It is estimated that marine microscopic algae – phytoplankton – are responsible for half of the Earth’s photosynthesis. In 2000, American scientists discovered that many marine bacteria contain a gene in their genome that encodes a new kind of light-harvesting pigment: proteorhodopsin. Proteorhodopsin is related to the pigment in the retina that enables human vision in less intense light. As many as half of the surface ocean bacteria have proteorhodopsins, which are membrane proteins that allow harvesting of energy from sunlight, implying a potentially significant role of non–chlorophyll-based phototrophy in oceanic carbon cycling and energy flows. Evidence of specific roles for proteorhodopsins in marine bacteria and the marine environment remains surprisingly scarce. One reason for this is the lack of marine bacteria (containing proteorhodopsin genes) that can be maintained in laboratory culture and that are tractable to genetic manipulation. Now, a decade later, the first direct evidence for the functioning of proteorhodopsin in native marine bacteria is published, based on mutational analysis in a marine bacterium.
A new study shows that a proteorhodopsin-containing member of the widespread marine genus Vibrio displays light-enhanced survival during starvation in seawater. At the same time the new study shows that proteorhodopsin-mediated phototrophy (the process of acquiring energy from light) allows marine bacteria to better survive periods of starvation in an often nutrient-depleted ocean. Growth recovery experiments showed that bacteria starving in the light could more rapidly respond to improved growth conditions than those incubated in the dark. A proteorhodopsin deficient Vibrio strain confirms that light-dependent survival of starvation is mediated by the proteorhodopsin. Proteorhodopsin phototrophy thus provides a physiological mechanism that allows surface ocean bacteria to manage an environment where resource availability fluctuates markedly.
The importance of understanding novel mechanisms for marine bacteria to efficiently use solar energy is obvious if one considers that a litre of seawater on average contains around a billion bacteria, many of which contain proteorhodopsin. The activity of these bacteria play a crucial role in the global carbon cycle by determining oceanic production of CO2 through respiration and determining how the fluxes of energy that are fixed by photosynthesis are channeled through marine food chains. Bacteria at the ocean surface are swimming in a sea of light, and it may not be all that surprising that evolution has favored microorganisms that can use this abundant energy source most efficiently.
Proteorhodopsin Phototrophy Promotes Survival of Marine Bacteria during Starvation. PLoS Biol 8(4): e1000358. doi:10.1371/journal.pbio.1000358
Proteorhodopsins are globally abundant photoproteins found in bacteria in the photic zone of the ocean. Although their function as proton pumps with energy-yielding potential has been demonstrated, the ecological role of proteorhodopsins remains largely unexplored. Here, we report the presence and function of proteorhodopsin in a member of the widespread genus Vibrio, uncovered through whole-genome analysis. Phylogenetic analysis suggests that the Vibrio strain AND4 obtained proteorhodopsin through lateral gene transfer, which could have modified the ecology of this marine bacterium. We demonstrate an increased long-term survival of AND4 when starved in seawater exposed to light rather than held in darkness. Furthermore, mutational analysis provides the first direct evidence, to our knowledge, linking the proteorhodopsin gene and its biological function in marine bacteria. Thus, proteorhodopsin phototrophy confers a fitness advantage to marine bacteria, representing a novel mechanism for bacterioplankton to endure frequent periods of resource deprivation at the ocean’s surface.