HETEROTROPHY AND NITROGEN FIXATION IN CHLOROGLOEA FRITSCHII.

Nitrogen fixation is mediated by a variety of autotrophic and heterotrophic bacteria. Cyanobacteria appear responsible for most planktonic fixation in aquatic ecosystems, and rates of fixation are high only when these organisms make up a major percentage of the planktonic biomass. Planktonic nitrogen fixation tends to be low in oligotrophic and mesotrophic lakes ≪ 0.1 g N m−2 yr−1) but is often high in eutrophic lakes (0.2–9.2 g N m−2 yr−1).We found no data on planktonic nitrogen fixation in estuaries or coastal seas except for the Baltic Sea and for the Peel‐Harvey estuary in Western Australia. Fixation rates are quite high in the Peel‐Harvey estuary; rates are low offshore in Baltic waters but can be high near shore. As in lakes, fixation in these systems is associated with major blooms of planktonic, heterocystic cyanobacteria. However, nitrogen‐fixing cyanobacteria are much more abundant in the Baltic Sea and in the Peel‐Harvey estuary than in other estuaries or coastal waters. In most estuaries and coastal waters, species of nitrogen‐fixing cyanobacteria are absent or make up a very small percentage of the phytoplankton biomass (< 1%), suggesting insignificant amounts of nitrogen fixation. Annual average rates of nitrogen fixation reported for blooms of Trichodesmium and for Rhizosolenia mats in oceanic waters are also low.Unlike nitrogen fixation by planktonic organisms, there appear to be no major differences between freshwater and marine ecosystems with regard to fixation by benthic bacteria. Rates of nitrogen fixation in the sediments of most lakes and estuaries are low to moderate, generally <0.25 g N m−2 yr−1 except in extremely organic‐rich estuarine sediments. In estuarine sediments which are organic‐rich, nitrogen fixation rates range from 0.4 to 1.6 g N m−2 yr−1. Most benthic fixation in mesotrophic and eutrophic lakes and estuaries is mediated by heterotrophic (and perhaps chemoautotrophic) bacteria, but benthic fixation in oligotrophic ecosystems is often dominated by cyanobacteria. Rates of nitrogen fixation in freshwater and marine wetlands and seagrass beds appear similar to or somewhat greater than those in nonvegetated, organic‐rich sediments. Rates of fixation in cyanobacterial mats are high to very high (1.3–76 g N m−2 yr−1), but these mats usually cover only a small area of the ecosystems in which they reside, limiting the importance of fixation in the mats to the mats themselves.The importance of nitrogen fixation to the nitrogen economy of aquatic ecosystems is quite variable. For example, fixation by planktonic organisms appears unimportant as a nitrogen source to most oligotrophic and mesotrophic lakes (generally <1% of total nitrogen inputs) but accounts for 6–82% of the nitrogen inputs to eutrophic lakes. Planktonic fixation provides ≪ 1% of the nitrogen inputs to surface waters of the world’s oceans and is probably also of little importance in most estuaries, including eutrophic estuaries. However, planktonic fixation provides >20% of the nitrogen input to the Asko region of the Baltic Sea and 17% of the nitrogen input to the Peel‐Harvey estuary in Australia.Fixation in sediments of estuaries and eutrophic and mesotrophic lakes usually constitutes a small percentage of the nitrogen inputs to these systems. However, benthic fixation appears to be a major source of nitrogen for many oligotrophic tropical lagoons and for some oligotrophic lakes, even though fixation rates are moderate because other nitrogen inputs tend to be low. Nitrogen fixation probably is a fairly minor input of nitrogen to marine wetlands, which are generally open to other inputs, but contributes roughly half the total nitrogen input to some freshwater wetlands (bogs, cypress domes), where other inputs are more limited.Nitrogen fixation appears important in making up deficits in nitrogen availability relative to phosphorus availability in many lakes, contributing to the phosphorus‐limited status of these systems. That many estuaries and coastal seas are nitrogen limited is due in part to the generally low rates of nitrogen fixation found in these systems.

We estimated the effects of temperature, moisture, and oxygen concentration on nitrogen fixation and respiration in woody debris and used this information to model seasonal variation in these processes. We measured acetylene reduction and CO2 evolution of wood samples to determine the relative effect of these abiotic factors on nitrogen fixation and respiration. The interactions of these abiotic factors were examined in a model to test whether temperature alone can be used as a predictor of seasonal changes in nitrogen fixation and respiration rates in woody debris. Nitrogen fixation rates were optimum near 30ºC, whereas respiration rates were optimum over a broader range, from 30°C to 50°C. Nitrogen fixation and respiration rates were greatest above 175% and 100% wood moisture content, respectively, with little activity below 50%. Nitrogen fixation was optimum at 2% O2, with activity much reduced above and below this concentration. Respiration was optimal when O2 exceeded 1%. In our simulations, annual nitrogen fixation and respiration rates were 7.8 and 1.7 times greater, respectively, when only temperature limitation was included than when moisture and oxygen limitations were also included. Therefore, seasonal interactions of abiotic factors need to be considered when estimating annual nitrogen fixation and respiration rates.

Field and laboratory studies are described on nitrogen fixation (acetylene reduction) and 14CO2 fixation during summer by a population of Rivularia growing in Red Sike, a stream in Upper Teesdale, northern England. An in situ diel study of carbon dioxide fixation showed a significant correlation (r = 0.84, P < 0.01) between fixation rates and light flux, but a relatively low correlation between nitrogen fixation and light flux (r = 0.23, P > 0.05). Although the rates of nitrogen fixation were much higher by day than night, between 8 and 16% of total activity occurred at night (5–8 h dark); a small nocturnal peak was evident in one of the three diel surveys. When colonies were shaded to reduce incident light to 15% of full irradiation, total daytime activity was doubled. This effect was also shown clearly in a laboratory study. Carbon dioxide fixation decreased with decreasing light intensity, whereas nitrogen fixation showed a bimodal response to light, with a low intensity peak at a photon flux density of about 10 μmol m−2 s−1. The highest ratio of nitrogen: carbon fixation occurred at this very low light level. Our hypothesis suggests that at high light fluxes nitrogen fixation may compete for energy with carbon dioxide fixation, whereas at low light fluxes it may use both most of the light-dependent energy as well as respiratory energy mobilizing glucan reserves synthesized under high light fluxes. This study showed that about 400 mol of CO2 to 1 mol of N2 was fixed by Rivularia during daylight. Nitrogen fixation may supply only a small percentage of the alga's nitrogen requirements, at least for the medium-sized (3–4 mm diameter) colonies studied.

Spatial and temporal variations in nitrogen fixation and denitrification rates were examined between July 1991 and September 1992 in the intertidal regions of Tomales Bay (California, USA). Microbial mat communities inhabited exposed mudflat and vegetated marsh surface sediments. Mudflat and marsh sediments exhibited comparable rates of nitrogen fixation. Denitrification rates were higher in marsh sediments. Nitrogen fixation rates were lowest during January at both sites, whereas highest rates occurred during summer and fall. Denitrification rates were highest during fall and winter months in marsh sediments, while rates in mudflat sediments were highest during summer and fall. In mudflat sediments, nitrogen fixation and denitrification rates, integrated over 24 h, ranged from 6 to 79 mg N m-1 d-1 and 1 to 10 mg N m-2 d-1, respectively. Rates of denitrification represented between 6 and 20% of nitrogen fixation rates during the day, but exceeded or were equivalent to nitrogen fixation rates at night. The highest integrated rates of both nitrogen fixation and denitrification occurred during July, whereas, the highest percent loss occurred during spring when denitrification rates amounted to 20% of nitrogen fixation rates during the day. Over an annual cycle, inputs of fixed N to mudflat communities occurred exclusively during daylight. These results underscore the importance of determining integrated diel rates of both nitrogen fixation and denitrification when constructing N budgets. Using this approach, it was shown that microbial denitrification can represent a significant loss of combined nitrogen from mats on daily as well as monthly time scales.

Rates of biological nitrogen fixation in two Wisconsin lakes arc given. In Lake Wingra, fixation continues at moderate rates throughout much of the ice‐free season. In Lake Mendota, the rate is normally zero throughout the ice‐free season, but occasional, sometimes high, rates are observed. Auxiliary experiments on the effects of incubation time, added glucose, added combined nitrogen, light intensity, and temperature on nitrogen fixation in lake water are discussed. The effect of incubation time on the rate of fixation varies from time to time. Glucose had no consistent effect on nitrogen fixation in the dark, although stimulation was observed in some experiments. In the light, glucose appeared to inhibit fixation. The effects of added ammonia and nitrate were inconsistent. At times, various added concentrations, particularly of ammonia, increased rates of nitrogen fixation. Rates of nitrogen fixation varied with light intensity. The rate increased to a maximum at 3,750 lux, then declined as the intensity increased further. Rates of nitrogen fixation increased at a constant rate with temperature in the range 16–30C.

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 432:17-29 (2011) - DOI: https://doi.org/10.3354/meps09147 Addition of inorganic or organic phosphorus enhances nitrogen and carbon fixation in the ­oligotrophic North Pacific Katie S. Watkins-Brandt1, Ricardo M. Letelier1, Yvette H. Spitz1, Matthew J. Church2, Daniela Böttjer2, Angelicque E. White1,* 1College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, Oregon, USA 2Department of Oceanography, School of Ocean and Earth Science and Technology, University of Hawai’i at Mãnoa, Honolulu, Hawaii, USA *Corresponding author. Email: awhite@coas.oregonstate.edu ABSTRACT: We present a spatially extensive record of dinitrogen (N2) fixation rates and distributions of N2 fixing microorganisms along with the results of exogenous phosphorus (P) addition experiments conducted during a series of cruises in the North Pacific Subtropical Gyre (NPSG). We measured the N2 and carbon (C) fixation rates of natural plankton assemblages in response to the addition of methylphosphonate (MPn), a dissolved organic phosphorus (DOP) compound, and dissolved inorganic phosphorus (DIP). Results are compared to parallel unamended controls. These experiments produced 3 major findings: (1) MPn and DIP were utilized with equal metabolic efficiency over a single photoperiod, (2) the bulk of the enhanced N2 fixation rates were within the range of those previously reported in the NPSG, suggesting that P levels in this region can be saturating but were not at the time of sampling and (3) MPn and DIP additions stimulated C fixation rates beyond estimated contributions by diazotrophs, and hence both DIP and bioavailable DOP additions could lead to enhancement of net primary productivity on short time-scales. Our results suggest that the rate of N2 fixation in our study region may have been restricted by the availability and/or the composition of the total P pool (inorganic and organic P) during our field season. KEY WORDS: Phosphorus · Nitrogen fixation · Dissolved organic matter Full text in pdf format PreviousNextCite this article as: Watkins-Brandt KS, Letelier RM, Spitz YH, Church MJ, Böttjer D, White AE (2011) Addition of inorganic or organic phosphorus enhances nitrogen and carbon fixation in the ­oligotrophic North Pacific. Mar Ecol Prog Ser 432:17-29. https://doi.org/10.3354/meps09147 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 432. Online publication date: June 27, 2011 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2011 Inter-Research.

ABSTRACTWe determined rates of decomposition and asymbiotic nitrogen fixation in the leaf litter of Cheirodendron spp. on the Hawaiian Islands. Leaf litter was collected from four sites on a long soil‐age gradient (300 yr to 4.1 M yr) and decomposed at two sites that differed widely in substrate age and nutrient availability. Rates of decomposition were higher in litter decomposed at the older site, where nutrient availability was greater. A substantial amount of nitrogen and phosphorus immobilization occurred in litter decomposed at the older site, with more immobilization occurring in litter with lower initial nitrogen and phosphorus concentrations, suggesting both supply and demand controls on nutrient immobilization. Potential rates of nitrogen fixation were very low in the first 25 d (0–5 nmol acetylene/gdw/h), rose to much higher rates by 70 d (20–45 nmol), and then declined by 140 d. We found no significant difference in rates of potential nitrogen fixation between sites of decomposition, but there was a strong substrate effect, with higher rates in litter with low lignin, low nitrogen, and high phosphorus. Where significant immobilization of nitrogen occurred for decomposing Cheirodendron, nitrogen fixation could have comprised no more than 10 percent of immobilized nitrogen. Overall, rates of nitrogen fixation were dependent on the source of the decomposing substrate but not on the site of decomposition, while short‐term decomposition and nutrient immobilization were strongly dependent on the site of decomposition but not as much on the source of the decomposing substrate.

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 359:25-36 (2008) - DOI: https://doi.org/10.3354/meps07241 Nitrogen fixation and growth rates of Trichodesmium IMS-101 as a function of light intensity E. Breitbarth1,2,*, J. Wohlers1, J. Kläs1, J. LaRoche1, I. Peeken1 1Leibniz-Institute of Marine Sciences, IFM-GEOMAR, Düsternbrooker Weg 20, 24105 Kiel, Germany 2Present address: Department of Chemistry, Analytical and Marine Chemistry, University of Gothenburg, Kemivägen 10, 412 96 Göteborg, Sweden *Email: eike@chem.gu.se ABSTRACT: The diazotrophic cyanobacterium Trichodesmium is a significant contributor to marine nitrogen and carbon cycles and has been incorporated in biogeochemical ocean circulation models. To date, parameterization of light as a controlling factor for nitrogen fixation has been based on field observations, where factors other than light also affect Trichodesmium physiology. Here we present data on light-dependent (15 to 1100 µmol quanta m–2 s–1) diazotrophic growth from controlled laboratory experiments and their implications for modeling approaches. We supply a simple empirical model to describe nitrogen fixation by Trichodesmium in batch cultures. Diazotrophic growth of axenic Trichodesmium IMS-101 was light saturated at 180 µmol quanta m–2 s–1 and did not vary significantly at higher photon irradiances up to 1100 µmol quanta m–2 s–1 (μcarbon based ≈ 0.26 d–1). Chlorophyll a (chl a) normalized N2 fixation rates were significantly affected by light intensity during mid-exponential growth (0.74 to 4.45 mol N fixed mol chl a–1 h–1) over the range of photon irradiances tested. In contrast, nitrogen fixation rates normalized to the cellular carbon content were relatively unaffected by light intensity (0.42 to 0.59, averaging 0.5 mmol N mol particulate organic carbon [POC]–1 h–1). Trichodesmium carbon biomass can be used to estimate the nitrogen input by this diazotroph into the ocean; the maximum input rate is 350 nmol N fixed l–1 h–1. KEY WORDS: Trichodesmium · Light · Nitrogen fixation · Marine nitrogen cycle · Marine carbon cycle · Marine cyanobacteria · Diazotrophic growth Full text in pdf format Supplementary appendix PreviousNextCite this article as: Breitbarth E, Wohlers J, Kläs J, LaRoche J, Peeken I (2008) Nitrogen fixation and growth rates of Trichodesmium IMS-101 as a function of light intensity. Mar Ecol Prog Ser 359:25-36. https://doi.org/10.3354/meps07241Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 359. Online publication date: May 05, 2008 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2008 Inter-Research.

Short-term manometric experiments with bacteria-free cultures of Anabaena cylindrica showed that the close dependency of nitrogen fixation upon photosynthesis could be temporarily eliminated in nitrogen-starved cells. Initial rates of nitrogen uptake by these cells in the absence of carbon dioxide were equally rapid in the light and dark, decreasing and finally ceasing after two hours. Continued steady nitrogen uptake was only maintained for long periods in the presence of carbon dioxide in the light. In the dark, nitrogen uptake was accompanied by carbon dioxide evolution. More oxygen was evolved in the light by cells fixing nitrogen than by those incubated under argon. This additional oxygen evolution could be accounted for by extra carbon dioxide fixation in the presence of nitrogen. Of a number of organic compounds tested, only sodium pyruvate stimulated nitrogen fixation. This stimulation was achieved both in the light and dark and in the presence and absence of carbon dioxide, showing that the role of pyruvate was other than acting as a carbon skeleton. Three metabolic inhibitors, cyanide and chlorpromazine (chiefly respiratory) and phenylurethane (photosynthetic) differentially inhibited photosynthesis and nitrogen fixation. The latter inhibitor had a more marked effect on photosynthesis while the two chiefly respiratory inhibitors had a stronger effect on nitrogen fixation.

Four cultivars of chickpea, two of them of Mediterranean origin (kabuli), CSG 9651, BG 267 and two Indian (desi) types, CSG 8962, DCP 92-3, differing in their salt sensitivities were identified after screening ten genotypes in saline soils. The cultivars CSG 9651 and CSG 8962 were salt tolerant while BG 267 and DCP 92-3 were salt sensitive, respectively. The seeds of different cultivars were inoculated with Mesorhizobium ciceri, strain F: 75 and the plants were grown in the greenhouse. After the establishment of symbiosis, 15-day-old seedlings were administered doses of saline at varying concentrations (0, 4, 6, 8 dSm-1 NaCl, Na2SO4, CaCl2). Plants were harvested at 40, 70 and 100 days after sowing, for analyses. The main aim was to compare the relative salt tolerance of both desi and kabuli cultivars in terms of nitrogen fixation and carbon metabolism, as well as to ascertain whether the negative effects of saline stress on nitrogen fixation were due to a limitation of photosynthate supply to the nodule or to a limitation on the nodular metabolism that sustains nitrogenase activity. Plant growth, nodulation and nitrogenase activity was more severely affected in BG 267 and DCP 92-3 under salinity treatments (6 and 8 dSm-1) compared with CSG 9651 and CSG 8962. Nodule number as well as nodule mass increased under salt stress in CSG 9651 and CSG 8962 which might be responsible for their higher nitrogen fixation. Salinity reduced leaf chlorophyll and Rubisco activities in all cultivars. However, tolerant cultivars CSG 9651 and CSG 8962 showed smaller declines than the sensitive ones. Phosphoenolpyruvate carboxylase (PEPCase) activity increased significantly in the nodules of tolerant cultivars under salt stress at all harvests, and this was clearly related to salt concentrations. Our results suggest that in salt-affected soils tolerant cultivars have more efficient nodulation and support higher rates of symbiotic nitrogen fixation than the sensitive cultivars.

In marine wetlands, nitrogen fixation is a potentially important nutrient source for nitrogen‐limited primary producers, but interactions between nitrogen fixers and different vascular plant species are not fully understood. Nitrogen fixation activity was compared in sediments vegetated by three plant species,Spartina foliosa,Salicornia virginica, andSalicornia bigeloviiin the Kendall Frost Reserve salt marsh in Mission Bay (CA). This study addressed the effects of plant type, day and night conditions, and sediment depths on nitrogen fixation. Higher rates of nitrogen fixation were associated withS. foliosathan with either of the twoSalicorniaspp., which are known to compete more effectively thanSpartinafor exogenous nitrogen in the salt marsh environment. Rates of nitrogen fixation, determined by acetylene reduction, in sediments vegetated byS. virginicawere low during the day (7.7 ± 1.2 μmol C2H4 m−2 h−1) but averaged 13 ± 6.6 μmol C2H4 m−2 h−1at night, with particularly high rates in samples from locations with visible cyanobacterial mats. The opposite diel pattern was found for sediments containingS. foliosaplants, in which average daytime and nighttime rates of nitrogen fixation were 62 ± 23 and 21 ± 15 μmol C2H4 m−2 h−1, respectively. ForS. foliosa, nitrogenase activity of rinsed roots and different sediment sections (0–1, or 4–5 cm depths) were measured. Although nitrogen fixation rates in vegetated sediment samples were substantial, all but one of rinsedS. foliosaroot samples (n = 12) and subsurface sediments at 4–5 cm depths failed to show nitrogen fixation activity after 2 h, suggesting that the most active nitrogen fixers in these systems likely reside in surface sediments. Further, nitrogenase activity in shaded and unshadedS. foliosasamples did not differ, suggesting that nitrogen fixers may not rapidly respond to changes in plant photosynthetic activity. Average nitrogen fixation rates inS. foliosa‐vegetated samples from the Mission Bay salt marsh were on the same order as those of highly productive Atlantic coast marshes, and this microbially‐mediated nitrogen source may be similarly substantial in other Mediterranean wetlands. Sediment abiotic variables seem to exert greater control upon nitrogen fixation activity than the effects of particular plant species. Nonetheless, dominant plant species may differ substantially in their reliance on nitrogen fixation as a nutrient source, with potentially important consequences for wetland conservation and restoration.

Purple nonsulfur photosynthetic bacteria have metabolic pathways for the fixation of both carbon and nitrogen and show great potential as sustainable hosts for the production of materials such as amino acids, carotenoids, and bioplastics. They are predominantly mixotrophs capable of utilizing both organic and inorganic carbon sources for growth. However, most past studies on purple photosynthetic bacteria have focused on nitrogen fixation pathways independent of the extent of carbon fixation. In this study, we analyzed the nitrogen fixation ability of Rhodovulum sulfidophilum, a purple nonsulfur photosynthetic bacterium commonly found in estuaries where organic carbon and sulfur compounds are relatively abundant, by monitoring the uptake and conversion of 15N-labeled N2 gas to amino acids under autotrophic or heterotrophic conditions. Greater growth and 15N uptake were observed in the heterotrophic carbon metabolic state. Regardless of the type of amino acid, the efficiency of 15N uptake was greater under heterotrophic conditions, with a difference of approximately 2.1–2.6-fold observed on the third day of culture. This also corresponded to the growth rates under the different conditions. Therefore, our current findings confirm the nitrogen fixation ability of R. sulfidophilum under both heterotrophic and autotrophic conditions, with the heterotrophic mode favoring greater nitrogen fixation and assimilation.

Nitrogen fixation rates of the globally distributed, biogeochemically important marine cyanobacterium Trichodesmium increase under high carbon dioxide (CO2) levels in short-term studies due to physiological plasticity. However, its long-term adaptive responses to ongoing anthropogenic CO2 increases are unknown. Here we show that experimental evolution under extended selection at projected future elevated CO2 levels results in irreversible, large increases in nitrogen fixation and growth rates, even after being moved back to lower present day CO2 levels for hundreds of generations. This represents an unprecedented microbial evolutionary response, as reproductive fitness increases acquired in the selection environment are maintained after returning to the ancestral environment. Constitutive rate increases are accompanied by irreversible shifts in diel nitrogen fixation patterns, and increased activity of a potentially regulatory DNA methyltransferase enzyme. High CO2-selected cell lines also exhibit increased phosphorus-limited growth rates, suggesting a potential advantage for this keystone organism in a more nutrient-limited, acidified future ocean.

Cyanobacteria are oxygenic photosynthetic prokaryotes with important roles in the global carbon and nitrogen cycles. Unicellular nitrogen-fixing cyanobacteria are known to be ubiquitous, contributing to the nitrogen budget in diverse ecosystems. In the unicellular cyanobacterium Cyanothece sp. strain ATCC 51142, carbon assimilation and carbohydrate storage are crucial processes that occur as part of a robust diurnal cycle of photosynthesis and nitrogen fixation. During the light period, cells accumulate fixed carbon in glycogen granules to use as stored energy to power nitrogen fixation in the dark. These processes have not been thoroughly investigated, due to the lack of a genetic modification system in this organism. In bacterial glycogen metabolism, the glgX gene encodes a debranching enzyme that functions in storage polysaccharide catabolism. To probe the consequences of modifying the cycle of glycogen accumulation and subsequent mobilization, we engineered a strain of Cyanothece 51142 in which the glgX gene was genetically disrupted. We found that the ΔglgX strain exhibited a higher growth rate than the wild-type strain and displayed a higher rate of nitrogen fixation. Glycogen accumulated to higher levels at the end of the light period in the ΔglgX strain, compared to the wild-type strain. These data suggest that the larger glycogen pool maintained by the ΔglgX mutant is able to fuel greater growth and nitrogen fixation ability.IMPORTANCE Cyanobacteria are oxygenic photosynthetic bacteria that are found in a wide variety of ecological environments, where they are important contributors to global carbon and nitrogen cycles. Genetic manipulation systems have been developed in a number of cyanobacterial strains, allowing both the interruption of endogenous genes and the introduction of new genes and entire pathways. However, unicellular diazotrophic cyanobacteria have been generally recalcitrant to genetic transformation. These cyanobacteria are becoming important model systems to study diurnally regulated processes. Strains of the Cyanothece genus have been characterized as displaying robust growth and high rates of nitrogen fixation. The significance of our study is in the establishment of a genetic modification system in a unicellular diazotrophic cyanobacterium, the demonstration of the interruption of the glgX gene in Cyanothece sp. strain ATCC 51142, and the characterization of the increased nitrogen-fixing ability of this strain.

Seasonal distribution, nitrogen fixation and primary productivity of Trichodesmium species were intermittently studied in the coastal waters of Tanzania. Samples were collected in 1975/6, 1980, 1993/4, 1994/5 and 1998/9. Four colony forming species were found, i.e. T. erythraeum, T. tenue, T. thiebautii and one unidentified Trichodesmium sp. while T. contortum was rarely encountered, and only as individual trichomes. T. erythraeum was most abundant, although other Trichodesmiumspecies dominated during particular periods of the year. The occurrence of Trichodesmium showed a consistent seasonal pattern. High Trichodesmium biomass was promoted by the NE monsoon (December–April) while it was low or absent during the SE monsoon (June–October). The biomass was highest at the surface especially during calm weather periods. The NE monsoon was characterized by elevated rainfall, temperature and nitrate concentrations while salinity, light intensity and turbidity tended to decrease. Phosphate concentrations did not show systematic variations with season. The rate of nitrogen fixation by Trichodesmium species in surface waters was 1.8 ± 1.6 pmol N trichome−1 h−1 giving an average N2 fixation of 42.7 mmol N m−3y−1. The mean rate of carbon fixation was 1.15 ± 0.3 ng C trichomes−1 h−1 in the upper 5 m depth. It is estimated that Trichodesmium contributes about 0.03–20% of the total CO2 fixation in the coastal surface waters during the SE and NE monsoon, respectively.