A review of: Gómez-Consarnau L, Akram N, Lindell K, Pedersen A, Neutze R, et al. (2010) Proteorhodopsin Phototrophy Promotes Survival of Marine Bacteria during Starvation. PLoS Biol 8(4)
Proteorhodopsins (PR’s) are light driven, membrane bound proton pumps that are found in some marine microbes at the photic level of the ocean. They allow a chemiosmotic gradient to be built up across their membranes resulting in the production of adenosine triphosphate (ATP) and therefore energy, without the need for chlorophyll. The ecological role of PR’s is largely unexplored and so they are being investigated as a contributor to energy flux and carbon cycling within our seas. Their role in bacterial reproduction success has also been investigated with little success. This journal, therefore, focuses on a member of the Vibrio species and investigates how PR’s allow the long term survival and growth of the AND4 strain when starved in seawater exposed to light compared to darkness.
Strain AND4 was isolated from the Andaman Sea and a PR encoding gene was identified from the whole genome sequence. After analysing its 16s rRNA it was found this gene shares a similarity of 87% with the PR encoded genome sequence of the V. haryeyi strain BAA – 1116, putting both within the Gammaproteobacteria group. However, their PR’s were found to cluster with Alphaproteobacteria when it came to similarity. This suggests that the PR genes and its chromophore have been acquired through lateral gene transfer from other, maybe distantly related marine bacteria. Does this mean that the genes do confer a competitive advantage to those which have obtained them? The PR gene has not being found in any other species of vibrio (to the authors knowledge). It was also discovered, from isolates of both Vibrio strains, that the amino acid Leu is found in a position that allows the PR light absorption peak to be concentrated within green light frequencies (absorption frequencies 535nm). This allows Phototrophy to be successful as this is the dominant light condition within sea waters.
To explore whether the PR’s do increase the success of the Vibrios that carry them, their effects on growth and survival was investigated. The AND4 growth experiments on a rich medium showed no difference in cell yields in light (133µmol photons m−2 s−1) or dark conditions. Transfer of these cells to a sterile and particle free natural sea water with low concentrations of nutrients also showed an increase in cells, however biomass did not increase. This means the cells were increasing in number but were smaller in size; this is called reductive division and is a technique shown in many Vibrios when nutrients become scarce. After 10 d of incubation however, although all bacterial numbers decreased, the PR cultures left in light were 2.5 times higher than those in darkness. This strongly suggests that PG phototrophy can improve marine bacteria survival when nutrients are not readily available in seawater.
Experiments completed at different light intensities also back up PR phototrophy as a plausible back – up energy source when there are limited nutrients. The cultures kept at high light intensity, rather than in 16:8h light:dark cycles, had optical densities that were higher by 40 – 60%. Differences between dark cultures and low light cultures were less pronounced but still showed PR’s would help during phases of starvation. Bacterial numbers peaked at day 3 of the experiments but decreased thereafter. However the cultures kept in high light remained nearly twice as high in numbers as all other experiments.
A photorhodopsin deficient Vibrio strain was also generated in this experiment and its growth recovery was compared to the wildtype. After 5 days the wildtype cultures kept at high light during starvation were at 3 -6 fold higher densities than those kept in darkness, there was no difference in density between the PR deficient strains kept in light or dark conditions during starvation.
All experiments completed in this paper suggest strongly that the presence of photorhodopsins and therefore photorhodopsin phototrophy can substantially help to increase life longevity in microbes when other resources are scarce. This is interesting as energy is created with light without the need for chlorophyll, a process that I have not come across before. It would be interesting to see which other microbes contain photorhodopsins and to study further how their presence will affect microbe communities in the future and what affect this could have on our seas.
6 comments:
This is a really good post. I understand what the experiment did and the conclusions. I think some algae species also use photorhodopsin. i wonder if any lateral dene transfer has happened during evolusion of these and if it was by viruses?
I agree with Alice that this is a really good post, I understood what the paper was about and I also liked the end of your blog where you said what other things good be done with the information obtained from this paper.
I agree that this is an interesting topic and it seems that little is known in the area. This topic was identified in a paper that I reviewed from a year earlier as an area in need of further investigation. However, even though it has been suggested as a possible new avenue of enquiry, it seems that interest in the topic is yet to catch up.
Ref. of paper I reviewed: Zubkov, M. V. (2009) Photoheterotrophy in marine prokaryotes. J. Plankton Res. 31: 933-938
In response to Alice's Q., there is definitely strong evidence of lateral gene transfer. Here's a quote from a paper by Frigaard et al. (2006) Nature 439, 847-850. "The genetic simplicity of these photosystems, their ability to assemble and function properly in the membranes of divergent microbial groups, and their potential to contribute to cellular energy metabolism, all are indicative of their likely predisposition for genetic mobility. Apparently, lateral gene dispersal mechanisms, coupled with strong selection for proteorhodopsin in the light, have contributed to the distribution of these photoproteins among various members of all three of life's domains."
Thankyou Colin. But is that gene tansfer between microbes and algae?
Alice - I had forgotten to comment on you earlier statement suggesting that algae had proteorhodopsin. I was going to say this was not true, but I just checked and realised that an amazing paper slipped under my radar a couple of months back. A phagotrophic dinoflagellate has been shown to posess PR and all theindications are that it uses it as a proton pump. All the signs are it has come from bacteria by horizontal gene transfer. The authors conclude by saying "O. marina presents an intriguing mix of feeding strategies that are
the sum of a winding path of evolution: the consummate predator
that has in the course of evolution shed photosynthesis, only to
regain a new version of phototrophy through horizontal gene transfer,
perhaps from bacteria on which its ancestors had fed." This would be a good one to follow up - here's a link to a PDF http://www.botany.ubc.ca/keeling/PDF/11OxyRhodop.pdf
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