Use of Genetically Improved Nitrogen-Fixing Cyanobacteria in Rice Paddy Fields: Prospects as a Source Material for Engineering Herbicide Sensitivity and Resistance in Plants

Cyanobacteria are the largest and most widely distributed group of photosynthetic prokaryotes on Earth, forming a prominent component of microbial populations in wetland soils, especially in rice paddy fields, and significantly contributing to fertility as a natural biofertilizer. Modern agricultural fields are generally treated with high doses of synthetic nitrogenous fertilizers and pesticides having adverse effects on the soil microflora of naturally occurring N₂-fixing cyanobacteria. This review deals with some of the advances made during the last few decades in the areas of developing ammonia derepressible pesticide-resistant cyanobacterial mutants for algalization of the wet agricultural fields as a viable and efficient N-photobiofertilizer.     

Nitrogen-Fixing (Acetylene Redution) Activity and Population of Aerobic Heterotrophic Nitrogen-Fixing Bacteria Associated with Wetland Rice

Nitrogen-fixing activity associated with different wetland rice varieties was measured at various growth stages by an in situ acetylene reduction method after the activities of blue-green algae (cyanobacteria) in the flood water and on the lower portion of the rice stem were eliminated. Nitrogen-fixing activities associated with rice varieties differed with plant growth stages. The activities increased with plant age, and the maximum was about at heading stage. The nitrogen fixed during the whole cropping period was estimated at 5.9 kg of N per ha for variety IR26 (7 days) and 4.8 kg of N per ha for variety IR36 (95 days). The population of aerobic heterotrophic N₂-fixing bacteria associated with rice roots and stems was determined by the most-probable-number method, using semisolid glucose-yeast extract and semisolid malate-yeast extract media. The addition of yeast extract to the glucose medium increased the number and activity of aerobic heterotrophic N₂-fixing bacteria. The glucose-yeast extract medium gave higher counts of aerobic N₂-fixing bacteria associated with rice roots than did the malate-yeast extract medium, on which Spirillum-like bacteria were usually observed. The lower portion of the rice stem was also inhabited by N₂-fixing bacteria and was an active site of N₂ fixation.     

Urban agricultural land use and characterization of mosquito larval habitats in a medium-sized town of C?te d'Ivoire.

Urban agriculture is common across Africa and contributes to the livelihoods of urban dwellers. Some crop systems create suitable mosquito breeding sites and thus might affect malaria transmission. The purpose of this study was to identify, map, and characterize potential mosquito breeding sites in agricultural land use zones in a medium-sized town of western C?te d'Ivoire and to assess risk factors for productive Anopheles breeding sites. Two surveys were carried out; one toward the end of the rainy season and the second one during the dry season. In all identified potential mosquito breeding sites, two experienced entomologists searched for the presence of Anopheles larvae and pupae with a standardized technique. Totals of 369 and 589 sites were found in the rainy and dry seasons, respectively, mainly in vegetable gardens and irrigated rice fields. Anopheles larvae were present in 50.7% and 42.4% of the sites investigated during the rainy and dry seasons, respectively. Typical Anopheles larval habitats were characterized by the presence of algae, the absence of floating vegetation, and the co-occurrence of Culex larvae. The highest Anopheles larval productivity was observed in rice paddies, agricultural trenches between vegetable patches, and irrigation wells. An indirect link could be established between the occurrence of productive Anopheles breeding sites and agricultural land use through specific man-made habitats, in particular agricultural trenches, irrigation wells, and rice paddies. Our findings have important bearings for the epidemiology and control of urban malaria in sub-Saharan Africa.     

新垦红壤坡地土壤水分有效性研究

针对南方红壤地区降雨时空分布不均的特点,以未开垦的自然植被为对照,对桂西北环境移民示范区不同季节(早季和雨季)一次性降雨前1d及降雨后4h、2d、4d、6d及8d新垦蔗地(中坡、下坡、谷地）0-20cm,20-40cm、40-60cm3个土层的土壤水分含量进行了测定．结果表明,雨后谷地蔗地的土壤有效水分增量几乎与降雨量相同,而中坡蔗地与未开垦的自然植被土壤有效水分增量仅相当于降雨量于80％．雨季雨后0-60cm土壤层次中土壤有效水分分布均匀,早季主要集中在表层．雨季一次性降雨后各哩理及各土层土壤有效水分饱和度均有显著差异;而早季3个新垦蔗地间无明显差异,3个土层以表层土壤有效水分饱和度最高,亚表层与心土层差异不明显．无论雨季还是旱季,0-60cm土层土壤有效水分的消耗速率都以自然植被处理为最低,由于雨季正是作物生长旺季,其0-60cm土层土壤有效水分的消耗速率比旱季快,按照早季雨后8d土壤有效水分的平均消耗速率,15d内0-60cm土层的有效水分将消耗殆尽。     

Contribution of N2 fixing cyanobacteria to rice production: availability of nitrogen from 15N-labelled cyanobacteria and ammonium sulphate to rice

This study investigate the potential contribution of nitrogen fixation by indigenous cyanobacteria to rice production in the rice fields of Valencia (Spain). N₂-fixing cyanobacteria abundance and N₂ fixation decreased with increasing amounts of fertilizers. Grain yield increased with increasing amounts of fertilizers up to 70 kg N ha^-1. No further increase was observed with 140 kg N ha^-1. Soil N was the main source of N for rice, only 8–14% of the total N incorporated by plants derived from ¹⁵N fertilizer. Recovery of applied ¹⁵N-ammonium sulphate by the soil–plant system was lower than 50%. Losses were attributed to ammonia volatilization, since only 0.3–1% of applied N was lost by denitrification. Recovery of ¹⁵N from labeled cyanobacteria by the soil–plant system was higher than that from chemical fertilizers. Cyanobacterial N was available to rice plant even at the tillering stage, 20 days after N application. This revised version was published online in June 2006 with corrections to the Cover Date.     

Biological nitrogen fixation: Investments,expectations and actual contributions to agriculture

Inputs of biologically fixed N into agricultural systems may be derived from symbiotic relationships involving legumes and Rhizobium spp., partnerships between plants and Frankia spp. or cyanobacteria, or from non-symbiotic associations between free-living diazotrophs and plant roots. It is assumed that these N₂-fixing systems will satisfy a large portion of their own N requirements from atmospheric N₂, and that additional fixed N will be contributed to soil reserves for the benefit of other crops or forage species. This paper reviews the actual levels of N₂ fixation attained by legume and non-legume associations and assesses their role as a source of N in tropical and sub-tropical agriculture. We discuss factors influencing N₂ fixation and identify possible strategies for improving the amount of N₂ fixed.     

Population dynamics of Meloidogyne incognita on black pepper plants in two agro-ecological regions in Vietnam

The objective of our study was to examine the effect of local environmental conditions, especially air temperature and rainfall, in two agro-ecological regions in Vietnam on the population dynamics of Meloidogyne incognita on black pepper plants and on percentage root galling. The two study sites were situated in Cam Lo, Quang Tri province (North Central Coast) and Buon Ma Thuot, Dac Lac province (Central Highlands). At the study sites, 13 plant-parasitic nematode taxa belonging to 12 genera were identified. Ten of these taxa were present in both study sites. Helicotylenchus certus, Hemicriconemoides cocophilus and Pratylenchus coffeae were only found in Buon Ma Thuot. Based on nematode population densities, M. incognita was the most abundant taxon present. Rainfall and air temperature differed significantly between the two study sites. The most important climatic difference was rainfall: in Buon Ma Thuot, the rainy season lasted 2 months longer and the monthly rainfall during the rainy season was much higher compared to Cam Lo. Although this difference resulted in some differences in the population dynamics of M. incognita in the soil and roots of the black pepper variety Vinh Linh, the highest root population densities were observed in both study sites during the first half of the dry season. In Cam Lo, the highest percentage root galling was observed during the first half of the dry season. In Buon Ma Thuot, the highest percentage root galling was observed towards the end of the rainy season. Estimating the population densities if M. incognita to decide on the application of a management strategy should be done during the first half of the dry season.     

Cyanobacterial biofertilizers in rice agriculture

Floodwater and the surface of soil provide the sites for aerobic phototrophic nitrogen (N) fixation by free-living cyanobacteria and theAzolla-Anabaena symbiotic N₂-fixing complex. Free-living cyanobacteria, the majority of which are heterocystous and nitrogen fixing, contribute an average of 20–30 kg N ha^-1, whereas the value is up to 600 kg ha^-1 for theAzollaAnabaena system (the most beneficial cyanobacterial symbiosis from an agronomic point of view). Synthesis and excretion of organic/growth-promoting substances by the cyanobacteria are also on record. During the last two or three decades a large number of studies have been published on the various important fundamental and applied aspects of both kinds of cyanobacterial biofertilizers (the free-living cyanobacteria and the cyanobacteriumAnabaena azollae in symbiotic association with the water fernAzolla), which include strain identification, isolation, purification, and culture; laboratory analyses of their N₂-fixing activity and related physiology, biochemistry, and energetics; and identification of the structure and regulation of nitrogenfixing (nif) genes and nitrogenase enzyme. The symbiotic biology of theAzolla-Anabaena mutualistic N₂-fixing complex has been clarified. In free-living cyanobacterial strains, improvement through mutagenesis with respect to constitutive N₂ fixation and resistance to the noncongenial agronomic factors has been achieved. By preliminary meristem mutagenesis inAzolla, reduced phosphate dependence was achieved, as were temperature tolerance and significant sporulation/spore germination under controlled conditions. Mass-production biofertilizer technology of free-living and symbiotic (Azolla-Anabaena) cyanobacteria was studied, as were the interacting and agronomic effects of both kinds of cyanobacterial biofertilizer with rice, improving the economics of rice cultivation with the cyanobacterial biofertilizers. Recent results indicate a strong potential for cyanobacterial biofertilizer technology in rice-growing countries, which opens up a vast area of more concerted basic, applied, and extension work in the future to make these self-renewable natural nitrogen resources even more promising at the field level in order to help reduce the requirement for inorganic N to the bare minimum, if not to zero.     

Simulium metallicum cytospecies E larval habitat characterization in the Altamira focus of onchocerciasis, northern Venezuela

Abstract. Simulium metallicum sibling species E was identified cytotaxonomically from an onchocerciasis focus at Altamira in northern Venezuela. S. metallicum E larvae were sampled monthly from two small mountain streams over a 15-month period (July 1990 to September 1991) and eleven habitat variables were measured at two altitudes. One stream consistently harboured much higher densities of S. metallicum E larvae than the other, with three annual peaks of abundance: during the dry season and at the beginning and end of the rainy season. These peak densities were correlated with high rainfall 4 months previously. Larvae were most abundant on submerged rocks and fallen leaves, in small shallow areas characterized by slow water current, high conductivity and sparse terrestrial vegetation cover. Stream variables which best explained the temporal changes in abundance were water discharge and conductivity. The population dynamics of S. metallicum E appeared to be influenced primarily by interactions between stream discharge and substrate stability. Relevance of these results to vector control with larvicides is discussed.     

A spatial and temporal approach to microevolutionary forces affecting population biology in the freshwater snail Biomphalaria pfeifferi

The limitations of both population demography and genetics highlight the need to combine these approaches when inferring the influence of demographic processes and modes of migration on genetic structure. The aim of this study was to use spatiotemporal genetic and demographic surveys to reveal the microevolutionary forces acting on the metapopulation dynamics of the freshwater snail Biomphalaria pfeifferi. We also analyzed the consequences of population turnover on temporal genetic differentiation, an aspect that has been little explored. Genetic drift was revealed by both the demographic survey, which indicated severe bottlenecks or extinction during the rainy (resp. dry) season in open (resp. closed) habitats, and the genetic approach, which indicated high selfing rates and strong temporal differentiation. Genetic reassignments and temporal differentiation both confirmed the results of the demographic survey, which suggests that migration occurs in closed (resp. open) habitats during the hot and dry (resp. rainy) season, and indicated that source-sink functioning may be envisaged. A propagule pool mode of colonization was inferred in the open habitats during the rainy season and a migrant pool in the closed habitats during the dry season. Our study also suggests that selection might be inferred from patterns of neutral genetic markers when recombination is limited.     

Leaf-cutting ant nests support less dense and impoverished seed assemblages in a human-modified Caatinga dry forest

Regenerating forests make up an increasingly large portion of tropical landscapes worldwide and regeneration dynamics may be influenced by leaf-cutting ants (LCA), which proliferate in disturbed areas and collect seeds for fungus culturing. Here, we investigate how LCA influences seed fate in human-modified areas of Caatinga dry forest. We evaluate the seed deposition and predation on Atta opaciceps nests, foraging habitat surrounding nest and control habitat away of nest influence of 15 colonies located along a forest cover gradient during the rainy and dry seasons. For each habitat, four 50-cm² plots were established and all seeds on the soil surface were collected along 1 year. We recorded 13,628 seeds distributed among 47 species and 36.57% of the total seeds did not show any sign of predation. Nest mound habitats supported low-density and species-poor seed assemblages, which were taxonomically distinct from the control habitats. These effects only occurred in the rainy season. The proportion of undamaged seeds were similar across the habitats. While forest cover did not influence seed assemblage in terms of species richness or seed predation, it did interact with habitat type via increments in seed abundance as forest cover increased across the nests. Forest cover also affected seed composition, but only in the rainy season. These results indicate that LCA decrease seed deposition in areas under their influence, particularly on the nest mounds. As LCA profit from human disturbance in the Caatinga, their role as seed ‘sinks’ should be enhanced in disturbed Caatinga patches, particularly during the rainy season, when most of the plant recruitment occurs. Our findings reinforce the importance of LCA as drivers of forest dynamics and resilience in human-modified landscapes.     

Cyanobacterial H<Subscript>2</Subscript> production — a comparative analysis

Several unicellular and filamentous, nitrogen-fixing and non-nitrogen-fixing cyanobacterial strains have been investigated on the molecular and the physiological level in order to find the most efficient organisms for photobiological hydrogen production. These strains were screened for the presence or absence of hup and hox genes, and it was shown that they have different sets of genes involved in H₂ evolution. The uptake hydrogenase was identified in all N₂-fixing cyanobacteria, and some of these strains also contained the bidirectional hydrogenase, whereas the non-nitrogen fixing strains only possessed the bidirectional enzyme. In N₂-fixing strains, hydrogen was mainly produced by the nitrogenase as a by-product during the reduction of atmospheric nitrogen to ammonia. Therefore, hydrogen production was investigated both under non-nitrogen-fixing conditions and under nitrogen limitation. It was shown that the hydrogen uptake activity is linked to the nitrogenase activity, whereas the hydrogen evolution activity of the bidirectional hydrogenase is not dependent or even related to diazotrophic growth conditions. With regard to large-scale hydrogen evolution by N₂-fixing cyanobacteria, hydrogen uptake-deficient mutants have to be used because of their inability to re-oxidize the hydrogen produced by the nitrogenase. On the other hand, fermentative H₂ production by the bidirectional hydrogenase should also be taken into account in further investigations of biological hydrogen production.Abbreviations Chl chlorophyll - MV methyl viologen     

Utility of some nitrogen-fixing microorganisms in the phyllosphere of crop plants

Summary The utility of spraying some known N₂-fixing microorganisms on rice leaves grown both in N-less sand culture and under field conditions was examined. The effect was compared with that of spraying a phyllosphere N₂-fixing isolate of Klebsiella, KUPBR₂, and application of nitrogenous fertilizers. All the growth parameters studied including dry weight and N-content were enhanced. Under field conditions number of tillers was increased by 26% withKlebsiella pneumoniae M5al and by 65% with Aphanothece. The dry weight of the plants was enhanced by 61–119%. The yield per 10 m² was almost doubled with Aphanothece, Beijerinckia 8007,Mycobacterium flavum, K. pneumoniae M5al and KUPBR₂. The increases observed withStreptomyces sp. G12 though less spectacular was significant at 1% level with respect to several growth parameters.K. pneumoniae M5al,M. flavum andStreptomyces sp. G12 exhibited nitrogenase activity both in laboratory culture and in association with rice plants.     

Biological N2 fixation by heterotrophic and phototrophic bacteria in association with straw

Much of the crop residues, including cereal straw, that are produced worldwide are lost by burning. Plant residues, and in particular straw, contain large amounts of carbon (cellulose and hemicellulose) which can serve as substrates for the production of microbial biomass and for biological N₂ fixation by a range of free-living, diazotrophic bacteria. Microorganisms with the dual ability to utilise cellulose and fix N₂ are rate, but some strains that utilize hemicellulose and fix N₂ have been found. Generally, cellulolysis and diazotrophy are carried out by a mixed microbial community in which N₂-fixing bacteria utilise cellobiose and glucose produced from straw by cellulolytic microorganisms. N₂-fixing bacteria include heterotrophic and phototrophic organisms and the latter are apparently more prominent in flooded soils such as rice paddies than in dryland soils. The relative contributions of N₂ fixed by heterotrophic diazotrophic bacteria compared with cyanobacteria and other phototrophic bacteria depend on the availability of substrates from straw decomposition and on environmental pressures. Measurements of asymbiotic N₂ fixation are limited and variable but, in rice paddy systems, rates of 25 kg N ha^-1 over 30 days have been found, whereas in dryland systems with wheat straw, in situ measurements have indicated up to 12 kg N ha^-1 over 22 days. Straw-associated N₂ fixation is directly affected by environmental factors such as temperature, moisture, oxygen concentration, soil pH and clay content as well as farm management practices. Modification of managements and use of inoculants offer ways of improving asymbiotic N₂ fixation.In laboratory culture systems, inoculation of straws with cellulolytic and diazotrophic microorganisms has resulted in significant increases in N₂ fixation in comparison to uninoculated controls and gains of N of up to 72 mg N fixed g^-1 straw consumed have been obtained, indicating the potential of inoculation to improve N gains in composts that can then be used as biofertilisers. Soils, on the other hand, contain established, indigenous microbial populations which tend to exclude inoculant microorganisms by competition. As a consequence, improvements in straw-associated N₂ fixation in soils have been achieved mostly by specific straw-management practices which encourage microbial activity by straw-decomposing and N₂-fixing microorganisms.Further research is needed to quantify more accurately the contribution of asymbiotic N₂ fixation to cropping systems. New strains of inoculants, including those capable of both cellulolytic and N₂-fixing activity, are needed to improve the N content of biofertilisers produced from composts. Developments of management practices in farming systems may result in further improvements in N₂ fixation in the field.     

Ecological and Biogeochemical Interactions Constrain Planktonic Nitrogen Fixation in Estuaries

Many types of ecosystems have little or no N₂ fixation even when nitrogen (N) is strongly limiting to primary production. Estuaries generally fit this pattern. In contrast to lakes, where blooms of N₂-fixing cyanobacteria are often sufficient to alleviate N deficits relative to phosphorus (P) availability, planktonic N₂ fixation is unimportant in most N-limited estuaries. Heterocystic cyanobacteria capable of N₂ fixation are seldom observed in estuaries where the salinity exceeds 8–10 ppt, and blooms have never been reported in such estuaries in North America. However, we provided conditions in estuarine mesocosms (salinity over 27 ppt) that allowed heterocystic cyanobacteria to grow and fix N₂ when zooplankton populations were kept low. Grazing by macrozooplankton at population densities encountered in estuaries strongly suppressed cyanobacterial populations and N₂ fixation. The cyanobacteria grew more slowly than observed in fresh waters, at least in part due to the inhibitory effect of sulfate (SO₄ ²⁻), and this slow rate of growth increased their vulnerability to grazing. We conclude that interactions between physiological (bottom–up) factors that slow the growth rate of cyanobacteria and ecological (top–down) factors such as grazing are likely to be important regulators excluding planktonic N₂ fixation from most Temperate Zone estuaries. Received 26 April 2002; Accepted 12 July 2002.     

Nitrogen fixation in the sub-Antarctic Macquarie Island

Summary The N₂-fixing biota of Macquarie Island are dominated by cyanobacteria growing epiphytically or symbolically with plants or lichens. Highest rates of C₂H₂-reducing activity were found in the leafy lichen Peltigera sp. colonizing herbfields and short grasslands and in the coastal angiosperm Colobanthus muscoides. Significant rates of C₂H₂ reduction were also found to be associated with the liverwort Jamesoniella colorata, commonly occurring in coastal and plateau mires, in a mossbed of Dicranella cardotii colonizing a land-slip face on the grassland slopes at 100 m altitude and within polsters of the mosses Ditrichum strictum and Andreaea sp. found in exposed localities on the plateau at 200–300 m altitude. It was concluded that the common feature of plants supporting active N₂ fixation in dry habitats was the dense packing of stems and leaves, enabling water translocation to the cyanobacterial zone by wick action. Epiphytic cyanobacterial C₂H₂ reduction in wet habitats was widespread and not restricted to any particular plant species. Notable N₂-fixing lichens of the plateau were Pseudocyphellaria delisea and Stereocaulon sp., although both were also occasionally found in coastal herbfields. No significant N₂-fixing activity was associated with any of the dominant grasses tested. Heterotrophic N₂ fixation was also found to be insignificant in the various habitats tested, however N₂-fixing Bacillus (B. macerans or B. polymyxa) were universally present in coastal, grassland slope, or plateau samples, including moss polster samples. A N₂fixing Clostridium sp. was isolated in only one instance, from soil in the vicinity of a seal wallow on the coast.     

Nitrogen enrichment during decomposition of mangrove leaf litter in an east African coastal lagoon (Kenya): Relative importance of biological nitrogen fixation

In situ decomposition of senescent leaves of twoabundant mangrove species (Rhizophora mucronataLamarck and Ceriops tagal (Perr) C.B. Rob),enrichment of nitrogen and activity of dinitrogenfixing bacteria during decomposition were investigatedduring both rainy and dry seasons in a tropicalcoastal lagoon (Gazi, Kenya). Rates of leafdecomposition were higher for R. mucronata thanfor C. tagal and were highest, for both species,during rainy season. Rates of decomposition, expressedas percentage dry mass loss, over a decompositionperiod of 50 days was: C. tagal (rainy season),69%; C. tagal (dry season), 27%; R.mucronata (rainy season), 98%; and R.mucronata (dry season), 48%. High rainfall anddiurnal tidal inundation appear to enhance the leafdecomposition process. Maximum rates of nitrogenfixation were 380 nmol N₂ h^-1 g^-1 dw forC. tagal (rainy season), 78 nmolN₂ h^-1 g^-1 dw for C. tagal (dryseason), 390 nmol N₂ h^-1 g^-1 dw for R. mucronata (rainy season) and 189 nmolN₂ h^-1 g^-1 dw for R. mucronata (dry season). Although N₂ fixation rates werehighest during rainy season, total nitrogenimmobilised in the leaves was highest during the dryseason. Biological nitrogen fixation can account forbetween 13 to 21% of the maximum nitrogen immobilisedin the decaying mangrove leaves. Nitrogen fixation, asa source of allochthonous nitrogen, sustains anitrogen input to the mangrove ecosystem, which addssignificantly to the nitrogen input through leaflitterfall.     

Relationship between abundance of N2-fixing cyanobacteria and environmental features of Spanish rice fields

In order to estimate the potential utilization of N₂-fixing (heterocystous) cyanobacteria as natural biofertilizers in the Valencian rice fields (Spain), the distribution and seasonal variation of these microorganisms in water and sediment samples were evaluated, and the relationships among cyanobacterial abundance and physical and chemical characteristics of soil and water were investigated. N₂-fixing cyanobacteria were present in all the samples analyzed (25 sampling points sampled three times per year during two years). The relative cyanobacterial abundance in soil and water followed contrasting patterns, maximum presence in soil coincided with minimum abundance in water. Correlation analysis showed that cyanobacterial abundance in the two phases (water and sediment) was influenced more by water than by soil properties. Salinity, mineralization variables, and soluble reactive phosphate (SRP) correlated positively with heterocystous cyanobacteria presence. Furthermore, dissolved inorganic nitrogen (DIN) and the ratio DIN: SRP correlated negatively with cyanobacterial abundance. However DIN: SRP ratio better described the cyanobacterial distribution, with a threshold effect: below the Redfield ratio value (7.2 in mass units) cyanobacterial abundance was clearly higher. Correspondence to: A. Quesada.     

Population Size and Migration of Anopheles gambiae in the Bancoumana Region of Mali and Their Significance for Efficient Vector Control

We present results of two intensive mark-release-recapture surveys conducted during the wet and dry seasons of 2008 in the villages of Fourda and Kenieroba, Mali. The former is a small fishing village by the Niger River with a moderate to high densities of Anopheles gambiae Giles s.s. (Diptera: Culicidae) throughout the year, while the latter is a large agricultural community 2 km inland that experiences strong seasonal fluctuation in An. gambiae densities. We estimate the population size of female An. gambiae in Fourda to be in less than 3,000 during the dry season. We found evidence of large population size and migration from Fourda in Kenieroba during the wet season, but very low numbers and no sign of migrants during the dry season. We suggest that malaria vector control measures aimed at adult mosquitoes might be made more efficient in this region and other seasonal riparian habitats by targeting disruption of mosquito populations by the river during the dry season. This would decrease the size of an already small population, and would be likely to delay the explosive growth in vector numbers in the larger inland villages as rainfall increases.     

Iron deficiency increases growth and nitrogen-fixation rates of phosphorus-deficient marine cyanobacteria

Marine dinitrogen (N₂)-fixing cyanobacteria have large impacts on global biogeochemistry as they fix carbon dioxide (CO₂) and fertilize oligotrophic ocean waters with new nitrogen. Iron (Fe) and phosphorus (P) are the two most important limiting nutrients for marine biological N₂ fixation, and their availabilities vary between major ocean basins and regions. A long-standing question concerns the ability of two globally dominant N₂-fixing cyanobacteria, unicellular Crocosphaera and filamentous Trichodesmium, to maintain relatively high N₂-fixation rates in these regimes where both Fe and P are typically scarce. We show that under P-deficient conditions, cultures of these two cyanobacteria are able to grow and fix N₂ faster when Fe deficient than when Fe replete. In addition, growth affinities relative to P increase while minimum concentrations of P that support growth decrease at low Fe concentrations. In Crocosphaera, this effect is accompanied by a reduction in cell sizes and elemental quotas. Relatively high growth rates of these two biogeochemically critical cyanobacteria in low-P, low-Fe environments such as those that characterize much of the oligotrophic ocean challenge the common assumption that low Fe levels can have only negative effects on marine primary producers. The closely interdependent influence of Fe and P on N₂-fixing cyanobacteria suggests that even subtle shifts in their supply ratio in the past, present and future oceans could have large consequences for global carbon and nitrogen cycles.