Planktonic marine cyanobacteria of the genus Trichodesmium occur throughout the oligotrophic tropical and subtropical oceans. Their unusual adaptations, from the molecular to the macroscopic level, contribute to their ecological success and biogeochemical importance. Trichodesmium fixes nitrogen gas (N2) under fully aerobic conditions while photosynthetically evolving oxygen. Its temporal pattern of N2 fixation results from an endogenous daily cycle that confines N2 fixation to daylight hours. Trichodesmium colonies provide a unique pelagic habitat that supports a complex assemblage of consortial organisms. These colonies often represent a large fraction of the plant biomass in tropical, oligotrophic waters and contribute substantially to primary production. N2 fixation by Trichodesmium is likely a major input to the marine and global nitrogen cycle.

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Trichodesmium, a Globally Significant Marine Cyanobacterium

During the past decade, the molecular mechanisms underlying the mammalian circadian clock have been defined. A core set of circadian clock genes common to most cells throughout the body code for proteins that feed back to regulate not only their own expression, but also that of clock output genes and pathways throughout the genome. The circadian system represents a complex multioscillatory temporal network in which an ensemble of coupled neurons comprising the principal circadian pacemaker in the suprachiasmatic nucleus of the hypothalamus is entrained to the daily light/dark cycle and subsequently transmits synchronizing signals to local circadian oscillators in peripheral tissues. Only recently has the importance of this system to the regulation of such fundamental biological processes as the cell cycle and metabolism become apparent. A convergence of data from microarray studies, quantitative trait locus analysis, and mutagenesis screens demonstrates the pervasiveness of circadian regulation in biological systems. The importance of maintaining the internal temporal homeostasis conferred by the circadian system is revealed by animal models in which mutations in genes coding for core components of the clock result in disease, including cancer and disturbances to the sleep/wake cycle.

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Mammalian circadian biology: elucidating genome-wide levels of temporal organization.

The surface water of the marine environment has traditionally been viewed as a nitrogen (N) limited habitat, and this has guided the development of conceptual biogeochemical models focusing largely on the reservoir of nitrate as the critical source of N to sustain primary productivity. However, selected groups of Bacteria, including cyanobacteria, and Archaea can utilize dinitrogen (N2) as an alternative N source. In the marine environment, these microorganisms can have profound effects on net community production processes and can impact the coupling of C-N-P cycles as well as the net oceanic sequestration of atmospheric carbon dioxide. As one component of an integrated ‘Nitrogen Transport and Transformations’ project, we have begun to re-assess our understanding of (1) the biotic sources and rates of N2 fixation in the world’s oceans, (2) the major controls on rates of oceanic N2 fixation, (3) the significance of this N2 fixation for the global carbon cycle and (4) the role of human activities in the alteration of oceanic N2 fixation. Preliminary results indicate that rates of N2 fixation, especially in subtropical and tropical open ocean habitats, have a major role in the global marine N budget. Iron (Fe) bioavailability appears to be an important control and is, therefore, critical in extrapolation to global rates of N2 fixation. Anthropogenic perturbations may alter N2 fixation in coastal environments through habitat destruction and eutrophication, and open ocean N2 fixation may be enhanced by warming and increased stratification of the upper water column. Global anthropogenic and climatic changes may also affect N2 fixation rates, for example by altering dust inputs (i.e. Fe) or by expansion of subtropical boundaries. Some recent estimates of global ocean N2 fixation are in the range of 100−200 Tg N (1−2 × 1014 g N) yr −1, but have large uncertainties. These estimates are nearly an order of magnitude greater than historical, pre-1980 estimates, but approach modern estimates of oceanic denitrification.

Dinitrogen fixation in the world's oceans

It is generally considered that nitrogen availability is one of the major factors regulating primary production in temperate coastal marine environments. Coastal regions often receive large anthropogenic inputs of nitrogen that cause eutrophication. The impact of these nitrogen additions has a profound effect in estuaries and coastal lagoons where water exchange is limited. Such increased nutrient loading promotes the growth of phytoplankton and fast growing pelagic macroalgae while rooted plants (sea-grasses) and benthic are suppressed due to reduced light availability. This shift from benthic to pelagic primary production introduces large diurnal variations in oxygen concentrations in the water column. In addition oxygen consumption in the surface sediments increases due to the deposition of readily degradable biomass. In this review the physico-chemical and biological factors regulating nitrogen cycling in coastal marine ecosystems are considered in relation to developing effective management programmes to rehabilitate seagrass communities in lagoons currently dominated by pelagic macroalgae and/or cyanobacteria.

Nitrogen cycling in coastal marine ecosystems.

Nitrogen is a quantitatively important bioelement which is incorporated into the biosphere through assimilatory processes carried out by microorganisms and plants. Numerous nitrogencontaining compounds can be used by different organisms as sources of nitrogen. These include, for instance, inorganic ions like nitrate or ammonium and simple organic compounds like urea, amino acids, and some nitrogen-containing bases. Additionally, many bacteria are capable of fixing N 2. Nitrogen control is a phenomenon that occurs widely among microorganisms and consists of repression of the pathways of assimilation of some nitrogen sources when some other, more easily assimilated source of nitrogen is available to the cells. Ammonium is the preferred nitrogen source for most bacteria, but glutamine is also a very good source of nitrogen for many microorganisms. Two thoroughly investigated nitrogen control systems are the NtrB-NtrC two-component regulatory system found in enterics and some other proteobacteria (80) and the GATA family global nitrogen control transcription factors of yeast and some fungi (75). Novel nitrogen control systems have, however, been identified in bacteria other than the proteobacteria, like Bacillus subtilis (26), Corynebacterium glutamicum (52), and the cyanobacteria. The cyanobacterial system is the subject of this review. The cyanobacteria are prokaryotes that belong to the Bacteria domain and are characterized by the ability to perform oxygenic photosynthesis. Cyanobacteria have a wide ecological distribution, and they occupy a range of habitats, which includes vast oceanic areas, temperate soils, and freshwater lakes, and even extreme habitats like arid deserts, frigid lakes, or hot springs. Photoautotrophy, fixing CO 2 through the Calvin cycle, is the dominant mode of growth of these organisms (109). A salient feature of the intermediary metabolism of cyanobacteria is their lack of 2-oxoglutarate dehydrogenase (109). As a consequence, they use 2-oxoglutarate mainly as a substrate for the incorporation of nitrogen, a metabolic arrangement that may have regulatory consequences. Notwithstanding their rather homogeneous metabolism, cyanobacteria exhibit remarkable morphological diversity, being found as either unicellular or filamentous forms and exhibiting a number of cell differentiation processes, some of which take place in response to defined environmental cues, as is the case for the differentiation of N 2-fixing heterocysts (109). Nitrogen control in cyanobacteria is mediated by NtcA, a transcriptional regulator which belongs to the CAP (the catabolite gene activator or cyclic AMP [cAMP] receptor protein) family and is therefore different from the well-characterized Ntr system. Interestingly, however, the signal transduction P II protein, which plays a key role in Ntr regulation, is found in cyanobacteria but with characteristics which differentiate it from proteobacterial P II. In the following paragraphs, we shall first briefly summarize our current knowledge of the cyanobacterial nitrogen assimilation pathways and of what is known about their regulation at the protein level. This description will introduce most of the known cyanobacterial nitrogen assimilation genes. We shall then describe the ntcA gene and the NtcA protein themselves to finally discuss NtcA function through a survey of the NtcA-regulated genes which participate in simple nitrogen assimilation pathways or in heterocyst differentiation and function.

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Nitrogen control in cyanobacteria.

Many, though not all, non-heterocystous cyanobacteria can fix N2. However, very few strains can fix N2 aerobically. Nevertheless, these organisms may make a substantial contribution to the global nitrogen cycle. In this general review, N2 fixation by laboratory cultures and natural populations of non-heterocystous cyanobacteria is considered. The properties and subcellular location of nitrogenase in these organisms is described, as is the response of N2 fixation to environmental factors such as fixed nitrogen, O2 and the pattern of illumination. The integration of N2 fixation with other aspects of cell metabolism (in particular photosynthesis) is also discussed. Similarities and differences between different individual strains of non-heterocystous cyanobacteria are highlighted.

https://pure.uva.nl/ws/files/3432970/2571_25298y.pdf

J.R Gallon

Papers

N2 Fixation by non-heterocystous cyanobacteria