page_title: Marine microorganisms
section_title: Microbial rhodopsin
hierarchical_section_title: Marine microorganisms / Marine microplankton / Phytoplankton / Microbial rhodopsin
caption_attribution_description: English: Model of the energy generating mechanism in marine bacteria. When sunlight strikes a rhodopsin molecule (1), it changes its configuration such that a proton is expelled from the cell (2). The chemical potential causes the proton to flow back into the cell (3), thus generating energy (4) in the form of adenosine triphosphate (5).
context_page_description: Marine microorganisms are defined by their habitat as the microorganisms living in a marine environment, that is, in the saltwater of a sea or ocean or the brackish water of a coastal estuary. A microorganism is any microscopic living organism, that is, any life form too small for the naked human eye to really see, needing a microscope. Microorganisms are very diverse. They can be single-celled or multicellular and include all bacteria and archaea and most protozoa, as well as some species of fungi, algae, and certain microscopic animals, such as rotifers and copepods. Many macroscopic animals and plants have microscopic juvenile stages. Some microbiologists also classify biologically active entities such as viruses and viroids as microorganisms, but others consider these as nonliving.
  Marine microorganisms make up about 70% of the biomass in the ocean. Taken together they form the marine microbiome. Over billions of years this microbiome has evolved many life styles and adaptations and come to participate in the global cycling of almost all chemical elements. Microorganisms are crucial to nutrient recycling in ecosystems as they act as decomposers.
context_section_description: Phototrophic metabolism relies on one of three energy-converting pigments: chlorophyll, bacteriochlorophyll, and retinal. Retinal is the chromophore found in rhodopsins. The significance of chlorophyll in converting light energy has been written about for decades, but phototrophy based on retinal pigments is just beginning to be studied.
  In 2000 a team of microbiologists led by Edward DeLong made a crucial discovery in the understanding of the marine carbon and energy cycles. They discovered a gene in several species of bacteria responsible for production of the protein rhodopsin, previously unheard of in bacteria. These proteins found in the cell membranes are capable of converting light energy to biochemical energy due to a change in configuration of the rhodopsin molecule as sunlight strikes it, causing the pumping of a proton from inside out and a subsequent inflow that generates the energy. The archaeal-like rhodopsins have subsequently been found among different taxa, protists as well as in bacteria and archaea, though they are rare in complex multicellular organisms.
  Research in 2019 shows these "sun-snatching bacteria" are more widespread than previously thought and could change how oceans are affected by global warming. "The findings break from the traditional interpretation of marine ecology found in textbooks, which states that nearly all sunlight in the ocean is captured by chlorophyll in algae. Instead, rhodopsin-equipped bacteria function like hybrid cars, powered by organic matter when available — as most bacteria are — and by sunlight when nutrients are scarce."
  There is an astrobiological conjecture called the Purple Earth hypothesis which surmises that original life forms on Earth were retinal-based rather than chlorophyll-based, which would have made the Earth appear purple instead of green.
caption: Model of the energy generating mechanism in marine bacteria      (1) When sunlight strikes a rhodopsin molecule
        (2) it changes its configuration so a proton is expelled from the cell
        (3) the chemical potential causes the proton to flow back to the cell
        (4) thus generating energy
        (5) in the form of adenosine triphosphate.[166]
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