Periodic Reporting for period 1 - NITROX (NITROX- Nitrogen regeneration under changing oxygen conditions)
Reporting period: 2016-06-01 to 2018-05-31
The specific goals of NITROX were to
1. identify which microbes are involved in N2 fixation in oxygen depleted waters using molecular genetic methods
2. measure how much N2 fixation takes place in those waters
3. understand how the N2 fixation in those waters is regulated, and how it will respond to further oxygen loss from the water.
Over the course of NITROX it has been found that a feedback regulation between N2 fixation, primary production by algae and cyanobacteria, and their degradation after they die and sink out of the surface water, exists. This means that oxygen poor conditions favor N2 fixation, through the higher availability of N algae can grow. As soon as those algae die and sink down to low oxygen waters, they are degraded, a process that consumes oxygen. Thus, oxygen is progressively being lost from those waters until they are completely anoxic and possibly a toxic substance, hydrogen sulfide can evolve. This development would in principle mean that oxygen poor waters would continuously expand, but as we found during NITROX, extreme anoxia with hydrogen sulfide being produced stops N2 fixation and with that primary production. In consequence, no more organic material is produced and exported, and oxygen consumption in deeper waters decreases. Thus, this is the first prove of a feedback cycle driven by bacteria, which can counteract one of the consequences of climate change, namely Ocean deoxygenation.
The sampling approach combined collection of water samples for incubation experiments to measure rates using stable isotopes of nitrogen and carbon. We further collected samples for genetic analysis, cell counting and we performed oxygen and nutrient measurements in parallel to every sample collection.
Laboratory analysis included mass spectrometry to determine the rates of N2 fixation, N loss and C fixation. We used a metagenomic approach, which means we sequenced all genes present in our sample, to understand the diversity of N2 fixers. From additional incubation experiments with changing oxygen and hydrogen sulfide concentrations, we defined the conditions under which N2 fixers are most effective, and those conditions, which make it unable for them to function.
Our major result so far is indeed that we found a limit in terms of hydrogen sulfide concentrations allowing for N2 fixation. This speaks for our hypothesis of too extreme anoxia decreasing N2 fixation and thus primary production. In conclusion, export and respiration of organic material would decrease, and the waters could recover to a less severe anoxia.