Amazon River enhances diazotrophy and carbon sequestration in the tropical North Atlantic Ocean.

The fresh water discharged by large rivers such as the Amazon is transported hundreds to thousands of kilometers away from the coast by surface plumes. The nutrients delivered by these river plumes contribute to enhanced primary production in the ocean, and the sinking flux of this new production results in carbon sequestration. Here, we report that the Amazon River plume supports N(2) fixation far from the mouth and provides important pathways for sequestration of atmospheric CO(2) in the western tropical North Atlantic (WTNA). We calculate that the sinking of carbon fixed by diazotrophs in the plume sequesters 1.7 Tmol of C annually, in addition to the sequestration of 0.6 Tmol of C yr(-1) of the new production supported by NO(3) delivered by the river. These processes revise our current understanding that the tropical North Atlantic is a source of 2.5 Tmol of C to the atmosphere [Mikaloff-Fletcher SE, et al. (2007) Inverse estimates of the oceanic sources and sinks of natural CO(2) and the implied oceanic carbon transport. Global Biogeochem Cycles 21, doi:10.1029/2006GB002751]. The enhancement of N(2) fixation and consequent C sequestration by tropical rivers appears to be a global phenomenon that is likely to be influenced by anthropogenic activity and climate change.

Nitrogen fixation in the marine environment.

Cyanobacteria of the genus Oscillatoria (Trichodesmium) account for annual inputs of nitrogen to the world's oceans of about 4.8 x 10(12) grams while benthic environments contribute 15 x 10(12) grams. The sum of these inputs is one-fifth of current estimates of nitrogen fixation in terrestrial environments and one-half of the present rate of industrial synthesis of ammonia. When the total of all nitrogen inputs to the sea is compared with estimated losses through denitrification, the marine nitrogen cycle approximates a steady state. Oceanic nitrogen fixation can supply less than 0.3 percent of the calculated demand of marine phytoplankton. The minor contribution by nitrogen fixation to the overall nitrogen economy of the sea is not consistent with the supposition that nitrogen is the primary limiting nutrient and suggests that factors other than nitrogen availability limit phytoplankton growth rates.

Marine nitrogen fixation: what's the fuss?

Biological nitrogen fixation is a much more important process in the nitrogen cycle of the oceans than previously thought. Further, nitrogen fixation may have an influence on the capacity of the oceans to sequester carbon. A greater diversity of marine nitrogen fixers has also been uncovered but their quantitative significance remains to be determined.

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.

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.

Hypolithic communities: important nitrogen sources in Antarctic desert soils.

Hypolithic microbial communities (i.e. cryptic microbial assemblages found on the undersides of translucent rocks) are major contributors of carbon input into the oligotrophic hyper-arid desert mineral soils of the Eastern Antarctic Dry Valleys. Here we demonstrate, for the first time, that hypolithic microbial communities possess both the genetic capacity for nitrogen fixation (i.e. the presence of nifH genes) and the ability to catalyse acetylene reduction, an accepted proxy for dinitrogen fixation. An estimate of the total contribution of these communities suggests that hypolithic communities are important contributors to fixed nitrogen budgets in Antarctic desert soils.

Impacts of atmospheric anthropogenic nitrogen on the open ocean.

Increasing quantities of atmospheric anthropogenic fixed nitrogen entering the open ocean could account for up to about a third of the ocean's external (nonrecycled) nitrogen supply and up to approximately 3% of the annual new marine biological production, approximately 0.3 petagram of carbon per year. This input could account for the production of up to approximately 1.6 teragrams of nitrous oxide (N2O) per year. Although approximately 10% of the ocean's drawdown of atmospheric anthropogenic carbon dioxide may result from this atmospheric nitrogen fertilization, leading to a decrease in radiative forcing, up to about two-thirds of this amount may be offset by the increase in N2O emissions. The effects of increasing atmospheric nitrogen deposition are expected to continue to grow in the future.

Denitrification in aquifer soil and nearshore marine sediments influenced by groundwater nitrate.

We estimated rates of denitrification at various depths in sediments known to be affected by submarine discharge of groundwater, and also in the parent aquifer. Surface denitrification was only measured in the autumn; at 40-cm depth, where groundwater-imported nitrate has been measured, denitrification occurred consistently throughout the year, at rates from 0.14 to 2.8 ng-atom of N g day. Denitrification consistently occurred below the zone of sulfate reduction and was sometimes comparable to it in magnitude. Denitrification occurred deep (14 to 40 cm) in the sediments along 30 km of shoreline, with highest rates occurring where groundwater input was greatest. Denitrification rates decreased with distance offshore, as does groundwater influx. Added glucose greatly stimulated denitrification at depth, but added nitrate did not. High rates of denitrification were measured in the aquifer (17 ng-atom of N g day), and added nitrate did stimulate denitrification there. The denitrification measured was enough to remove 46% of the nitrate decrease observed between 40- and 14-cm depth in the sediment.

Effects of co-occurring aromatic hydrocarbons on degradation of individual polycyclic aromatic hydrocarbons in marine sediment slurries.

Rates of polycyclic aromatic hydrocarbon (PAH) degradation and mineralization were influenced by preexposure to alternate PAHs and a monoaromatic hydrocarbon at relatively high (100 ppm) concentrations in organic-rich aerobic marine sediments. Prior exposure to three PAHs and benzene resulted in enhanced [14C]naphthalene mineralization, while [14C]anthracene mineralization was stimulated only by benzene and anthracene preexposure. Preexposure of sediment slurries to phenanthrene stimulated the initial degradation of anthracene. Prior exposure to naphthalene stimulated the initial degradation of phenanthrene but had no effect on either the initial degradation or mineralization of anthracene. For those compounds which stimulated [14C]anthracene or [14C]naphthalene mineralization, longer preexposures (2 weeks) to alternative aromatic hydrocarbons resulted in an even greater stimulation response. Enrichment with individual PAHs followed by subsequent incubation with one or two PAHs showed no alteration in degradation patterns due to the simultaneous presence of PAHs. The evidence suggests that exposure of marine sediments to a particular PAH or benzene results in the enhanced ability of these sediments to subsequently degrade that PAH as well as certain other PAHs. The enhanced degradation of a particular PAH after sediments have been exposed to it may result from the selection and proliferation of specific microbial populations capable of degrading it. The enhanced degradation of other PAHs after exposure to a single PAH suggests that the populations selected have either broad specificity for PAHs, common pathways of PAH degradation, or both.

Nitrate requirement for acetylene inhibition of nitrous oxide reduction in marine sediments.

The inhibition of nitrous oxide (N2O) reduction by acetylene (C2H2) in saltmarsh sediment was temporary; we investigated this phenomenon and possible causes. The reduction of N2O in the presence of C2H2 was biological. N2O consumption in the presence of C2H2 began when nitrate concentration became very low. The time course of N2O consumption after periods of N2O accumulation was unaffected by initial nitrate concentrations between 16 and 200µM, or C2H2 concentrations between 10 and 100% of the gas phase. Sulfide had no effect on the kinetics of N2O reduction in the presence of C2H2. In more dilute slurries of saltmarsh sediments and in estuarine sediment, N2O persisted in the presence of C2H2 unless sufficient organic carbon was added to deplete nitrate. In saltmarsh sediments, the rate of N2O consumption in the presence of C2H2 was not changed by preincubation with C2H2. Initial positive rates of N2O production in the presence of C2H2 occurred only when the block was apparently effective (i.e., at nitrate concentrations greater than about 5-10µM) and appeared to represent a valid estimate of denitrification. Conversely, and in agreement with previous studies, concentrations of NO3 (-) below these levels resulted in reduced efficiency of C2H2 blockage of N2O reductase.

Microbial transformations of methylated sulfur compounds in anoxic salt marsh sediments.

Anoxic salt marsh sediments were amended with several methylated sulfur compounds. Sediment microbes transformed the added compounds into other volatile methylated sulfur compounds and eventually mineralized the compounds to CH4 and presumably to CO2 and H2S. The principal methyl-sulfur product of dimethylsulfoniopropionate (DMSP) was found to be dimethylsulfide (DMS), with only small amounts of methane thiol (MSH) produced. By contrast, methionine and S-methyl cysteine were degraded mostly to MSH and to lesser amounts of DMS. Dimethylsulfoxide (DMSO) was biologically converted to DMS. Dimethyldisulfide (DMDS) was rapidly reduced to MSH by the sediment microflora, and some DMS was also produced. DMS, whether added directly or when derived from other precursors, was metabolized with the production of MSH. Methane thiol was also metabolized, and evidence suggests that MSH may be biologically methylated to form DMS. Experiments with selective microbial inhibitors were used to ascertain which microbial groups were responsible for the observed transformations. Based on these experiments, it appears that both sulfate-reducing and methane-producing bacteria may be involved in transforming and mineralizing methylated sulfur compounds. A simple scheme of how methylated sulfur compounds may be transformed in the environment is presented.

Effects of four aromatic organic pollutants on microbial glucose metabolism and thymidine incorporation in marine sediments.

The metabolism of D-[U-14C]glucose and the incorporation of [methyl-3H]thymidine by aerobic and anaerobic marine sediment microbes exposed to 1 to 1,000 ppm anthracene, naphthalene, p,p'-dichlorodiphenyltrichloroethane, and pentachlorophenol were examined. Cell-specific rates of [14C]glucose metabolism averaged 1.7 X 10(-21) and 0.5 X 10(-21) mol/min per cell for aerobic and anaerobic sediment slurries, respectively; [3H]thymidine incorporation rates averaged 43 X 10(-24) and 9 X 10(-24) mol/min per cell for aerobic and anaerobic slurries, respectively. Aerobic sediments exposed to three of the organic pollutants for 2 to 7 days showed recovery of both activities. Anaerobic sediments showed little recovery after 2 days of pre-exposure to the pollutants. We conclude that (i) anaerobic sediments are more sensitive than aerobic sediments to pollutant additions; (ii) [3H]thymidine incorporation is more sensitive to pollutant additions than is [14C]glucose metabolism; and (iii) the toxicity of the pollutants increased in the following order: anthracene, p,p'-dichlorodiphenyltrichloroethane, naphthalene, and pentachlorophenol.

Degassing of Pore Water Methane during Sediment Incubations.

Laboratory experiments were used to examine the degassing of CH(4) from a muddy sediment. Sediment containing dissolved CH(4) showed a hyperbolic time course of CH(4) release when allowed to degas in stoppered 20-ml vials. Equilibration required ca. 24 h for 5 ml of sediment. The rate of CH(4) release was found to be dependent on the ratio of exposed sediment surface area to sediment volume. The water content of the sediment was a factor in the total amount of CH(4) released but did not affect the rate of degassing. Addition of water to sediment samples (to form a slurry) accelerated CH(4) release, with a 1:1 dilution giving ca. 80% of maximum release after only 2 min. Shaking (vortexing) the sediments also facilitated CH(4) exchange, with 2 min of vigorous agitation giving 77% of maximum release. The organic content of the sediment did not affect either the amount or the rate of CH(4) degassing. Rubber stoppers exposed to CH(4) were found to absorb CH(4) rapidly and to subsequently release it in proportion to the concentration to which they were exposed. Artifacts may be associated with CH(4) production measurements if sediment and stopper degassing are not considered. It is recommended that any study of methane production or distribution include preliminary experiments to determine the degassing kinetics for the specific sediment system being used.

Degradation and mineralization of the polycyclic aromatic hydrocarbons anthracene and naphthalene in intertidal marine sediments.

The degradation of the polynuclear aromatic hydrocarbons (PAHs) anthracene and naphthalene by the microbiota of intertidal sediments was investigated in laboratory studies. No mineralization of either PAH was observed in the absence of oxygen. Both rates and total amounts of PAH mineralization were strongly controlled by oxygen content and temperature of the incubations. Inorganic nitrogen and glucose amendments had minimal effects on PAH mineralization. The rates and total amounts of PAH mineralized were directly related to compound concentration, pre-exposure time, and concentration. Maximum mineralization was observed at the higher concentrations (5 to 100 mug/g [ppm]) of both PAHs. Optimal acclimation to anthracene and naphthalene (through pre-exposures to the compounds) occurred at the highest acclimation concentration (1,000 ppm). However, acclimation to a single concentration (100 ppm) resulted in initial relative mineralization rates over a range of re-exposure concentrations (1 to 1,000 ppm) being nearly identical. Maximum mineralization of both PAHs occurred after intermediate periods (1 to 2 weeks) of pre-exposure. The fraction of the total heterotrophic population capable of utilizing anthracene or naphthalene as sole carbon source was also greatest after 2 weeks.

Heterotrophic microbial activity in shallow aquifer sediments of Long Island, New York.

Bacterial numbers and activities (as estimated by glucose uptake and total thymidine incorporation) were investigated at two sites in Long Island, New York aquifer sediments. In general, bacterial activities were higher in shallow (1.5-4.5 m below the water table or BWT), oxic sediments than in deep (10-18 m BWT), anoxic sediments. The average total glucose uptake rates were 0.18 ± 0.10 ng gdw(-1) h(-1) in shallow sediments and 0.09 ± 0.11 ng gdw(-1) h(-1) in deep sediments; total thymidine incorporation rates were 0.10 ± 0.13 pmol gdw(-1) h(-1) and 0.03 ± 0.03 pmol gdw(-1) h(-1) in shallow and deep sediments, respectively. Incorporation of glucose was highly efficient, as only about 10% of added label was recovered as CO2. Bacterial abundance (estimated from acridine orange direct counts) was 2.5 ± 2.0 × 10(7) cells gdw(-1) and 2.0 ± 1.3 × 10(7) cells gdw(-1) in shallow and deep sediments, respectively. These bacterial activity and abundance estimates are similar to values found in other aquifer environments, but are 10- to 1000-fold lower than values in soil or surface sediment of marine and estuarine systems. In general, cell specific microbial activities were lower in sites from Connetquot Park, a relatively pristine site, when compared to activities found in sites from Jamesport, which has had a history of aldicarb (a pesticide) contamination. To our knowledge, this is the first report of bacterial activity measurements in the shallow, sandy aquifers of Long Island, New York.

Perfusion method for assaying microbial activities in sediments: applicability to studies of n(2) fixation by c(2)h(2) reduction.

A perfusion method for assaying nitrogenase activity (acetylene reduction) in marine sediments was developed. The method was used to assay sediment cores from Spartina alterniflora (salt marsh), Zostera marina (sea grass), and Thalassia testudinum (sea grass) communities, and the results were compared with those of conventional sealed-flask assays. Rates of ethylene production increased progressively with time in the perfusion assays, reaching plateau values of 2 to 3 nmol . g of dry sediment . h by 10 to 20 h. Depletion of interstitial NH(4) was implicated in this stimulation of nitrogenase activity. Initial acetylene reduction rates determined by the perfusion assay of cores from the Spartina community ranged from 0.15 to 0.60 nmol of C(2)H(4) . g of dry sediment . h. These rates were similar to those for sediments assayed in sealed flasks without seawater when determined over linear periods of C(2)H(4) production. Initial values obtained by using the perfusion method were 0.66 nmol of C(2)H(4) . g of dry sediment . h for sediments from Zostera communities and 0.70 nmol of C(2)H(4) . g of dry sediment . h for sediments from Thalassia communities. In all cases, rates determined by simultaneous slurry assays were lower than those determined by the perfusion method.

Nitrogen Fixation Associated with Rinsed Roots and Rhizomes of the Eelgrass Zostera marina.

Nitrogen fixation was associated with the rinsed roots and rhizomes of the seagrass, Zostera marina L. Nitrogenase activity (acetylene reduction) was greater on rhizomes compared to roots, and on older roots and rhizomes relative to younger tissue. Compared to aerobic assays, anaerobic or microaerobic conditions enhanced the rate of acetylene reduction by rhizomes with attached roots, with the highest activity (100 nanomoles per gram dry weight per hour) occurring at pO(2) = 0.01 atmosphere. Addition of glucose, sucrose, or succinate also increased the rate of acetylene reduction under anaerobic conditions, with glucose providing the most stimulation. In one experiment, comparison of acetylene reduction assays with (15)N(2) incorporation yielded a ratio of about 2.6:1. Seagrass communities are thought to be limited by the availability of nitrogen and, therefore, nitrogenase activity directly associated with their roots and rhizomes suggests the possibility of a N(2)-fixing flora which may subsidize their nutritional demand for nitrogen.

Stimulation of methanogenesis by aldicarb and several other N-methyl carbamate pesticides.

Aldicarb and several other N-methyl carbamate pesticides stimulated methane production in anaerobic salt marsh soils and organic-rich aquifer soils. Stimulation was biological and linearly related to the amount of carbamate added. Of the four carbamates studied, methomyl gave the greatest stimulation followed by carbaryl, aldicarb, and baygon. The percent conversions [(moles of CH(4) in excess of control/mole of carbamate added) x 100] for methomyl, carbaryl, aldicarb, and baygon were 88, 57, 40, and 11, respectively. Using aldicarb as a model carbamate, we found that monomethylamine (MA) accumulated in sediments as a result of aldicarb addition. MA arises from the N-methyl carbamoyl portion of the carbamates as a result of presumptive biological hydrolysis. MA levels decreased as CH(4) production was stimulated, and 2-bromoethane sulfonic acid (a specific inhibitor of mathanogenesis) partially inhibited the loss of MA. These findings suggest that N-methyl carbamates are readily hydrolyzed to MA in the presence of an active microbial population under anaerobic conditions and that methanogenesis is stimulated as a result of the consumption of MA by methanogenic bacteria.

Microbial aldicarb transformation in aquifer, lake, and salt marsh sediments.

The microbial transformation of [N-methyl-(sup14)C]aldicarb, a carbamate pesticide, occurred in aquifer, lake, and salt marsh sediments. Microbial degradation of aldicarb took place within 21 days in aquifer sediments from sites previously exposed to aldicarb (Jamesport, Long Island, N.Y.) but did not occur in sediments which were not previously exposed (Connetquot State Park, Long Island, N.Y.). At the Jamesport sites, higher aldicarb transformation rates occurred in deep, anoxic sediments than in shallow, oxic sediments. There was a significant negative relationship (P < 0.05) between transformation rates and ambient dissolved O(inf2) levels. Aldicarb hydrolysis rates in Jamesport sediments were 10- to 1,000-fold lower than rates previously reported for soils. In addition, aldicarb degradation rates were not significantly correlated with measurements of bacterial activity and density previously determined in the same sediments. Substantially higher aldicarb degradation rates were found in anoxic lake and salt marsh than in aquifer sediments. Furthermore, we investigated the anaerobic microbial processes involved in aldicarb transformation by adding organic substrates (acetate, glucose), an alternative electron acceptor (nitrate), and microbial inhibitors (molybdate, 2-bromoethanesulfonic acid) to anoxic aquifer, lake, and salt marsh sediments. The results suggest that a methanogenic consortium was important in aldicarb transformation or in the use of aldicarb-derived products such as methylamine. In addition, microbial aldicarb transformation proceeded via different pathways under oxic and anoxic conditions. In the presence of O(inf2), aldicarb transformation was mainly via an oxidation pathway, while in the absence of O(inf2), degradation took place through a hydrolytic pathway (including the formation of methylamine precursors). Under anoxic conditions, therefore, aldicarb can be transformed by microbial consortia to yield products which can be of direct benefit to natural populations of methanogens present in sediments.