Evidence of Phototrophy in Vibrio sp. by Expression of Proteorhodopsin: Starvation Response

Evidence of Phototrophy in Vibrio sp. by Expression of Proteorhodopsin: Starvation Response

The class of proteins known as the rhodopsins are the simplest light harvesting pigments, originally isolated from Archea as bacteriorhodopsin. These membrane embedded proteins produce an electrochemical gradient across the membrane when activated by light, where proton a proton gradient drives ATP synthesis. The discovery of proteorhodopsin among planktonic bacteria lead to the isolation of the gene cluster from the gammeoproteobacteria linage SAR 86 and transfer to heterologous bacteria E.coli to reveal light driven phototrophy similar to that of the Archeal protein bacterial rhodopsin. Further studies revealed the ubiquitous presence of proteorhodopsin among other marine bacteria, now thought to be present in 13-80% of bacteria. An inability to culture many marine bacteria meant that functioning of proteorhdopsin was only experimentally possible in E.coli until whole-genome-sequencing revealed the presence of proteorhodopsin in several culturable marine bacteria, including a Vibrio.

The is great significance to this investigation by Gomez-Consarnau et al (2010). Previously Vibrio’s have been described as organoheterotrophs, gaining nutrition from detritus or as opportunistic parasites. This investigation describes the functioning of proteorhodopsin in-situ by several mechanisms showing that vibrio’s enhance their survivability during periods of starvation by the expression of proteorhodopsin induced by light. Phylogenetic analysis has suggested that the lateral transfer of genes may be responsible for the presence of proteorhodopsin in Vibrio ADN4. It is also suggested that the presence of Leucine at position 105 modifies the the proteorhodopsin protein to absorb light maximally in the green part of the electromagnetic spectrum.

Gomez-Consarneu and colleges first measured growth and survival of  Vibrio Strain AND4 in light and dark regimes. No difference in cell yields was observed between light and ark regimes grown on a rich media. When AND4, grown on rich media, was transferred to natural seawater with low concentrations of nutrients reductive division was observed, characteristic of the starvation response in vibrio’s. The important value is that after 10 days starvation the culture in light was 2.5x higher than the culture in the dark.  

The optical densities were also measured in differing light regimes. In the dark the OD dropped off rapidly, but in the light the decrease was much less pronounced. After 7-13 days after starvation OD was 40-60% higher in light compared to dark. Further investigation took place to determine if it was light that was inducing the enhanced survivability of AND4 by producing a mutant deficient in the proteorhodopsin gene cluster by in-frame deletion. In line with prediction, the deficient mutant showed no significant survivability compared to control groups. Recovery time was also investigated with starved bacteria grown on rich-media. The results indicated that the wild-type AND4 grew 3-6 fold  faster after 5 day t compaerd the dark incubated bacteria.

This investigation adds a new dimension to our understanding of phototrophy in the ocean and what we thought we knew about the Vibrios. The functioning of proteorhodopsin was also been investigated by Wang et al (2012) in Vibrio campbellii , interestingly the results showed that continuous illumination actually decreased bacterial yields which the author suggests could be the result of photodegradation of cellular components or, the activation of lytic phase of embedded in the genome. Another possibility for the differences observed in cell growth under illumination between Gomez-Conarneu et al (2010) and Wang et al (2012) could be the luminescence used in the experiments. Gomez-Conarneu et al (2010) used luminescence values for continuous light of , 133 and 150 µmol photons m⁻² s⁻¹ whereas the luminescence used by Wang et al (2012) was considerable lower 42 µmol photons s⁻¹ m⁻².  

Gomez-Consarnau. L, Neelam.  A, Kristoffer. L, Pederson. A, Neutez. R, Milton. D. L, Gonzalez J. M, Pinhasi. J., (2012). Proteorhodopsin Phototrophy Promotes Survival of Marine Bacteria During Starvation. PLOoS Biol 8(4): e1000358. doi:10.1371/journal.pbio.1000358.  

http://www.plosbiology.org/article/info%3Adoi%2F10.1371%2Fjournal.pbio.1000358

Wang Z, O'Shaughnessy TJ, Soto CM, Rahbar AM, Robertson KL, et al. (2012) Function and Regulation of Vibrio campbellii Proteorhodopsin: Acquired Phototrophy in a Classical Organoheterotroph. PLoS ONE 7(6): e38749. doi:10.1371/journal.pone.0038749

http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0038749