Trichodesmium is an important dinitrogen (N~2~)-fixing cyanobacterium in marine ecosystems. Recent nucleic acid analyses indicate that Trichodesmium colonies with their diverse epibionts support various nitrogen (N) transformations beyond N~2~-fixation. However, rates of these transformations and concentration gradients of N-compounds in Trichodesmium colonies remain largely unresolved. We combined isotope-tracer incubations, micro-profiling, and numeric modelling to explore carbon fixation, N-cycling processes, as well as oxygen, ammonium and nitrate concentration gradients in individual field-sampled Trichodesmium colonies. Colonies were net-autotrophic, with carbon and N~2~-fixation occurring mostly at day-time. Ten percent of the fixed N was released as ammonium after 12-hour incubations. Nitrification was not detectable but nitrate consumption was high when nitrate was added. The consumed nitrate was partly reduced to ammonium, while denitrification was insignificant. Thus, the potential N-transformation network was characterized by fixed N gain and recycling processes rather than denitrification. Oxygen concentrations within colonies were 60–200% air-saturation. Moreover, our modelling predicted steep concentration gradients, with up to 6-fold higher ammonium concentrations, and nitrate depletion in the colony centre compared to the ambient seawater. These gradients created a chemically heterogeneous microenvironment, presumably facilitating diverse microbial metabolisms in millimetre-sized Trichodesmium colonies.

The dataset includes measurements of oxygen profiles in Trichodesmium colonies, and modelled profiles and concentration fields of oxygen, ammonium and nitrate. The data included are plotted in Figure 3 and 4 in the indicated original article.

Supplement to: Klawonn, Isabell; Eichner, Meri J; Wilson, Samuel T; Moradi, Nasrollah; Thamdrup, Bo; Kümmel, Steffen; Gehre, Matthias; Khalili, Arzhang; Grossart, Hans-Peter; Karl, David Michael; Ploug, Helle (2020): Distinct nitrogen cycling and steep chemical gradients in Trichodesmium colonies. The ISME Journal, 14(2), 399-412