Bacterial genetic fitness help regulate ocean carbon

The world’s oceans are dominated by microscopic organisms that power the Earth’s biogeochemical processes. These microbial ecosystems sustain life in the oceans by forming the basis of the ocean food web and balance the composition of our atmosphere – though scientists are just beginning to understand and study these ecosystems.

A new research study led by the University of Georgia examines reproductive fitness of bacteria critical to the world-wide microbial carbon flow, providing a new tool for understanding how interactions within microbial communities influence an organism’s success.

“The factors that determine the rate at which bacteria contribute their genes to the next generation have a critical impact on the carbon fixed by phytoplankton in the oceans,” said Mary Ann Moran, UGA Foundation Distinguished Professor in the Franklin College of Arts and Sciences department of marine sciences and senior author on the paper.

The study focuses on this fitness to pass on genes. In the case of bacteria, which reproduce by dividing, fitness can be quantified by the rate at which new cells are produced. For example, a bacterial species that grows twice as fast in one condition than it does in another has higher fitness in the first condition. The research used thousands of different mutant bacteria (each with a different gene unable to function) mixed together in a pool to provide a wide array of conditions that could be tested at the same time.

About 40% of the carbon fixed by phytoplankton in the ocean is passed to the marine bacteria that live around them in a nutrient-rich region surrounding the phytoplankton cell called the phycosphere. On a global basis, this constitutes 20% of the total, as marine and terrestrial carbon fixation are roughly equal.

The results contribute a new measure for microbial ecologists to take into account when assessing ecological interactions that occur between phycosphere-associated bacteria at the micron scale and their potential to influence a major fraction of annual carbon flux at the global scale.

“Scientists who study microbial ecology of the ocean at the level of community interactions are quite limited by the methods available, and this approach serves as a new tool for the tool box,” said Jeremy Schreier, UGA doctoral researcher and lead author on the paper. “Our study indeed revealed new information about the processes that affect bacteria in a particular type of ocean microenvironment. We suspect there is a considerable amount of further information that can be gained in complex environments using this approach.”

“The use of bacterial reporters – bacteria that produce a signal indicating the conditions they are experiencing – has been successful in many areas of environmental science, from detecting toxic compounds or measuring oxygen concentrations in soil,” said Moran, who was elected to the National Academy of Sciences in 2021. “This study takes a similar approach, but instead of measuring a fluorescent signal or color change in the bacterium, we use their ability to grow as the indicator.” 

The study, A Mutant Fitness Assay Identifies Bacterial Interactions in a Model Ocean Hot Spot, was published in the Proceedings of the National Academy of Sciences.

Image: Schreier(l), and Moran examine laboratory samples. Photo by Andrew Tucker, UGA