Award: OCE-1924424

Oxygen-depleted marine habitats are globally-important systems of intense chemical cycling by microorganisms and sensitive to climate change (Fig. 1). Oxygen minimum zones (OMZs) display a sharp transition from fully oxygenated to nearly or completely oxygen-free water, known as an oxycline (Fig. 2). These regions sustain microbial webs mediating critical transformations of carbon, nitrogen and sulfur. OMZs are known to release nitrous oxide, a potent greenhouse gas, so understanding the dynamics of the system that governs critical chemical transformations is paramount to predicting the effects of climate change. However, our ability to fully comprehend consequences for the marine ecosystem is hampered by our limited knowledge of actual rates of key microbiological processes and dynamics of the microorganisms mediating them.

We investigated key microbial metabolic processes across a range of geochemical conditions in the Eastern Tropical Pacific (ETNP) OMZ on both the community and single-cell level. Our study began with a five-week research cruise to the ETNP OMZ in February/March of 2023 aboard the R/V Atlantis. We conducted a suite of experiments at selected depths at two stations in the ETNP (Fig. 3). For each depth, we profiled community chemoautotrophy rates, which is a microbial process that fixes CO₂ like photosynthesis, but uses inorganic chemicals instead of light as an energy source. We also tested if alternative energy sources stimulated these rates. From the same depths, we also profiled community heterotrophic potential which is a microbial process that uses organic chemicals as both an energy and carbon source. Figure 4 presents an overview of the complementary measurements performed by both the Pachiadaki and Taylor labs. As expected, we found that microbial chemoautotrophy and heterotrophy were elevated in the oxycline relative to the OMZ core. Additionally, we found evidence for a functioning cryptic sulfur cycling in the OMZ core via increased chemoautotrophic activity upon amendment with hydrogen sulfide (HS^-). The significance of this is that we demonstrated that organisms needing a chemical energy source (HS^-) that is undetectable in OMZs are present and active. The hypothesis is that these bacteria use HS^- as soon as other organisms produce it, hence HS^- is chemically undetectable ana “cryptic” cycle.

We prepared samples to evaluate single-cell chemoautotrophic, heterotrophic and biosynthetic rates employing ¹³C-bicarbonate, ¹³C-phenylalanine and deuterated water tracers, respectively (Fig. 5). Target microorganisms critical to nitrogen and sulfur cycling were selected based on metagenomic and metatranscriptomic analyses performed by the Pachiadaki lab (Fig. 4). These experiments allow us to evaluate the contributions of key groups of microorganisms to chemoautotrophy, heterotrophy and elemental cycling within the ETNP OMZ. Due to significant delays associated with the COVID-19 pandemic, ship scheduling, and many unexpected methodological hurdles, we have required more time to finalize our single-cell analyses. This will be accomplished at no additional cost to NSF, as Butkevich’s Ph.D. dissertation hinges on acquisition of these results. She will produce a follow-up report for NSF upon completion of her defense and relay any resulting publications. Graduation is projected within two years.

Broader Impacts: Thus far, our findings have provided critical insights into the rates of microbial metabolic processes that govern carbon cycling in the Eastern Tropical North Pacific OMZ. Additionally, the single-cell methods developed and refined to achieve the goals set out in this project have the potential for application to other systems with challenging, unculturable organisms, e.g., deep ocean environs, gut microbiome, etc. We hope the information produced by this project will help inform climate models and clarify microbial metabolic rates for key organisms in their natural environment, as well as produce metabolic rates for organisms that are unculturable in the lab.

This project has facilitated a range of training and professional development opportunities. It partially supported an SBU graduate student in acquiring advanced instrumentation skills, cruise preparation experience and expanding her expertise in microbial ecology. This student showcased her skills with an oral presentation at the international Ocean Sciences Meeting in New Orleans in February 2024 and in Glasgow, Scotland in February 2026. This project has fundamentally bolstered her career trajectory. High school and undergraduate volunteers also gained hands-on experience in several state-of-the-art laboratory techniques. These experiential learning experiences significantly influenced their educational paths.

Last Modified: 07/23/2026
Modified by: Gordon T Taylor