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Oxygen-starved "dead zones" in the ocean seem like they should only support life that can survive without oxygen. Yet scientists have uncovered a surprising contradiction: bacteria that require oxygen are actually flourishing in these seemingly inhospitable waters. For example, nitrite-oxidizing bacteria (NOB) appear to defy logic by thriving where oxygen levels should be lethal to them.

Researchers recently used U.S. National Science Foundation (NSF) ACCESS allocations on the Expanse system at the San Diego Supercomputer Center (SDSC) at the University of California San Diego School of Computing, Information and Data Sciences (SCIDS) to develop a model to explain this paradox — revealing that these bacteria follow a dramatic "boom-and-bust" survival strategy. During rare moments when oxygen briefly enters these zones, the bacterial populations explode in a frenzy of growth and reproduction. As oxygen levels plummet again, most bacteria that require oxygen die off — but crucially, enough individuals endure to kickstart the cycle anew when the next oxygen pulse arrives.

“Our breakthrough realization is that the intermittent oxygen intrusions are critical to explaining the very high abundances of NOB that have been measured in these anoxic zones,” said Emily Zakem, a theoretical and computational biogeochemist and microbial ecologist at Carnegie Science. “These fleeting oxygen bursts provide critical lifelines that allow NOB to thrive, in contrast to other types of bacteria that need oxygen, because their primary source of energy – nitrite – accumulates in these zones. This accumulation is key for NOB to “boom” when oxygen does become available.”

To uncover this survival strategy, Zakem led a team that implemented a new ecosystem model into a complex three-dimensional ocean model replicating conditions from the prominent dead zone in the Eastern Tropical South Pacific. Zakem said that the most exciting takeaway from the model is that NOB were most prosperous at the boundaries of the zones — a result that relates to a much bigger picture.

“The NOB regulate the dead zone borders by consuming a very large fraction of the oxygen there. Any additional incoming oxygen would be also rapidly devoured. And vice versa, if less oxygen is supplied, less NOB would grow. This maintains the size and intensity of the zones,” Zakem said. “With this stabilizing feedback, we speculate that the nitrogen cycling inside is shielded from climate change stressors – one of which is changes in oxygen supply to these zones.”

She said that these resilient bacteria populations continue to amaze scientists with not only their spontaneous lifestyle, but their contribution to the functioning of the waters they inhabit.

“Our results have impacts beyond the question of how these bacteria were able to live in conditions that didn’t seem possible,” Zakem said. “Thanks to NSF ACCESS and our sims using SDSC’s Expanse, we can now tie NOB interactions within these zones to their effects on larger ocean processes.”

The research was published in Science.

Funding was provided by the Simons Foundation and NSF (grant nos. OCE-1847687 and 2125142). Computational work on Expanse was provided by NSF ACCESS (allocation nos. TG-OCE170017 and EES220053).