Breitbarth, Eike (2005) Ecophysiology of the marine cyanobacterium Trichodesmium. (PhD/ Doctoral thesis), Christian-Albrechts-Universität, Kiel, 149 pp.

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Abstract

The aim of this thesis is to improve the mechanistic understanding of nitrogen fixation by the marine non-heterocystous cyanobacterium Trichodesmium. This diazotroph is a major contributor to the marine nitrogen cycle and thus is important for parameterizations for nitrogen fixation in Ocean Biogeochemical Circulation Models. A wide-ranging amount of information on ecophysiological characteristics of Trichodesmium has been published and is summarized in a literature review (chapter I). This review stresses that Trichodesmium abundance in the open ocean can be limited by the nutrients iron and/or phosphorus and is constrained by the physical parameters temperature and light. Nevertheless, chapter I also identifies that the effects of temperature and light availability on diazotrophic growth of Trichodesmium were not fully understood and required further investigation. Laboratory results (chapter III) demonstrate that Trichodesmium N2 fixation is confined to water temperatures of 20 – 34 °C with an optimum range of 24 – 30 °C. These findings are of particular interest with respect to global warming, considering the important role of Trichodesmium in the marine nitrogen cycle. Combining these data with climate models (HadCM3 and GFDL R30) chapter III predicts a future decline in the fixed nitrogen input by Trichodesmium that could significantly affect marine nitrogen cycling within this century. Chapters IV and V address the role of light for diazotrophic growth of Trichodesmium. Results show that Trichodesmium is well adapted to the light regimes throughout the euphotic zone of tropical and subtropical oceans. The carbon specific growth rate increases up to an irradiance of 180 μmol quanta m-2 s-1, and is constant (0.26 d-1) thereafter up to 1100 μmol quanta m-2 s-1, where light inhibition sets in. The maximum nitrogen fixation rate measured was 350 nmol N2 fixed l-1 h-1. Chapter IV further provides a simple numerical model to describe nitrogen input into seawater by Trichodesmium as a function of light intensity. This outcome is complemented by a conceptual model of nitrogen and carbon fixation of Trichodesmium presented in Chapter V. Additionally, a reassessment of a commonly applied method to measure nitrogen fixation (Acetylene Reduction Assay) was conducted (chapter II). This method was improved by providing newly derived ethylene gas solubility coefficients that are required to accurately calculate nitrogen fixation rates. These were previously unavailable from published literature and thus the presented publication contributes to a standardization of nitrogen fixation measurements. Further methodological approaches to assess nitrogen fixation and release of fixed nitrogen by diazotrophs are provided in an outlook for future work in chapter V.

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