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Distinct nitrogen cycling and steep chemical gradients in Trichodesmium colonies.

Journal article
Authors Isabell Klawonn
Meri Eichner
Samuel T Wilson
Nasrollah Moradi
Bo Thamdrup
Steffen Kümmel
Matthias Gehre
Arzhang Khalili
Hans-Peter Grossart
David M Karl
Helle Ploug
Published in The ISME journal
Volume 14
Pages 399-412
ISSN 1751-7370
Publication year 2020
Published at Department of marine sciences
Pages 399-412
Language en
Links dx.doi.org/10.1038/s41396-019-0514-...
www.ncbi.nlm.nih.gov/entrez/query.f...
Subject categories Water in nature and society

Abstract

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.

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