Oscillating behavior of carbohydrate granule formation and dinitrogen fixation in the cyanobacterium Cyanothece sp. strain ATCC 51142

Abstract

It has been shown that some aerobic, unicellular, diazotrophic cyanobacteria temporally separate photosynthetic 02 evolution and oxygen-sensitive N2 fixation. Cyanothece sp. ATCC strain 51142 is an aerobic, unicellular, diazotrophic cyanobacterium that fixes N2 during discrete periods of its cell cycle. When the bacteria are maintained under diurnal light-dark cycles, N2 fixation occurs in the darlk Similar cycling is observed in continuous light, implicating a circadian rhythm. Under N2-fixing conditions, large inclusion granules form between the thylakoid membranes. Maximum granulation, as observed by electron microscopy, occurs before the onset of N2 fixation, and the granules decrease in number during the period of N2 fixation.The granules can be purified from cell homogenates by differential centrifugation. Biochemical analyses of the granules indicate that these structures are primarily carbohydrate, with some protein. Further analyses of the carbohydrate have shown that it is a glucose polymer with some characteristics of glycogen. It is proposed that N2 fixation is driven by energy and reducing power stored in these inclusion granules. Cyanothece sp. strain ATCC 51142 represents an excellent experimental organism for the study of the protective mechanisms of nitrogenase, metabolic events in cyanobacteria under normal and stress conditions, the partitioning of resources between growth and storage, and biological rhythms.Some cyanobacteria use energy acquired through oxygenic photosynthesis to drive N2 fixation. Nitrogenase, the enzyme that catalyzes the six-electron reduction of dinitrogen to ammonia, is notoriously sensitive to molecular oxygen, so much so that it seems incompatible with the oxygen-evolving nature of photosynthetic organisms. However, cyanobacteria have devised remarkably elegant methods of protecting nitrogenase from contacting oxygen, including spatial separation, temporal separation, and efficient enzyme systems that destroy reactive 02 by-products (8).Aerobic, unicellular, diazotrophic cyanobacteria exhibit temporal separation of oxygenic photosynthetic activities and oxygen-sensitive N2 fixation (8) Dinitrogen fixation is an energy-demanding process, requiring several ATPs per molecule of combined nitrogen produced (41). Thus, for N2 fixation to continue in the dark, without light-driven energy production or exogenous nutrient sources, the cells must utilize internal stores of energy and reducing power. In fact, it has been shown that, under diazotrophic conditions, the aerobic, unicellular cyanobacteria accumulate large quantities of carbohydrates (9, 24-26). These pools of photosynthate accumulate during a phase of rapid CO2 fixation in the light and are then apparently utilized during the dark phase to drive N2 fixation (and presumably for maintenance energy). Work with both Synechococcus sp. strain Miami BG 043511 (24, 25) and Gloeothece spp. (9,26) has demonstrated that carbohydrates accumulate toward the end of the light phase and dissipate in the dark in cultures grown diazot...

Genome-wide analysis of diel gene expression in the unicellular N2-fixing cyanobacterium Crocosphaera watsonii WH 8501

“…The expression of nitrogenase genes in Trichodesmium and in the intertidal or coastal marine unicellular cyanobacterium Cyanothece sp. strain 51142 has been shown to be under circadian control (Schneegurt et al, 1994;Chen et al, 1998;BermanFrank et al, 2001;Toepel et al, 2008). Genome-wide diel expression analysis may further elucidate the mechanism mediating the spatial/temporal segregation of oxygenic photosynthesis and N 2 fixation in diazotrophic cyanobacteria.…”

Convergent Evolution of Polysaccharide Debranching Defines a Common Mechanism for Starch Accumulation in Cyanobacteria and Plants

“…However, more recent studies revealed the existence of starch-like structures in unicellular diazotrophic cyanobacteria belonging to the order Chroococcales (Nakamura et al, 2005;Deschamps et al, 2008;Suzuki et al, 2013). The presence of anomalous glycogen particles had already been identified previously in this clade, while it is only very recently that this material was recognized as starch-like and the term "semiamylopectin" was coined to describe the major amylopectin-like fraction within these granules (Schneegurt et al, 1994;Suzuki et al, 2013). Four out of the six reported starch-accumulating cyanobacteria strains accumulate only this polysaccharide fraction.…”

“…Indeed, in glycogen, the Glc of the outer chains may be easier to mobilize following fluctuations of physiology both in light and darkness. The crystalline starch-like granules, in turn, will offer a larger pool of osmotically inert carbohydrate stores for delayed use by nitrogenase or cell division at night (Schneegurt et al, 1994). It will be of interest to check for the presence of an analogous fraction in other starch-storing cyanobacteria.…”

No Promoter Left Behind: Global Circadian Gene Expression in Cyanobacteria

“…This observation suggests that circadian cycles allow cellular processes to occur in a temporal sequence that is most beneficial to the organism. For example, some species of cyanobacteria (e.g., the genera Trichodesmium, Cyanothece, and Synechococcus) show a daily rhythm of photosynthesis that is in reverse phase to the daily cycle of nitrogenase expression, a key enzyme for nitrogen fixation Schneegurt et al, 1994;Huang et al, 1999). The peak of nitrogenase expression is during the dark phase of the cycle, allowing the temporal separation of expression of the oxygen-sensitive nitrogenase enzyme during periods when oxygen-evolving photosynthesis is not occurring.…”