MicrobiologyBytes

The availability of nitrogen (N) is one of the factors that controls the productivity of the oceans, and there has been great interest in determining the magnitudes and pathways of N inputs to the world’s oceans through biological N₂ fixation. Biological N₂ fixation is the reduction of N₂ gas to biologically available ammonium, and this is performed by a diverse but limited number of bacterial and archaeal genera. Cyanobacteria are generally assumed to be the major N₂-fixing microorganisms in the open ocean. Atmospheric N₂ is one of the important external sources of N to the surface waters of the oceans, and thus provides the stoichiometric nutrient flux to support the export of carbon to the deep ocean. This is of importance in ocean–atmosphere fluxes and feedbacks that help to constrain the atmospheric concentrations of the greenhouse gas CO₂. There continues to be controversy over whether the oceanic denitrification losses of N are balanced by N₂ fixation inputs. There are large uncertainties in the estimates of basin- and global-scale denitrification and N₂ fixation rates from biogeochemical calculations, perhaps due to the many assumptions required to scale these processes globally from either biogeochemical or biological data. Conversely, there is also a general lack of data on the distributions and activities of N₂-fixing microorganisms over the vast scales of the ocean. At the core of resolving these issues is the identification of the organisms involved and determining how they function, the factors that limit their growth, and their roles in food webs. The difficulties in determining the roles of open ocean microorganisms in N₂ fixation are the nature of N₂-fixing microorganisms themselves, the dilute nature of microbial populations in the oligotrophic ocean, and the general difficulty in cultivating microorganisms from the ocean. Despite these hurdles, over the past few years much has been learned about the microorganisms primarily responsible for N₂ fixation in the surface waters of the open ocean.

Nitrogen fixation by marine cyanobacteria. Trends Microbiol. Jan 10 2011
Discrepancies between estimates of oceanic N₂ fixation and nitrogen (N) losses through denitrification have focused research on identifying N₂-fixing cyanobacteria and quantifying cyanobacterial N₂ fixation. Previously unrecognized cultivated and uncultivated unicellular cyanobacteria have been discovered that are widely distributed, and some have very unusual properties. Uncultivated unicellular N₂-fixing cyanobacteria (UCYN-A) lack major metabolic pathways including the tricarboxylic acid cycle and oxygen-evolving photosystem II. Genomes of the oceanic N₂-fixing cyanobacteria are highly conserved at the DNA level, and genetic diversity is maintained by genome rearrangements. The major cyanobacterial groups have different physiological and ecological constraints that result in highly variable geographic distributions, with implications for the marine N-cycle budget.

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• Cyanobacteria
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Cyanobacteria are the most environmentally significant group of bacteria on Earth. In this article in Microbiology Today, David Adams explains how in many ways life on Earth owes its very existence to this ancient group of micro-organisms:

Cyanobacteria are a huge group of photosynthetic bacteria found in almost every environment on Earth, including many of those most inhospitable to life, such as hot springs, deserts and the Antarctic. They are also enormously abundant, particularly in the oceans, and are primary producers, meaning that they fix CO₂ and in many cases also N₂; as a consequence they have an immense influence on the planet’s nutrient cycles and even its weather. Life on Earth owes a further great debt to this group of bacteria because their evolution of oxygenic photosynthesis, in which oxygen is released from the splitting of water, resulted in the eventual oxygenation of the atmosphere, providing the stimulus for the evolution of complex life forms. In addition, cyanobacteria are the ancestors of plastids, the photosynthetic organelles of today’s algae and plants.

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• Calcifying cyanobacteria and carbon capture
• A day in the life of a cyanobacterium
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Strategies to reduce emissions of carbon dioxide (CO₂) from fossil fuels, and hence mitigate climate change, include energy savings, development of renewable biofuels, and carbon capture and storage (CCS). For CCS, several scenarios are being considered. One approach is capture of point-source CO₂ from power plants or other industrial sources and subsequent injection of the concentrated CO₂ underground or into the ocean. An alternative to this point-source CCS method is expansion of biological carbon sequestration of atmospheric CO₂ by measures such as reforestation, changes in land use practices, increased carbon allocation to underground biomass, production of biochar, and enhanced biomineralization. In addition to geological or oceanic CO₂ injection, novel models for point-source CCS based on accelerated weathering and biomineralization are emerging, utilizing either abiotic or biotic processes. Biomineralization of CO₂ by calcium carbonate (CaCO3) precipitation is a common phenomenon in marine, freshwater, and terrestrial ecosystems and is a fundamental process in the global carbon cycle.

Employment of cyanobacteria in biomineralization of carbon dioxide by calcium carbonate precipitation offers novel and self-sustaining strategies for point-source carbon capture and sequestration. Although details of this process remain to be elucidated, a carbon-concentrating mechanism, and chemical reactions in exopolysaccharide or proteinaceous surface layers are assumed to be of crucial importance. Cyanobacteria can utilize solar energy through photosynthesis to convert carbon dioxide to recalcitrant calcium carbonate. Calcium can be derived from sources such as gypsum or industrial brine. A better understanding of the biochemical and genetic mechanisms that carry out and regulate cynaobacterial biomineralization should put us in a position where we can further optimize these steps by exploiting the powerful techniques of genetic engineering, directed evolution, and biomimetics.

Calcifying cyanobacteria-the potential of biomineralization for carbon capture and storage. Curr Opin Biotechnol. Apr 22 2010

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Most organisms exhibit daily cycles that are driven by endogenous circadian clocks. Until recently, transcription/translation feedback on central clock genes has been proposed as the core mechanism of circadian rhythm generation in any organism. A similar model was proposed for the functions of three essential clock genes, kaiA, kaiB and kaiC, in the cyanobacterium Synechococcus elongatus PCC 7942 (hereinafter, Synechococcus) under continuous light conditions. The model proposed the importance of feedback regulation in which KaiC inhibits its own (kaiBC) transcription, while being enhanced by KaiA. However, the circadian rhythm of KaiC phosphorylation persisted for at least three cycles after the rapid disappearance of kaiABC mRNA under continuous dark conditions even in the presence of excess transcription and translation inhibitors.

The Synechococcus genome seems to be primarily regulated by light/dark cycles and is dramatically modified by the protein-based circadian oscillator. Although bacterial transcription has been considered to correlate well with proteomic profiles, this work suggests that there is a much greater discrepancy between the two profiles than previously thought.

Cyanobacterial daily life with Kai-based circadian and diurnal genome-wide transcriptional control in Synechococcus elongatus. PNAS USA July 30, 2009 doi: 10.1073/pnas.0902587106
In the unicellular cyanobacterium Synechococcus elongatus PCC 7942, essentially all promoter activities are under the control of the circadian clock under continuous light (LL) conditions. Here, we used high-density oligonucleotide arrays to explore comprehensive profiles of genome-wide Synechococcus gene expression in wild-type, kaiABC-null, and kaiC-overexpressor strains under LL and continuous dark (DD) conditions. In the wild-type strains, >30% of transcripts oscillated significantly in a circadian fashion, peaking at subjective dawn and dusk. Such circadian control was severely attenuated in kaiABC-null strains. Although it has been proposed that KaiC globally represses gene expression, our analysis revealed that dawn-expressed genes were up-regulated by kaiC-overexpression so that the clock was arrested at subjective dawn. Transfer of cells to DD conditions from LL immediately suppressed expression of most of the genes, while the clock kept even time in the absence of transcriptional feedback. Thus, the Synechococcus genome seems to be primarily regulated by light/dark cycles and is dramatically modified by the protein-based circadian oscillator.

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• Saturday Cinema: Cyanobacteria
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