Belt and Suspenders: Alpine Lake Bacteria Deploy 2 Light-Harvesting Systems

Though human beings, along with other vertebrate and invertebrate organisms, do not photosynthesize, we’re certainly the downstream beneficiaries of the life forms that do. Phototrophic organisms at the bottom of the food chain convert plentiful sunlight into the energy that ultimately powers all other life.

The 2 metabolic systems for harvesting light energy are fundamentally different. The most acquainted is the chlorophyll-based photosynthesis, by which plant life utilizes light to power the conversion of co2 and water into sugars and also starches; the other system consists of proton-pumping rhodopsins.

Microbial rhodopsins, retinal-binding proteins, offer ion transport driven by light (and, incidentally, sensory functions). It is a family that includes light-driven proton pumps, ion pumps, ion channels, and light sensors. Microbial rhodopsins are discovered in archaea, bacteria, and Eukaryota and are widespread in oceans and freshwater lakes.

Generally speaking, varieties tend to choose one or the other metabolic system, the PC/Mac dichotomy of phototrophic organisms. Nevertheless, a multi-institutional group of molecular biologists currently reports finding an alpine lake bacterium that utilizes both bacteriochlorophyll-based photosynthetic complexes and proton-pumping rhodopsins. Their research is published in PNAS.

Based on flash photolysis measurements, the writers report that both systems are photochemically active in Sphingomonas glacialis AAP5, discovered in the alpine lake Gossenköllesee, located in the Tyrolean Alps. Specifically, in low-light conditions in between four and 22 degrees Celsius, the bacterium expresses bacteriochlorophyll, and also, in light conditions at temperatures below sixty degrees Celsius, describes xanthorhodopsin, a proton pump.

The organisms S. glacialis

S. glacialis utilizes harvested light to synthesize ATP and to stimulate development. The authors write, “This recommends that the use of 2 systems for light harvesting might represent an evolutionary adjustment to the specific environmental conditions discovered in alpine lakes and other analogous ecosystems,” namely a response to large seasonal temperature and light changes.

As the authors note, bacteriochlorophyll-based systems are big, complex, and pigment-driven, needing complex molecular machinery for synthesis, assembly, and regulation. However, once assembled, they comprise a “set-it-and-forget-it” system that works even under low-light conditions. Rhodopsins, on the other hand, are far easier and less expensive to express; their disadvantage is that they are just assembled and function in the presence of greater irradiance levels.

The Photoheterotrophs

Loaded with all the genetic hardware for both chlorophototrophy and retinalphototrophy, these photoheterotrophic little guys have a decreased requirement for aerobic respiration and can therefore use available carbon for development, a scarce commodity in the alpine lake environment they call home.

Wondering regarding the presence of similarly adaptable “belt-and-suspenders” organisms in other environments with big seasonal temperature modifications and fluctuations in light availability, the scientists surveyed 215,874 bacterial genomes, identifying both sets of genes in 55 bacteria; almost half came from alpine environments. They note that one species was recently recognized in Yellowstone springs, a vastly distinct physicochemical environment; however, another in which environmental extremes have a high delta.

Bacteriochlorophyll systems are moved primarily vertically; nevertheless, rhodopsin genes are inexpensively and typically acquired horizontally. Thus, the authors compose, “this process might have happened repeatedly during development. However, whether these species retain and also express the obtained rhodopsin gene will depend upon the recent genes offering a competitive advantage in a particular environment. Therefore, dual phototrophy may also be advantageous in other environments with extremely dynamic physicochemical conditions with extremes favoring one system over the other.”

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