Datensatz

Zusammenfassung

Trichodesmium is an important dinitrogen (N2)-fixing cyanobacterium in
marine ecosystems. Recent nucleic acid analyses indicate that
Trichodesmium colonies with their diverse epibionts support various
nitrogen (N) transformations beyond N2 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
N2 fixation occurring mostly during the day. Ten percent of the fixed N
was released as ammonium after 12-h 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
characterised 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.