MARUM Research Academy

Modelling N2 fixation by oceanic free-living heterotrophic bacteria

Nitrogen concentration is an important limiting factor for phytoplankton growth. Since phytoplankton is a primary driver of the marine food web, variations in the nitrogen budget can have profound consequences for marine livelihood and food security. Although nitrogen is found at high concentrations in seawater in the form of inert dissolved gas (N2), only specialized groups of bacteria and archaea (N2 fixers) can convert gaseous N2 into bioavailable (fixed) forms of nitrogen, a process known as “N2 fixation”. Being the largest source of fixed nitrogen in the ocean,marine N2 fixation maintains ocean fertility by compensating for nitrogen losses via denitrification. In many parts of the low-latitude oceans, N2 fixation supports a significant portion (up to 50%) of oceanic primary production. Although cyanobacteria are traditionally considered the dominant N2 fixers in the open ocean, recent analysis of nifH gene, a marker gene used to assess the presence and diversity of N2 fixers, revealed active (non-cyanobacterial) heterotrophic N2 fixation across the world’s oceans. However, the distributioon of heterotrophic N2 fixers and their contribution to the oceanic nitrogen cycle are still unknown.

With this project, we aim to develop a trait-based model to investigate (1) the cellular mechanisms of N2 fixation by free-living heterotrophic bacteria and (2) the role played by environmental factors in regulating heterotrophic N2 fixation. This project will shed lights on the seasonal variations of N2 fixation rates in low latitude oceans, on the distribution of heterotrophic N2 fixers on oceanic surface waters, and on the contribution of heterotrophic bacteria in sustaining the marine food web.