaii). Additionally Hawaiian samples were also taken from 75m depths. 

Their results showed 15 variants of proteorhodopsin when compared with the original, within the Monterey Bay site; they fall into three clusters that share at least 97% identity over 248 amino acids. However the Antarctic proteorhodopsin genes were different from those found in Monterey (78% over 248 amino acids). This variance in the amino acid sequence was found across the entire protein. However despite differences in amino acid structure and absorption spectra it was found that Antarctic proteorhodopsin was highly related to that of Monterey bay.

Results from spectral analysis showed that Monterey bay and Hawaii surface samples absorb light from green elements of the spectrum. Whereas those from the Antarctic and Hawaii deep samples used blue, (see diagram).

In conclusion the paper shows that proteorhodopsin is attuned to different light environments or wavelengths at different depths and across vast geographical distribution. There is also variation in proteorhodopsin within the same environment.  

Within the paper there is reference to an “original proteorhodopsin” however it is not always clear as to whether this is in reference to the “original archaeal” rhodopsin, or an isolate from Monterey Bay. Additionally there a numerous references to unpublished data or observations, which makes the conclusions hard to track.

Oded Béjà1,2, Elena N. Spudich2,3, John L. Spudich3, Marion Leclerc1 & Edward F. DeLong1. (14 June 2001). Proteorhodopsin phototrophy in the ocean. Nature. 411 (Letters to Nature), 786-789.

Available at: http://www.nature.com/nature/journal/v411/n6839/full/411786a0.html