Microsensor measurements of oxygen concentration profiles, and computer-simulated concentration profiles of oxygen, ammonium and nitrate oxygen in Trichodesmium colonies

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.