When Blue-Green Algae Get a Virus

Half the oxygen in the world is produced not by plants on earth, but by single-cell “plants” that live in the ocean. One of the major groups of these “plants” are the cyanobacteria (also formerly known as “blue-green algae”). Like land plants, cyanobacteria perform photosynthesis: they trap CO₂ to produce oxygen and organic compounds (fats, sugars, etc.). Like any other living organism, sometimes cyanobacteria get infected by viruses. Now a team headed by Professor Debbie Lindell from the Technion Faculty of Biology finds that just like humans, cyanobacteria can experience viral epidemics that significantly affect their population. These findings were published in Nature Microbiology.

Prof. Debbie Lindell

Dr. Michael Carlson, a postdoctoral fellow in Prof. Lindell’s lab, sailed along the Pacific Ocean to study the populations of two common cyanobacteria: Prochlorococcus and Synechococcus. The two genera live in different latitudes: Prochlorococcus lives in warmer but less nutrient-rich waters, while Synechococcus prefers colder and more nutrient-rich latitudes. In the area between, both thrive, creating a hotspot, or “cyanobacteria-city.”

Dr. Michael Carlson

This hotspot, it turns out, is also a hotspot of viral activity. Much like a bustling city sees considerably more viral infection than a remote village, in the “cyanobacteria-city” more cyanobacteria are infected. Thrice more, normally, as Prof. Lindell and Dr. Carlson saw in 2015 and 2016. But when the team arrived at the same location in 2017, they found the Prochlorococcus population in the hotspot significantly reduced and displaying 10 times more infection than normal. In 2017, the Prochlorococcus population declined at 17^oC, when normally these cyanobacteria are comfortable at temperatures as low as 12^oC. The Prochlorococcus, in short, suffered a viral outbreak, causing a high percentage of them to die.

Until now, it had not been known that viral infection could have such a dramatic effect on cyanobacterial populations. It was known that viruses infected and killed cyanobacteria, certainly. But among the other factors affecting cyanobacterial population size, such as being eaten by bigger organisms, water temperature, and nutrient availability, to name a few, viral infection was not known to be significant. Prof. Lindell’s group’s findings are comparable in their impact to someone who, after having known for years about the existence of the flu, suddenly discovers the 1918 Spanish influenza.

Research vessel

This discovery was made possible by technologies Prof. Lindell’s lab had developed earlier. The group devised novel methods to quantify the groups of viruses that infect the cyanobacteria, and the extent to which these viruses infect their hosts. Sailing northwards from Hawaii, the group was able to sample the same locations over three years at high spatial resolution, and discover the 2017 infection event. Satellite data on water temperature and chlorophyll concentration allowed the group to infer that the phenomenon they observed was spread across the North Pacific Ocean, and not limited to the single cruise track they sailed along.

The lab on the ship

While the Prochlorococcus population suffered in 2017, the population of Synechococcus was less affected, and in fact increased in size and spread, benefitting from the reduced competition. Prof. Lindell and her team believe this to be due to the fact that Synechococcus reproduce faster: the viruses killed Prochlorococcus before they were able to reproduce, but it didn’t have the same impact on Synechococcus.

At least half the earth’s oxygen production and primary production of organic compounds comes from the ocean. Oceans cover 70% of the planet’s surface, they drive weather and regulate temperature. Yet much remains unknown about the oceans and the organisms that live in them. The work of Prof. Lindell’s lab helps shed a little light into what goes on in the depths of the ocean.

Graph from the Nature Microbiology article: cyanophage abundance in the North Pacific, as mapped by the Technion team. Warmer colours represent higher abundance. The black lines indicate the cruise track in 2016-2017. The grey areas represent regions with no values due to cloud cover or that were beyond the limits of the predictive model. The hotspot peak corresponds to yellow regions

This study was led by Prof. Lindell and Dr. Carlson, in collaboration with researchers from the University of Washington and the University of Hawaii. It was supported by the European Research Council (ERC) and the Simons Foundation as part of the Simons Collaboration on Ocean Processes and Ecology (SCOPE). Dr. Carlson was supported by a Fulbright Postdoctoral Fellowship.

For the full article in Nature Microbiology click here