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N2 fixation in free-floating filaments of Trichodesmium is higher than in transiently suboxic colony microenvironments

Journal article
Authors Meri Eichner
Silke Thoms
Björn Rost
Wiebke Mohr
Soeren Ahmerkamp
Helle Ploug
Marcel Kuypers
Dirk deBeer
Published in New Phytologist
Volume 222
Issue 2
Pages 852-863
Publication year 2019
Published at Department of marine sciences
Pages 852-863
Language en
Links doi.org/10.1111/nph.15621
Subject categories Biological Sciences

Abstract

To understand the role of micrometer‐scale oxygen (O2) gradients in facilitating dinitrogen (N2) fixation, we characterized O2 dynamics in the microenvironment around free‐floating trichomes and colonies of Trichodesmium erythraeum IMS101. Diurnal and spatial variability in O2 concentrations in the bulk medium, within colonies, along trichomes and within single cells were determined using O2 optodes, microsensors and model calculations. Carbon (C) and N2 fixation as well as O2 evolution and uptake under different O2 concentrations were analyzed by stable isotope incubations and membrane inlet mass spectrometry. We observed a pronounced diel rhythm in O2 fluxes, with net O2 evolution restricted to short periods in the morning and evening, and net O2 uptake driven by dark respiration and light‐dependent O2 uptake during the major part of the light period. Remarkably, colonies showed lower N2 fixation and C fixation rates than free‐floating trichomes despite the long period of O2 undersaturation in the colony microenvironment. Model calculations demonstrate that low permeability of the cell wall in combination with metabolic heterogeneity between single cells allows for anoxic intracellular conditions in colonies but also free‐floating trichomes of Trichodesmium. Therefore, whereas colony formation must have benefits for Trichodesmium, it does not favor N2 fixation.

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