Though humans, along with other vertebrate and invertebrate organisms, don’t photosynthesize, we’re definitely the downstream beneficiaries of the life forms that do. Phototrophic organisms at the bottom of the food chain convert abundant sunlight into the energy that ultimately powers all other life.

The two metabolic systems for harvesting light energy are fundamentally different. The most familiar is the chlorophyll-based photosynthesis by which plant life uses light to power the conversion of carbon dioxide and water into sugars and starches; the other system consists of proton-pumping rhodopsins.

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

Generally speaking, species tend to pick one or the other metabolic system, the PC/Mac dichotomy of phototrophic organisms. However, a multi-institutional team of molecular biologists now reports finding an alpine lake bacterium that uses both bacteriochlorophyll-based photosynthetic complexes and proton-pumping rhodopsins. Their study is published in PNAS.

Based on flash photolysis measurements, the authors report that both systems are photochemically active in Sphingomonas glacialis AAP5, found in the alpine lake Gossenköllesee, located in the Tyrolean Alps. Specifically, in low-light conditions between 4 and 22 degrees Celsius, the bacterium expresses bacteriochlorophyll, and in light conditions at temperatures below 16 degrees Ce