Proteorhodopsin Promotes Anaerobic Growth.

Proteorhodopsin is a simple light driven pump, used by microorganisms within the surface waters of the ocean to generate additional ATP, without carbon and other typical ATP generation processes. This means the light driven pump could produce ATP under anaerobic, energy-starved conditions. This is exactly what Wang and his colleagues investigated in 2015. The ability to generate ATP by proteorhodopsin under anaerobic conditions has useful applications in bio-chemical production, such as the production of succinate and 1,4-butanediol. Anaerobic pathways theoretically have higher yields than aerobic at lower costs, but will often have lower biomass yields, usually attributed to less available ATP. If there was a way to obtain higher biomass because of higher ATP generation under anaerobic conditions, it would be much cheaper. Until Wang et al. (2015) it was not known if proteorhodopsin could produce enough ATP under anaerobic conditions to promote cell growth.

Wang et al. inserted the SAR86 proteorhodopsin gene into an Escherichia coli strain, prevented respiration by inhibiting a cyctochrome (hemD) and measured growth along with ATP production. They found 50% more ATP production when exposed to light than those kept in the dark, demonstrating that photophosphorylation can and does occur. To prove that anaerobic ATP production can produce enough energy to fuel growth they used lactate as a carbon source and found 10% more growth was exhibited in the first three hours, which positively correlated with ATP production. Thus, proteorhodopsin can produce enough ATP to allow growth under anaerobic conditions.

Although growth did occur, it was only for a limited window. This may have been because of a limited number of proteorhodopsin molecules, compared to ATP synthase. Thus, may not be quite as useful for bio-chemical applications as Wang et al. made out. Also, only one strain of E. coli was used and of three strains of proteorhodopsin genes , the SAR86 proteorhodopsin gene was the only one to be successful in producing enough ATP to promote cell growth. However, the paper is accessible and gives experimental limitations with explanations, such as not using Sodium Azide because it decomposes under light and can kill E. coli at high concentrations; a useful feature for others who wish to further study this idea.

The promotion of growth in E. coli under anaerobic, energy-starving conditions, may provide a potential explanation as to why the proteorhodopsin suite of genes is so ubiquitous among microbes, though not all experience these conditions.

Wang, Y., Li, Y., Xu, T., Shi, Z., & Wu, Q. (2015). Experimental evidence for growth advantage and metabolic shift stimulated by photophosphorylation of proteorhodopsin expressed in Escherichia coli at anaerobic condition. Biotechnology and bioengineering, 112(5), 947-956.