Reactive oxygen species (ROS) are key components in the physiology of organisms, allowing for cell development, cellular communication, and wound repair. They can also be toxic. ROS degrade DNA, proteins, and cell membranes. Formation of ROS is an unfortunate consequence of breathing oxygen, so all oxygen-breathing organisms, including humans, must strictly control the concentration of ROS inside our cells in order to survive. This is accomplished by producing enzymes (proteins) that degrade ROS to benign products so they can be excreted. If cells don’t properly remove these ROS, there are consequences – cancer and premature aging in humans, cell death in macro- and micro-organisms.  

The ROS superoxide and hydrogen peroxide are also ubiquitous in the ocean, where reaction between sunlight and organic matter has been presumed the primary source. Phytoplankton (algae, diatoms) have also been shown to produce ROS – yet, their abundance and distribution in nature is also fundamentally constrained by sunlight. More recent discoveries have revealed that the majority of ROS production in the ocean is produced within the dark – yet the sources and reasons for this production were unknown. ROS are key controls in biogeochemistry, where for instance they influence the bioavailability of metals to support microbial life, degrade carbon to more labile sources that may then enter into the foodweb, and induce the bleaching and demise of coral reefs. It is therefore of great need to obtain a better understanding of the processes that control their formation and lifetime in marine systems.

The goals of this research were to determine if ROS are common to dark marine waters and, if so, who is making ROS in the ocean and why. We focused here on the oligotrophic ocean as these nutrient-poor regions represent key areas where carbon-mediated photochemical ROS production is limited. As a comparison, we also studied deep coastal waters, where carbon is not limited but light will be attenuated. This research involved a number of research cruises in the Pacific and Atlantic Oceans, seasonal sampling in coastal waters off Cape Cod, Massachusetts, and laboratory incubations of key microbial species. In brief, we discovered that dark production of superoxide is ubiquitous in marine systems, where both bacteria and phytoplankton are key sources. Interestingly, for the common nitrogen-fixing microorganism Trichodesmium obtained from various sites in the oligotrophic Sargasso Sea, we find that superoxide production is decoupled from photosynthesis and has an inverse relationship with colony density. These results hint at a role for superoxide in cell signaling and communication in these organisms. Trichodesmium forms substantial blooms in oligotrophic ocean waters, where superoxide production has been implicated in programmed cell death and subsequent bloom demise. Thus, our findings point to a complex role for superoxide in phytoplankton bloom dynamics. We have also identified the extracellular enzymes involved in superoxide production by a common marine bacterium and two diatom species. Surprisingly, the enzymes responsible for superoxide production by these organisms differ but are all classified as antioxidants, enzymes that are expected to degrade (not produce) ROS.

This research has revealed the importance of microbial processes and extracellular enzymes in the formation of superoxide in dark regions of the ocean – regions that have historically been assumed free of ROS. It has been long appreciated that these same ROS are important for controlling the biogeochemistry of the atmosphere and surface ocean – we now need to think about them in the 95% of our global habitat that is in the dark. We also need to revisit the function of enzymes that are canonically considered antioxidants – in fact, these enzyme...

Please report errors in award information by writing to: awardsearch@nsf.gov.