Predator interactions determine where Prochlorococcus thrive

A new study led by the Massachusetts Institute of Technology (MIT) has found that the global distribution of Prochlorococcus, the smallest and most abundant photosynthesizing organism on the planet, is not influenced by changes in temperature, as previously thought, but rather by this microbe’s complex cycles of interactions with some of its predators.

Prochlorococcus is most abundant in the ocean’s warm surface waters, and its numbers drop off significantly closer to the poles. Until recently, scientists thought that, as with many other marine organisms, Prochlorococcus’ range is influenced by temperature: the colder the waters, the less likely to live there.

However, new findings suggest that this may not be the case. As a byproduct of its photosynthesis, Prochlorococcus produces carbohydrates, which are essential nutrients for heterotrophic bacteria – single-celled organisms that do not photosynthesize but live off the organic matter produced by phytoplankton.

While Prochlorococcus is the main producer of carbohydrates in warmer waters, there are various other species of phytoplankton that produce these essential nutrients closer to the poles. By analyzing data collected from the northeast Pacific Ocean and employing simulations of ocean circulation and marine ecosystem interactions, the MIT researchers have found that, in nutrient-rich areas where bacteria can feast and grow on more food sources, Prochlorococcus populations tend to be quickly decimated.

According to study lead author Stephanie Dutkiewicz, an oceanographer at MIT, “as soon as you reach an environment where other species add carbohydrates to the mix, bacteria and grazers grow up and annihilate Prochlorococcus.” Thus, it appears to be purely coincidental that the colder the waters, the lower the Prochlorococcus populations. Instead, the complex relationship with a shared predator (the carbohydrate-consuming bacteria) is the main factor setting Prochlorococcus’ range.

Incorporating these findings into models will be crucial in predicting how this microbe and other marine species will shift due to climate change. “Prochlorococcus is a big harbinger of changes in the global ocean,” said Dutkiewicz. “If its range expands, that’s a canary – a sign that things have changed in the ocean by a great deal.”

“There are reasons to believe its range will expand with a warming world,” added study co-author Christopher Follett, an expert in Oceanography and Biogeochemistry at MIT.” But we have to understand the physical mechanisms that set these ranges. And predictions just based on temperature will not be correct.”

The study is published in the journal Proceedings of the National Academy of Sciences.

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By Andrei Ionescu, Earth.com Staff Writer