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High cell-specific rates of nitrogen and carbon fixation by the cyanobacterium Aphanizomenon sp. at low temperatures in the Baltic Sea

Journal article
Authors Jennie B Svedén
Birgit Adam
Jakob Walve
Nurun Nahar
Niculina Musat
Gaute Lavik
Martin J. Whitehouse
Marcel M. M. Kuypers
Helle Ploug
Published in FEMS Microbiology Ecology
Volume 91
Issue 12
Pages 1-10
ISSN 0168-6496
Publication year 2015
Published at Department of marine sciences
Pages 1-10
Language en
Links dx.doi.org/10.1093/femsec/fiv131
Keywords Aphanizomenon; SIMS; isotopes; O2-microsensors; C-fixation; N2-fixation
Subject categories Marine ecology

Abstract

Aphanizomenon is a widespread genus of nitrogen (N2)-fixing cyanobacteria in lakes and estuaries, accounting for a large fraction of the summer N2-fixation in the Baltic Sea. However, information about its cell-specific carbon (C)- and N2-fixation rates in the early growth season has not previously been reported. We combined various methods to study N2-fixation, photosynthesis and respiration in field-sampled Baltic Sea Aphanizomenon sp. during early summer at 10◦C. Stable isotope incubations at in situ light intensities during 24 h combined with cell-specific secondary ion mass spectrometry showed an average net N2-fixation rate of 55 fmol N cell−1 day−1. Dark net N2-fixation rates over a course of 12 h were 20% of those measured in light. C-fixation, but not N2-fixation, was inhibited by high ambient light intensities during daytime. Consequently, the C:N fixation ratio varied substantially over the diel cycle. C- and N2-fixation rates were comparable to those reported for Aphanizomenon sp. in August at 19◦C, using the same methods. High respiration rates (23% of gross photosynthesis) were measured with 14C-incubations and O2-microsensors, and presumably reflect the energy needed for high N2-fixation rates. Hence, Aphanizomenon sp. is an important contributor to N2-fixation at low in situ temperatures in the early growth season.

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