Glutamine synthetase and nitrogen cycling in colonies of the marine diazotrophic cyanobacteria Trichodesmium spp

Distinct nitrogen cycling and steep chemical gradients in Trichodesmium colonies

Trichodesmium is an important dinitrogen (N₂)-fixing cyanobacterium in marine ecosystems. Recent nucleic acid analyses indicate that Trichodesmium colonies with their diverse epibionts support various nitrogen (N) transformations beyond N₂ fixation. However, rates of these transformations and concentration gradients of N compounds in Trichodesmium colonies remain largely unresolved. We combined isotope-tracer incubations, micro-profiling and numeric modelling to explore carbon fixation, N cycling processes as well as oxygen, ammonium and nitrate concentration gradients in individual field-sampled Trichodesmium colonies. Colonies were net-autotrophic, with carbon and N₂ fixation occurring mostly during the day. Ten percent of the fixed N was released as ammonium after 12-h incubations. Nitrification was not detectable but nitrate consumption was high when nitrate was added. The consumed nitrate was partly reduced to ammonium, while denitrification was insignificant. Thus, the potential N transformation network was characterised by fixed N gain and recycling processes rather than denitrification. Oxygen concentrations within colonies were ~60-200% air-saturation. Moreover, our modelling predicted steep concentration gradients, with up to 6-fold higher ammonium concentrations, and nitrate depletion in the colony centre compared to the ambient seawater. These gradients created a chemically heterogeneous microenvironment, presumably facilitating diverse microbial metabolisms in millimetre-sized Trichodesmium colonies.

Levels of daily light doses under changed day-night cycles regulate temporal segregation of photosynthesis and N2 Fixation in the cyanobacterium Trichodesmium erythraeum IMS101

While the diazotrophic cyanobacterium Trichodesmium is known to display inverse diurnal performances of photosynthesis and N₂ fixation, such a phenomenon has not been well documented under different day-night (L-D) cycles and different levels of light dose exposed to the cells. Here, we show differences in growth, N₂ fixation and photosynthetic carbon fixation as well as photochemical performances of Trichodesmium IMS101 grown under 12L:12D, 8L:16D and 16L:8D L-D cycles at 70 μmol photons m^-2 s^-1 PAR (LL) and 350 μmol photons m^-2 s^-1 PAR (HL). The specific growth rate was the highest under LL and the lowest under HL under 16L:8D, and it increased under LL and decreased under HL with increased levels of daytime light doses exposed under the different light regimes, respectively. N₂ fixation and photosynthetic carbon fixation were affected differentially by changes in the day-night regimes, with the former increasing directly under LL with increased daytime light doses and decreased under HL over growth-saturating light levels. Temporal segregation of N₂ fixation from photosynthetic carbon fixation was evidenced under all day-night regimes, showing a time lag between the peak in N₂ fixation and dip in carbon fixation. Elongation of light period led to higher N₂ fixation rate under LL than under HL, while shortening the light exposure to 8 h delayed the N₂ fixation peaking time (at the end of light period) and extended it to night period. Photosynthetic carbon fixation rates and transfer of light photons were always higher under HL than LL, regardless of the day-night cycles. Conclusively, diel performance of N₂ fixation possesses functional plasticity, which was regulated by levels of light energy supplies either via changing light levels or length of light exposure.

Distributions of iron, phosphorus and sulfur along trichomes of the cyanobacteria Trichodesmium

Changes in the elemental composition within trichomes of the nonheterocystous cyanobacteriaTrichodesmiumare potentially related to N₂-fixation.

Potential Trace Metal Co-Limitation Controls on N2 Fixation and [Formula: see text] Uptake in Lakes with Varying Trophic Status

The response of N2 fixation and [Formula: see text] uptake to environmental conditions and nutrient enrichment experiments in three western U.S. lake systems was studied (eutrophic Clear Lake; mesotrophic Walker Lake; oligotrophic Lake Tahoe). We tested the effect of additions of bioactive trace metals molybdenum as Mo(V) and iron (Fe) as well as phosphate (P), N2 fixation, [Formula: see text], carbon (C) fixation, chlorophyll a (Chla), and bacterial cell counts under both natural conditions and in mesocosm experiments. We found distinct background N2 fixation and [Formula: see text] uptake rates: highest at Clear Lake (N2 fixation: 44.7 ± 1.8 nmol N L(-1) h(-1)), intermediate at Walker Lake (N2 fixation: 1.7 ± 1.1 nmol N L(-1) h(-1); [Formula: see text] uptake: 113 ± 37 nmol N L(-1) h(-1)), and lowest at Lake Tahoe (N2 fixation: 0.1 ± 0.07 nmol N L(-1) h(-1); [Formula: see text] uptake: 37.2 ± 10.0 nmol N L(-1) h(-1)). N2 fixation was stimulated above control values with the addition of Fe and Pin Clear Lake (up to 50 and 63%, respectively); with Mo(V), Fe, and P in Walker Lake (up to 121, 990, and 85%, respectively); and with Mo(V) and P in Lake Tahoe (up to 475 and 21%, respectively). [Formula: see text] uptake showed the highest stimulation in Lake Tahoe during September 2010, with the addition of P and Mo(V) (∼84% for both). High responses to Mo(V) additions were also observed at some sites for C fixation (Lake Tahoe: 141%), Chla (Walker Lake: 54% and Clear Lake: 102%), and bacterial cell counts (Lake Tahoe: 61%). Overall our results suggest that co-limitation of nutrients is probably a common feature in lakes, and that some trace metals may play a crucial role in limiting N2 fixation and [Formula: see text] uptake activity, though primarily in non-eutrophic lakes.

Trichodesmium--a widespread marine cyanobacterium with unusual nitrogen fixation properties

The last several decades have witnessed dramatic advances in unfolding the diversity and commonality of oceanic diazotrophs and their N₂-fixing potential. More recently, substantial progress in diazotrophic cell biology has provided a wealth of information on processes and mechanisms involved. The substantial contribution by the diazotrophic cyanobacterial genus Trichodesmium to the nitrogen influx of the global marine ecosystem is by now undisputable and of paramount ecological importance, while the underlying cellular and molecular regulatory physiology has only recently started to unfold. Here, we explore and summarize current knowledge, related to the optimization of its diazotrophic capacity, from genomics to ecophysiological processes, via, for example, cellular differentiation (diazocytes) and temporal regulations, and suggest cellular research avenues that now ought to be explored.

Diazocyte development in the marine diazotrophic cyanobacterium Trichodesmium

The establishment of non-diazotrophic cultures of the filamentous marine cyanobacterium Trichodesmium erythraeum IMS101 enabled the first detailed investigation of the process leading to the development of its unique nitrogen-fixing cell type, the diazocyte. Trichome heterogeneity was apparent already within 3-8 h, while the differentiation of mature diazocytes, containing the nitrogenase enzyme, required 27 h after the removal of combined nitrogen. The distribution of 'pro-diazocytes' within the trichomes correlates with the localization of mature diazocytes, which suggests that pattern regulation is an early event during diazocyte development. The development was initially identified as changes in the subcellular ultrastructure, most notably the degradation of glycogen granules and gas vacuoles. These changes were preceded by the induced expression of the global nitrogen regulator ntcA at an early stage of combined nitrogen deprivation, followed by elevated expression of genes related to nitrogen metabolism and their corresponding proteins. The strongest induction (10-fold) was related to the transcription of the respiratory gene coxB2, apparent already at an early stage, which suggests an important role for respiration and the subsequent energy generation in the subcellular changes found, and in the creation of the reducing environment required for nitrogen fixation in diazocytes.

Amino Acid Cycling in Colonies of the Planktonic Marine Cyanobacterium Trichodesmium thiebautii

We examined diel trends in internal pools and net efflux of free amino acids in colonies of the nonheterocystous, diazotrophic cyanobacterium Trichodesmium thiebautii, freshly collected from waters of the Caribbean and the Bahamas. The kinetics of glutamate uptake by whole colonies were also examined. While intracolonial pools of most free amino acids were relatively constant through the day, pools of glutamate and glutamine varied over the diel cycle, with maxima during the early afternoon. This paralleled the daily cycle of nitrogenase activity. We also observed a large net release of these two amino acids from intact colonies. Glutamate release was typically 100 pmol of N colony h. This is about one-fourth the concurrent rate of N(2) fixation during the day. However, while nitrogenase activity only occurs during the day, net release of glutamate and glutamine persisted into the night and may therefore account for a greater loss of recently fixed N on a daily basis. This release may be an important route of new N input into tropical, oligotrophic waters. Whole colonies also displayed saturation kinetics with respect to glutamate uptake. The K(s) for whole colonies varied from 1.6 to 3.2 muM, or about 100-fold greater than typical ambient concentrations. Thus, uptake systems appear to be adapted to the higher concentrations of glutamate found within the intracellular spaces of the colonies. This suggests that glutamate may be a vehicle for N exchange among trichomes in the colony.

Temporal Variability in Nitrogenase Gene Expression in Natural Populations of the Marine Cyanobacterium Trichodesmium thiebautii

We report a distinct diel periodicity in the abundance of nifH (dinitrogenase reductase) mRNA in natural populations of the nonheterocystous marine cyanobacterium Trichodesmium thiebautii. Our observations show that in addition to translational and posttranslational controls, Trichodesmium nitrogenase expression is also regulated at the transcriptional and/or posttranscriptional level.

Coevolution of metal availability and nitrogen assimilation in cyanobacteria and algae

Marine primary producers adapted over eons to the changing chemistry of the oceans. Because a number of metalloenzymes are necessary for N assimilation, changes in the availability of transition metals posed a particular challenge to the supply of this critical nutrient that regulates marine biomass and productivity. Integrating recently developed geochemical, biochemical, and genetic evidence, we infer that the use of metals in N assimilation - particularly Fe and Mo - can be understood in terms of the history of metal availability through time. Anoxic, Fe-rich Archean oceans were conducive to the evolution of Fe-using enzymes that assimilate abiogenic NH(4)(+) and NO(2)(-). The N demands of an expanding biosphere were satisfied by the evolution of biological N(2) fixation, possibly utilizing only Fe. Trace O(2) in late Archean environments, and the eventual 'Great Oxidation Event' c. 2.3 Ga, mobilized metals such as Mo, enabling the evolution of Mo (or V)-based N(2) fixation and the Mo-dependent enzymes for NO(3)(-) assimilation and denitrification by prokaryotes. However, the subsequent onset of deep-sea euxinia, an increasingly-accepted idea, may have kept ocean Mo inventories low and depressed Fe, limiting the rate of N(2) fixation and the supply of fixed N. Eukaryotic ecosystems may have been particularly disadvantaged by N scarcity and the high Mo requirement of eukaryotic NO(3)(-) assimilation. Thorough ocean oxygenation in the Neoproterozoic led to Mo-rich oceans, possibly contributing to the proliferation of eukaryotes and thus the Cambrian explosion of metazoan life. These ideas can be tested by more intensive study of the metal requirements in N assimilation and the biological strategies for metal uptake, regulation, and storage.

DIAZOTROPHIC GROWTH OF THE MARINE CYANOBACTERIUM TRICHODESMIUM IMS101 IN CONTINUOUS CULTURE: EFFECTS OF GROWTH RATE ON N2 -FIXATION RATE, BIOMASS, AND C:N:P STOICHIOMETRY(1)

Trichodesmium N2 fixation has been studied for decades in situ and, recently, in controlled laboratory conditions; yet N2 -fixation rate estimates still vary widely. This variance has made it difficult to accurately estimate the input of new nitrogen (N) by Trichodesmium to the oligotrophic gyres of the world ocean. Field and culture studies demonstrate that trace metal limitation, phosphate availability, the preferential uptake of combined N, light intensity, and temperature may all affect N2 fixation, but the interactions between growth rate and N2 fixation have not been well characterized in this marine diazotroph. To determine the effects of growth rate on N2 fixation, we established phosphorus (P)-limited continuous cultures of Trichodesmium, which we maintained at nine steady-state growth rates ranging from 0.27 to 0.67 d(-1) . As growth rate increased, biomass (measured as particulate N) decreased, and N2 -fixation rate increased linearly. The carbon to nitrogen ratio (C:N) varied from 5.5 to 6.2, with a mean of 5.8 ± 0.2 (mean ± SD, N = 9), and decreased significantly with growth rate. The N:P ratio varied from 23.4 to 45.9, with a mean of 30.5 ± 6.6 (mean ± SD, N = 9), and remained relatively constant over the range of growth rates studied. Relative constancy of C:N:P ratios suggests a tight coupling between the uptake of these three macronutrients and steady-state growth across the range of growth rates. Our work demonstrates that growth rate must be considered when planning studies of the effects of environmental factors on N2 fixation and when modeling the impact of Trichodesmium as a source of new N to oligotrophic regions of the ocean.

Red tides in the Gulf of Mexico: Where, when, and why?

[1] Independent data from the Gulf of Mexico are used to develop and test the hypothesis that the same sequence of physical and ecological events each year allows the toxic dinoflagellate Karenia brevis to become dominant. A phosphorus-rich nutrient supply initiates phytoplankton succession, once deposition events of Saharan iron-rich dust allow Trichodesmium blooms to utilize ubiquitous dissolved nitrogen gas within otherwise nitrogen-poor sea water. They and the co-occurring K. brevis are positioned within the bottom Ekman layers, as a consequence of their similar diel vertical migration patterns on the middle shelf. Upon onshore upwelling of these near-bottom seed populations to CDOM-rich surface waters of coastal regions, light-inhibition of the small red tide of ~1 ug chl l(-1) of ichthytoxic K. brevis is alleviated. Thence, dead fish serve as a supplementary nutrient source, yielding large, self-shaded red tides of ~10 ug chl l(-1). The source of phosphorus is mainly of fossil origin off west Florida, where past nutrient additions from the eutrophied Lake Okeechobee had minimal impact. In contrast, the P-sources are of mainly anthropogenic origin off Texas, since both the nutrient loadings of Mississippi River and the spatial extent of the downstream red tides have increased over the last 100 years. During the past century and particularly within the last decade, previously cryptic Karenia spp. have caused toxic red tides in similar coastal habitats of other western boundary currents off Japan, China, New Zealand, Australia, and South Africa, downstream of the Gobi, Simpson, Great Western, and Kalahari Deserts, in a global response to both desertification and eutrophication.

Modeling the dynamic regulation of nitrogen fixation in the cyanobacterium Trichodesmium sp

A physiological, unbalanced model is presented that explicitly describes growth of the marine cyanobacterium Trichodesmium sp. at the expense of N(2) (diazotrophy). The model involves the dynamics of intracellular reserves of carbon and nitrogen and allows the uncoupling of the metabolism of these elements. The results show the transient dynamics of N(2) fixation when combined nitrogen (NO(3)(-), NH(4)(+)) is available and the increased rate of N(2) fixation when combined nitrogen is insufficient to cover the demand. The daily N(2) fixation pattern that emerges from the model agrees with measurements of rates of nitrogenase activity in laboratory cultures of Trichodesmium sp. Model simulations explored the influence of irradiance levels and the length of the light period on fixation activity and cellular carbon and nitrogen stoichiometry. Changes in the cellular C/N ratio resulted from allocations of carbon to different cell compartments as demanded by the growth of the organism. The model shows that carbon availability is a simple and efficient mechanism to regulate the balance of carbon and nitrogen fixed (C/N ratio) in filaments of cells. The lowest C/N ratios were obtained when the light regime closely matched nitrogenase dynamics.

Marine enzymes

Marine enzyme biotechnology can offer novel biocatalysts with properties like high salt tolerance, hyperthermostability, barophilicity, cold adaptivity, and ease in large-scale cultivation. This review deals with the research and development work done on the occurrence, molecular biology, and bioprocessing of marine enzymes during the last decade. Exotic locations have been accessed for the search of novel enzymes. Scientists have isolated proteases and carbohydrases from deep sea hydrothermal vents. Cold active metabolic enzymes from psychrophilic marine microorganisms have received considerable research attention. Marine symbiont microorganisms growing in association with animals and plants were shown to produce enzymes of commercial interest. Microorganisms isolated from sediment and seawater have been the most widely studied, proteases, carbohydrases, and peroxidases being noteworthy. Enzymes from marine animals and plants were primarily studied for their metabolic roles, though proteases and peroxidases have found industrial applications. Novel techniques in molecular biology applied to assess the diversity of chitinases, nitrate, nitrite, ammonia-metabolizing, and pollutant-degrading enzymes are discussed. Genes encoding chitinases, proteases, and carbohydrases from microbial and animal sources have been cloned and characterized. Research on the bioprocessing of marine-derived enzymes, however, has been scanty, focusing mainly on the application of solid-state fermentation to the production of enzymes from microbial sources.

Glutamine synthetase from the marine cyanobacteria Prochlorococcus spp: characterization, phylogeny and response to nutrient limitation

The regulation of glutamine synthetase (EC 6.3.1.2) from Prochlorococcus was previously shown to exhibit unusual features: it is not upregulated by nitrogen starvation and it is not inactivated by darkness (El Alaoui et al. (2001) Appl Environ Microbiol 67: 2202-2207). These are probably caused by adaptations to oligotrophic environments, as confirmed in this work by the marked decrease in the enzymatic activity when cultures were subjected to iron or phosphorus starvation. In order to further understand the adaptive features of ammonium assimilation in this cyanobacterium, glutamine synthetase was purified from two Prochlorococcus strains: PCC 9511 (high-light adapted) and SS120 (low-light adapted). We obtained approximately 100-fold purified samples of glutamine synthetase electrophoretically homogeneous, with a yield of approximately 30%. The estimated molecular mass of the subunits was roughly the same for both strains: 48.3 kDa. The apparent Km constants for the biosynthetic activity were 0.30 mM for ammonium, 1.29 mM for glutamate and 1.35 mM for ATP; the optimum pH was 8.0. Optimal temperature was surprisingly high (55 degrees C). Phylogenetic analysis of glnA from three Prochlorococcus strains (MED4, MIT9313 and SS120) showed they group closely with marine Synechococcus isolates, in good agreement with other studies based on 16 S RNA sequences. All of our results suggest that the structure and kinetics of glutamine synthetase in Prochlorococcus have not been significantly modified during the evolution within the cyanobacterial radiation, in sharp contrast with its regulatory properties.

Diurnal expression of hetR and diazocyte development in the filamentous non-heterocystous cyanobacterium Trichodesmium erythraeum

The marine non-heterocystous cyanobacterium Trichodesmium fixes atmospheric N(2) aerobically in light. In situ immunolocalization/light microscopy of NifH revealed that lighter, non-granulated cell regions observed correspond to the nitrogenase-containing diazocyte clusters in Trichodesmium IMS101. The number of diazocyte clusters per trichome varied from 0 to 4 depending on trichome length. The constant percentage of diazocytes (approx. 15 %) in cultured strains and five natural populations suggests a developmentally regulated differentiation process. Real-time RT-PCR showed that ntcA, encoding the global nitrogen regulator in cyanobacteria, and hetR, the key regulatory gene in heterocyst differentiation, are both constitutively expressed during a 12 h/12 h light/dark cycle. hetR in addition showed a distinct peak in the dark (close to midnight) while nifH expression commenced 6-8 h later. The expression of all three genes was negatively affected by addition of ammonia. Some early heterocyst differentiation genes were also identified in the genome of Trichodesmium. The data suggest that hetR and ntcA may be required for development and function of diazocytes in Trichodesmium.

The nitrogen physiology of the marine N2-fixing cyanobacteria Trichodesmium spp

Trichodesmium spp. have proved to be enigmatic organisms, and their ecology and physiology are unusual among diazotrophs. Recent research shows that they can simultaneously fix N2 and take up combined nitrogen. The co-occurrence of these two processes is thought to be incompatible, but they could be obligatory in Trichodesmium spp. if only a small fraction of cells within a colony or along a filament are capable of N2 fixation. Combined nitrogen is released from cells during periods of active growth and N2 fixation, and concomitantly taken up by Trichodesmium spp. or cells living in association with colonies. Although the nitrogenase of Trichodesmium spp. is affected by high concentrations of combined nitrogen, it might be relatively less sensitive to low concentrations of combined nitrogen typical of the oligotrophic ocean and culture conditions. Nitrogenase activity and synthesis exhibits an endogenous rhythm in Trichodesmium spp. cultures, which is affected by the addition of nitrogen.

Modification of the Fe Protein of Nitrogenase in Natural Populations of Trichodesmium thiebautii

The Fe protein of nitrogenase in the marine nonheterocystous cyanobacterium Trichodesmium thiebautii is interconverted between two forms, which is reminiscent of the ADP-ribosylation described in the purple bacterium Rhodospirillum rubrum. In natural populations of T. thiebautii during the day, when nitrogenase activity (NA) is present and while photosynthetic rates are high, a low-molecular-mass form of the Fe protein is present. In the late afternoon, the low-molecular-mass form is partially converted to a higher-molecular-mass form (approximately equal distribution of high- and low-molecular-mass forms of the Fe protein subunits), concurrent with cessation of NA. Some of the higher-molecular-mass form persists through the night until the very early morning, when the lower-molecular-mass form appears. New synthesis of both the Fe and MoFe proteins of nitrogenase appears to occur at this time. The higher-molecular-mass form of the Fe protein is also produced rapidly in response to artificially elevated external O 2 levels (40%) during the day. T. thiebautii is capable of recovery of NA in less than 1 h following exposure to 40% O 2 , which is correlated with the return of the Fe protein to the lower-molecular-mass form. Recovery from exposure to O 2 is not dependent upon protein synthesis. The modification of the Fe protein is clearly involved in regulation of NA during the diel cycle of NA in T. thiebautii but may also be involved in protecting the Fe protein during transient O 2 concentration increases.