The dark side of the biological pump: the impact of iron availability on heterotrophic bacterial carbon consumption in the mesopelagic

Below the surface, in the ocean’s “twilight zone” – the water from roughly 150 to 600 meters deep – sinking organic matter from surface waters is consumed by bacteria. The more organic carbon is consumed in the twilight zone, the less sinks to the deep ocean and to sediments. Thus, the activity of bacteria in the twilight zone is an important control on the ocean carbon cycle. In this project, a team of researchers will test the idea that the metabolism of bacteria in the twilight zone may be partly controlled by iron, an essential nutrient. This work will have implications for understanding ocean biology and chemistry. The project will promote workforce development through support of an early career research scientist and a graduate student. The lead investigator will develop a related laboratory module for an undergraduate course based on this work. Preliminary data demonstrate that iron availability impacts the total carbon consumption rate by heterotrophic bacteria in the mesopelagic ocean, and that heterotrophic bacteria in this region of the water column are co-limited by iron and carbon. The team will use a combination of laboratory culturing studies and field experiments to test three related hypotheses about why and how this co-limitation occurs. The experiments will span a range in iron and carbon conditions, will test steady state versus pulse conditions, and will examine varying degrees of carbon lability. The three hypotheses they will test are: H1) Heterotrophic bacteria maintain high iron quotas in steady state conditions in the mesopelagic due to changes in the availability of iron and carbon in the transition from the surface to the mesopelagic and the cellular allocation of iron varies depending on the iron to carbon ratio at steady-state conditions, H2) The high iron quotas maintained by copiotrophic bacteria in the mesopelagic allows them to respond efficiently to pulses of new carbon, and the response is proportional to the quotas established in the steady-state environment, the magnitude of the carbon pulse, and the lability of the carbon substrate, and H3) The composition of organic material and iron homeostasis of heterotrophic bacteria affects total carbon consumption in the mesopelagic. The expected outcomes of the work are an increased mechanistic understanding of how the iron status of heterotrophic bacteria in the mesopelagic ocean impacts their ability to consume organic carbon, and how and why there is widespread co-limitation throughout the upper mesopelagic ocean. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.