Jun 30, 2022
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People think that the world’s oxygen is produced via photosynthesis by green plants growing on earth. But in fact, half of the precious gas that we need to breathe is manufactured 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 plants on land, cyanobacteria trap carbon dioxide to produce oxygen and organic compounds such as fats and sugars. Like any other living organism, cyanobacteria sometimes get infected by viruses. Now a team headed by Prof. Debbie Lindell from the Biology Faculty of the Technion-Israel Institute of Technology in Haifa has found that just like humans, cyanobacteria can experience viral epidemics that significantly affect their population. 


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 therefore helps shed a little light onto the depths of the oceans.


These findings were published in Nature Microbiology under the title “Viruses affect picocyanobacterial abundance and biogeography in the North Pacific Ocean.” 


Dr. Michael Carlson, a postdoctoral fellow in Lindell’s lab, sailed along the Pacific Ocean to study the populations of two common cyanobacteria – Prochlorococcus and Synechococcus. The two groups live in different latitudes; Prochlorococcus lives in warmer waters that have fewer nutrients, while Synechococcus prefers colder and more nutrient-rich latitudes. In the area between, both thrive and create a hotspot or “cyanobacteria-city.”


They studied the amount of cyanophage and infected picocyanobacteria in 87 surface water samples from five narrow sections that covered some 2,200 kilometers in the North Pacific Ocean on three cruises that lasted for two to four weeks each between 2015 and 2017.


It turned out this 550-kilometer-wide hotspot, is unfortunately also a focal point 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 – three times more, normally, as Lindell and Carlson saw in 2015 and 2016. 


But when the team arrived at the same location a year later, they found the Prochlorococcus population in the hotspot significantly reduced and suffering from 10 times more infection than normal. In 2017, the Prochlorococcus population declined at 17o Celsius, when normally these cyanobacteria are comfortable at temperatures as low as 12o C. The Prochlorococcus, in short, suffered a virus 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 certainly known that viruses infected and killed cyanobacteria, but among the other factors affecting cyanobacterial population size – like being eaten by bigger organisms, water temperature and nutrient availability – viral infection was not known to be significant. The Technion team’s findings are comparable to after having known for years about the flu and then suddenly discovering the 1918 Spanish influenza.

This discovery was made possible by technologies Lindell’s lab had developed earlier. The group constructed 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, they were able to sample the same locations over three years at high spatial resolution, and discover the 2017 infection event. Satellite data on the water’s temperature and concentration of chlorophyll enabled the group to reach their conclusion that the phenomenon they observed was spread across the North Pacific Ocean and not limited to the single cruise along which they sailed.

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