Development and application of proxies for past cyanobacterial N₂ fixation

Abstract

N₂ fixation adds bioavailable nitrogen to the global oceans and therewith drives modern-day marine primary productivity. The diazotrophs mainly responsible for the fixation of nitrogen are principally found in the cyanobacterial lineage with unicellular and filamentous non-heterocystous species dominating. There is evidence that diazotrophic cyanobacteria were of similar importance in the past nitrogen cycling, in particular during the formation of organic-rich deposits of the Phanerozoic (e.g. Pleistocene Mediterranean sapropels and Cretaceous black shales). However, the poor preservation potential and the lack of suitable geochemical tracers that are specific for N₂-fixing cyanobacteria have hampered their rigorous identification in the geological record. This thesis describes investigations aimed at a better understanding of the presence and past distribution of N₂-fixing cyanobacteria and their significance in the past nitrogen cycling. For this, a multiplicity of cyanobacterial cultures, including unicellular, filamentous non-heterocystous and heterocystous species, were analyzed for their isotopic and biogeochemical inventory in order to identify geochemical tracers specific for diazotrophic cyanobacteria. Heterocystous species confine the fixation of N₂ to specialized cells, the heterocysts, which are wrapped in a thick cell envelope limiting the diffusion of oxygen into the cell to a level that respiration can maintain an anoxic microenvironment that is needed to perform N₂ fixation. The major structural components of this cell envelope are heterocyst glycolipids (HGs), consisting of a sugar head group glycosidically bound to long-chain diols, triols, keto-ols and keto-diols. It was found that these components were remarkably well preserved in a variety of recent to ancient lacustrine and marine sediments. For example, HGs were identified in core-top sediments of the Baltic Sea, which is well known for its annual blooms of heterocystous cyanobacteria. These components were also detected in microbial mats growing along the coastline of the North Sea barrier island Schiermonnikoog (The Netherlands). They were particularly abundant in Pleistocene sapropels of the eastern Mediterranean Sea suggesting that heterocystous cyanobacteria, likely as epiphytes in symbiosis with planktonic diatoms, played a major role in adding ‘new’ nitrogen to the stratified surface waters of the Mediterranean Sea and thus sustained the high primary productivity invoked to explain the organic-rich nature of these deposits. The present study showed the general applicability of heterocyst glycolipids as biological marker for N₂-fixing cyanobacteria in miscellaneous present-day environmental settings. It also showed that heterocystous cyanobacteria – in contrast to the modern day – may have had a profound effect on the past marine nitrogen cycling and sustained times of high primary productivity. Heterocyst glycolipids may thus form useful molecular fossils to improve our understanding of the evolution and importance of these N₂-fixing microorganisms in the past nitrogen cycling.