Blue-green algae source sugar from the oceans

English: Stromatolites growing in Hamelin Pool...
Stromatolites – ancient cyanobacteria – in Shark Bay, Western Australia. (Photo credit: Wikipedia)

Cyanobacteria – commonly referred to by the misnomer ‘blue-green algae’ – are photoautotrophs, which is to say that they have the nifty ability to make their own food from sunlight. In fact, their predecessors were probably some of the first organisms on Earth to master this feat.

The ability to use light energy from the sun to power reactions that turn carbon dioxide and water into sugars (a process known as photosynthesis) assured these tiny organisms a permanent role on the evolutionary stage. At 3.5 billion years old, stromatolites – the clumpy, fossilised remains of ancient cyanobacteria – are some of the oldest records of life that we have.

Today, cyanobacteria are a diverse bunch. They can be found everywhere from rainforests to deserts, Antarctic rocks to hot springs, freshwater rivers to the open ocean, and together, their contribution to global photosynthetic output is a whopping 20-30%. When you consider that photosynthesis in plants and green algae is the result of an ancient cyanobacterium having been engulfed long ago to form the photosynthesising chloroplast, cyanobacteria have certainly made their mark on our blue-green planet.

But apparently, photosynthesis is not the only way that cyanobacteria can source their food. A team of Spanish and UK scientists have found that Prochlorococcus, the most abundant marine cyanobacterium, is able to scavenge glucose directly from their surrounds. The team have previously shown that Prochlorococcus  takes up glucose when grown in the lab, but with no predicted glucose transporters annotated in the Prochlorococcus genome, the scientists were curious to discover how the cyanobacterium conducted its scavenging operations.

The team discovered that a single gene, dubbed Pro1404, is responsible. The gene was originally assumed to code for a protein that transports melibiose (a two-sugar molecule of glucose and galactose) in the presence of sodium. But when grown in the presence of glucose, Prochlorococcus ramps up expression of the Pro1404 gene, suggesting a role in glucose transport. Confirmation of the role of Pro1404 in glucose transport came from an experiment where the scientists genetically engineered a strain of cyanobacterium incapable of glucose uptake – Synechoccuc elongatus strain PCC 7942 – to contain the Pro1404 gene. The addition of the Pro1404 gene turned this normally sugar-shunning strain into a glucose leech.

Far from being a simple quirk of life in the laboratory, the scientists were also able to show that Prochlorococcus utilises glucose in its native environment. Despite glucose concentrations in the Atlantic Ocean being in the range of 0.5- 2.7 nM (roughly 90 – 480 parts per billion), the team were able to detect glucose uptake in Prochlorococcus sampled from three locations in the Atlantic Ocean.

Equipped with the efficient photosynthetic machinery that blue-green algae are known for, as well as a specific molecular apparatus for glucose uptake, Prochlorococcus can tailor its diet with the changing tides. If the oceans bring a sudden influx of glucose, they dine on the fruits of someone else’s labour; when glucose is scarce, they turn to the sun and make their own.

Reference: Muñoz-Marín, Luque, Zubkov, Hill, Diez & García-Fernández. 2013. Prochlorococcus can use the Pro1404 transporter to take up glucose at nanomolar concentrations in the Atlantic Ocean. Proceedings of the National Academy of Sciences USA.

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