Any living organism that directly harnesses the sun's energy uses one of three types of energy-converting pigments: chlorophyll, which gives plants their green color, bacteriochlorophyll, or retinal pigments, which help form microbial rhodopsins.
Rhodopsins capture solar energy in the ocean's surface and power microbial metabolism at a scale ... [+]
Scientists from Australia, China, Mexico, Spain, the U.K., and the U.S. conducted a new study examining proteorhodopsin concentrations. They sampled across various depths and nutrient concentrations in the eastern Atlantic Ocean and Mediterranean Sea.
Proteorhodopsins are molecular pumps powered by light. Microbes use these pumps to generate energy-carrying molecules or uptake nutrients and resources. The vast majority of marine bacteria carry genes for proteorhodopsins (other rhodopsin types are found in just a fraction of bacteria). Proteorhodopsins are referred to as rhodopsins here for simplicity. Moreover, microbial rhodopsins are different from the rhodopsins you might have heard of before—and actually carry in your eye. The rhodopsins used by microbes are different than the ones encoded in your genes.
Cyanobacteria can use chlorophyll (pictured) or other pigments to produce energy from sunlight.
The researchers found where there are less organic nutrients in the ocean, there is more rhodopsin-based energy production. There also tends to be less chlorophyll-based energy production. In addition, the scientists found that the rhodopsin amount in each cell was an order of magnitude greater than previously thought.
On a cell-by-cell basis, energy yields from rhodopsin were less than yields from chlorophyll, but when scientists accounted for the total number of cells across the depths they sampled, energy capture from rhodopsin-based systems was higher.
The rhodopsins absorb enough light energy to power basic bacterial metabolism in some regions of the eastern Mediterranean Sea. The research findings support the idea that rhodopsins are a major mechanism used to harvest solar energy in the ocean's surface.
Harnessing solar energy using rhodopsin is an important survival mechanism in regions of the ocean where resources are scarce. Nutrient-scarce regions in the ocean are likely to expand under climate change, therefore, rhodopsin will become increasingly important for generating energy in ocean microbes.
