Partially solar powered proteobacteria via proteorhodopsin

The conversion of light energy into chemical energy via photosynthesis is a very important role carried out by aquatic ecosystems. However, in the aquatic environment this respiration can be inhibited, whether by oxygen depletion or by a respiratory poison. In 2000, the integral membrane protein proteorhodopsin (PR) was discovered, a protein that is hugely abundant in PR expressing bacteria in the worlds oceans. This protein has been discovered to be a light powered protein pump that helps the bacteria to survive when respiration is inhibited.

Walter et al (2006) tested the bacteria Escherichia coli, which is known to contain PR, to see whether once respiration became impaired, the bacteria could use this light driven proton pump to continue respiring and so combat respiratory inhibition. Respiratory impairment was stimulated by introducing the bacteria to azide, a respiratory poison. The bacteria were then exposed to green light at a number of frequencies.

Cells that were PR positive showed a definite response to the light. When the light was present, there was an increase in cell velocity and once the light was removed the velocity decreased. In the absence of azide light had no effect, showing a direct link between the light and the increased cell velocity. The rate of movement of the cells was found to increase with light intensity. The reason behind the bacteria’s ability to become light powered is due to the light creating a proton motive force, creating the potential to move protons across the membrane.

Therefore we can draw that PR positive bacteria can cope with the issue of respiratory impairment in the environment, a huge advantage in comparison to those bacteria which lack PR. It is also suggested that the PR is spectrally tuned to its environment, meaning that they best suit the light levels that they receive in their environment.

The possible uses of PR are still uncertain. Walter et al suggest that we may be able to synthesise these light powered PR bacteria for uses by expressing the PR and the cells natural ability to generate proton motive force. There are substantial amounts of research still needed in this area to assess whether this is indeed possible.

This study provides incredibly strong evidence that some marine bacteria posess the ability to use light to produce energy when respiration is impaired and suggest that this may in some way be utilized. However it doesn’t suggest in what ways we may be able to use the PR. I believe that there are implications for PR expressing bacteria, possibly in such fields as solar power, though more research into the viability of this is needed.

Walter, J.M., Greenfield, D., Bustamante, C., Liphardt, J. . (2006). Light-powering Escherichia coli with proteorhodopsin. PNAS. 104 2408-2412.