Partitioning Effects during Terminal Carbon and Electron Flow in Sediments of a Low-Salinity Meltwater Pond near Bratina Island, McMurdo Ice Shelf, Antarctica

A study of anaerobic sediments below cyanobacterial mats of a low-salinity meltwater pond called Orange Pond on the McMurdo Ice Shelf at temperatures simulating those in the summer season (<5°C) revealed that both sulfate reduction and methane production were important terminal anaerobic processes. Addition of [2-¹⁴C]acetate to sediment samples resulted in the passage of label mainly to CO₂. Acetate addition (0 to 27 mM) had little effect on methanogenesis (a 1.1-fold increase), and while the rate of acetate dissimilation was greater than the rate of methane production (6.4 nmol cm⁻³ h⁻¹ compared to 2.5 to 6 nmol cm⁻³ h⁻¹), the portion of methane production attributed to acetate cleavage was <2%. Substantial increases in the methane production rate were observed with H₂ (2.4-fold), and H₂ uptake was totally accounted for by methane production under physiological conditions. Formate also stimulated methane production (twofold), presumably through H₂ release mediated through hydrogen lyase. Addition of sulfate up to 50-fold the natural levels in the sediment (interstitial concentration, ~0.3 mM) did not substantially inhibit methanogenesis, but the process was inhibited by 50-fold chloride (36 mM). No net rate of methane oxidation was observed when sediments were incubated anaerobically, and denitrification rates were substantially lower than rates for sulfate reduction and methanogenesis. The results indicate that carbon flow from acetate is coupled mainly to sulfate reduction and that methane is largely generated from H₂ and CO₂ where chloride, but not sulfate, has a modulating role. Rates of methanogenesis at in situ temperatures were four- to fivefold less than maximal rates found at 20°C.     

Effects of Temperature on Methane Consumption in a Forest Soil and in Pure Cultures of the Methanotroph Methylomonas rubra

Methane oxidation in soil cores from a mixed hardwood-coniferous forest varied relatively little as a function of incubation temperatures from −1 to 30°C. The increase in oxidation rate was proportional to T^2.4 (in kelvins). This relationship was consistent with limitation of methane transport through a soil gas phase to a subsurface zone of consumption by diffusion. The Q₁₀ for CO₂ production, 3.4, was substantially higher than that for methane oxidation, 1.1, and indicated that the response of soil respiration to temperature was limited by enzymatic processes and not diffusion of either organic substrates or molecular oxygen. When grown under conditions of phase-transfer limitation, cultures of Methylomonas rubra showed a minimal response to temperature changes between 19 and 38°C, as indicated by methane oxidation rates; in the absence of phase-transfer limitations, M. rubra oxidized methane at rates strongly dependent on temperature.     

Sulfate Reduction in Peat from a New Jersey Pinelands Cedar Swamp

Microbial sulfate reduction rates in acidic peat from a New Jersey Pine Barrens cedar swamp in 1986 were similar to sulfate reduction rates in freshwater lake sediments. The rates ranged from a low of 1.0 nmol cm⁻³ day⁻¹ in February at 7.5- to 10.0-cm depth to 173.4 nmol cm⁻³ day⁻¹ in July at 5.0- to 7.5-cm depth. The presence of living Sphagnum moss at the surface generally resulted in reduced rates of sulfate reduction. Pore water sulfate concentrations and water table height also apparently affected the sulfate reduction rate. Concentrations of sulfate in pore water were nearly always higher than those in surface water and groundwater, ranging from 26 to 522 μM. The elevated pore water sulfate levels did not result from the evapotranspiratory concentration of infiltrating stream water or groundwater, but probably resulted from oxidation of reduced sulfur compounds, hydrolysis of ester sulfates present in the peat, or both. The total sulfur content of peat that had no living moss at the surface was 164.64 ± 1.5 and 195.8 ± 21.7 μmol g (dry weight)⁻¹ for peat collected from 2.5 to 5.0 and 7.5 to 10.0 cm, respectively. Organosulfur compounds accounted for 84 to 88% of the total sulfur that was present in the peat. C-bonded sulfur accounted for 91 to 94% of the organic sulfur, with ester sulfate being only a minor constituent. Reduced inorganic sulfur species in peat from 2.5 to 7.5 cm were dominated by H₂S-FeS (68%), while pyritic sulfide was the predominant inorganic sulfur species in the peat from depths of 7.5 to 10.0 cm (75%).     

Sediment Nitrification, Denitrification, and Nitrous Oxide Production in a Deep Arctic Lake

We used a combination of ¹⁵N tracer methods and a C₂H₂ blockage technique to determine the role of sediment nitrification and denitrification in a deep oligotrophic arctic lake. Inorganic nitrogen concentrations ranged between 40 and 600 nmol · cm⁻³, increasing with depth below the sediment-water interface. Nitrate concentrations were at least 10 times lower, and nitrate was only detectable within the top 0 to 6 cm of sediment. Eh and pH profiles showed an oxidized surface zone underlain by more reduced conditions. The lake water never became anoxic. Sediment Eh values ranged from −7 to 484 mV, decreasing with depth, whereas pH ranged from 6.0 to 7.3, usually increasing with depth. The average nitrification rate (49 ng of N · cm⁻³ · day⁻¹) was similar to the average denitrification rate (44 ng of N · cm⁻³ · day⁻¹). In situ N₂O production from nitrification and denitrification ranged from 0 to 25 ng of N · cm⁻³ · day⁻¹. Denitrification appears to depend on the supply of nitrate by nitrification, such that the two processes are coupled functionally in this sediment system. However, the low rates result in only a small nitrogen loss.     

Dynamics of Bacterial Sulfate Reduction in a Eutrophic Lake

Bacterial sulfate reduction in the surface sediment and the water column of Lake Mendota, Madison, Wis., was studied by using radioactive sulfate (³⁵SO₄²⁻). High rates of sulfate reduction were observed at the sediment surface, where the sulfate pool (0.2 mM SO₄²⁻) had a turnover time of 10 to 24 h. Daily sulfate reduction rates in Lake Mendota sediment varied from 50 to 600 nmol of SO₄²⁻ cm⁻³, depending on temperature and sampling date. Rates of sulfate reduction in the water column were 10³ times lower than that for the surface sediment and, on an areal basis, accounted for less than 18% of the total sulfate reduction in the hypolimnion during summer stratification. Rates of bacterial sulfate reduction in the sediment were not sulfate limited at sulfate concentrations greater than 0.1 mM in short-term experiments. Although sulfate reduction seemed to be sulfate limited below 0.1 mM, Michaelis-Menten kinetics were not observed. The optimum temperature (36 to 37°C) for sulfate reduction in the sediment was considerably higher than in situ temperatures (1 to 13°C). The response of sulfate reduction to the addition of various electron donors metabolized by sulfate-reducing bacteria in pure culture was investigated. The degree of stimulation was in this order: H₂ > n-butanol > n-propanol > ethanol > glucose. Acetate and lactate caused no stimulation.     

Role of Sulfate Reduction Versus Methanogenesis in Terminal Carbon Flow in Polluted Intertidal Sediment of Waimea Inlet, Nelson, New Zealand

An investigation of the terminal anaerobic processes occurring in polluted intertidal sediments indicated that terminal carbon flow was mainly mediated by sulfate-reducing organisms in sediments with high sulfate concentrations (>10 mM in the interstitial water) exposed to low loadings of nutrient (equivalent to <10² kg of N · day⁻¹) and biochemical oxygen demand (<0.7 × 10³ kg · day⁻¹) in effluents from different pollution sources. However, in sediments exposed to high loadings of nutrient (>10² kg of N · day⁻¹) and biochemical oxygen demand (>0.7 × 10³ kg · day⁻¹), methanogenesis was the major process in the mediation of terminal carbon flow, and sulfate concentrations were low (≤2 mM). The respiratory index [¹⁴CO₂/(¹⁴CO₂ + ¹⁴CH₄)] for [2-¹⁴C]acetate catabolism, a measure of terminal carbon flow, was ≥0.96 for sediment with high sulfate, but in sediments with sulfate as little as 10 μM in the interstitial water, respiratory index values of ≤0.22 were obtained. In the latter sediment, methane production rates as high as 3 μmol · g⁻¹ (dry weight) · h⁻¹ were obtained, and there was a potential for active sulfate reduction.     

Methanol Promotes Atmospheric Methane Oxidation by Methanotrophic Cultures and Soils

Two methanotrophic bacteria, Methylobacter albus BG8 and Methylosinus trichosporium OB3b, oxidized atmospheric methane during batch growth on methanol. Methane consumption was rapidly and substantially diminished (95% over 9 days) when washed cell suspensions were incubated without methanol in the presence of atmospheric methane (1.7 ppm). Methanotrophic activity was stimulated after methanol (10 mM) but not methane (1,000 ppm) addition. M. albus BG8 grown in continuous culture for 80 days with methanol retained the ability to oxidize atmospheric methane and oxidized methane in a chemostat air supply. Methane oxidation during growth on methanol was not affected by methane deprivation. Differences in the kinetics of methane uptake (apparent K_m and V_max) were observed between batch- and chemostat-grown cultures. The V_max and apparent K_m values (means ± standard errors) for methanol-limited chemostat cultures were 133 ± 46 nmol of methane 10⁸ cells⁻¹ h⁻¹ and 916 ± 235 ppm of methane (1.2 μM), respectively. These values were significantly lower than those determined with batch-grown cultures (V_max of 648 ± 195 nmol of methane 10⁸ cells⁻¹ h⁻¹ and apparent K_m of 5,025 ± 1,234 ppm of methane [6.3 μM]). Methane consumption by soils was stimulated by the addition of methanol. These results suggest that methanol or other nonmethane substrates may promote atmospheric methane oxidation in situ.     

Populations of Methane-Producing Bacteria and In Vitro Methanogenesis in Salt Marsh and Estuarine Sediments

Most probable numbers (MPNs) of methanogens in various salt marsh and estuarine sediments were determined with an anaerobic, habitat-simulating culture medium with 80% H₂ plus 20% CO₂ as substrate. Average MPNs for the short Spartina (SS) marsh sediments of Sapelo Island, Ga., were maximal at the 5- to 7-cm depth (1.2 × 10⁷/g of dry sediment). Populations decreased to approximately 880/g of dry sediment at the 34- to 36-cm depth. There was no significant difference between summer and winter populations. In tall Spartina (TS) marsh sediments, average populations were maximal (1.2 × 10⁶/g of dry sediment) in the upper 0- to 2-cm zone; populations from the 5- to 36-cm zones were similar (average of 9 × 10⁴/g of dry sediment). Methanogenic populations for TS sediments of James Island Creek marsh, Charleston, S.C., were similar (average of 3 × 10⁶/g of dry sediment) for all depths tested (0 to 22 cm), which was comparable to the trend observed for TS sediments at Sapelo Island, Ga. Sediment grab samples collected along a transect of James Island Creek and its adjacent Spartina marsh had MPNs that were approximately 20 times greater for the region of Spartina growth (average of 10⁶/g of dry sediment) compared with the channel (approximately 5 × 10⁴ methanogens per g of dry sediment). A similar trend was found at Pawley's Island marsh, S.C., but populations were approximately one order of magnitude lower. In vitro rates of methanogenesis with SS sediments incubated under 80% H₂-20% CO₂ showed that the 5- to 7-cm region exhibited maximal activity (51 nmol of CH₄ g⁻¹ h⁻¹), which was greater than rates for sediments above and below this depth. SS sediment samples (5 to 7 cm) incubated under 100% N₂ and supplemented with formate exhibited rates of methanogenesis similar to those generated by samples under 80% H₂-20% CO₂. Replacing the N₂ atmosphere with H₂ resulted in an eightfold decrease in the rate of methanogenesis. In vitro methanogenic activity by TS salt marsh sediments, incubated under 80% H₂-20% CO₂, was similar for all depths tested (0 to 22 cm). TS sediment samples (0 to 7 cm) supplemented with formate and incubated under 100% N₂ had greater rates of methanogenesis compared with unsupplemented samples.     

Nitrate-Dependent Ferrous Iron Oxidation by Anaerobic Ammonium Oxidation (Anammox) Bacteria

We examined nitrate-dependent Fe²⁺ oxidation mediated by anaerobic ammonium oxidation (anammox) bacteria. Enrichment cultures of “Candidatus Brocadia sinica” anaerobically oxidized Fe²⁺ and reduced NO₃⁻ to nitrogen gas at rates of 3.7 ± 0.2 and 1.3 ± 0.1 (mean ± standard deviation [SD]) nmol mg protein⁻¹ min⁻¹, respectively (37°C and pH 7.3). This nitrate reduction rate is an order of magnitude lower than the anammox activity of “Ca. Brocadia sinica” (10 to 75 nmol NH₄⁺ mg protein⁻¹ min⁻¹). A ¹⁵N tracer experiment demonstrated that coupling of nitrate-dependent Fe²⁺ oxidation and the anammox reaction was responsible for producing nitrogen gas from NO₃⁻ by “Ca. Brocadia sinica.” The activities of nitrate-dependent Fe²⁺ oxidation were dependent on temperature and pH, and the highest activities were seen at temperatures of 30 to 45°C and pHs ranging from 5.9 to 9.8. The mean half-saturation constant for NO₃⁻ ± SD of “Ca. Brocadia sinica” was determined to be 51 ± 21 μM. Nitrate-dependent Fe²⁺ oxidation was further demonstrated by another anammox bacterium, “Candidatus Scalindua sp.,” whose rates of Fe²⁺ oxidation and NO₃⁻ reduction were 4.7 ± 0.59 and 1.45 ± 0.05 nmol mg protein⁻¹ min⁻¹, respectively (20°C and pH 7.3). Co-occurrence of nitrate-dependent Fe²⁺ oxidation and the anammox reaction decreased the molar ratios of consumed NO₂⁻ to consumed NH₄⁺ (ΔNO₂⁻/ΔNH₄⁺) and produced NO₃⁻ to consumed NH₄⁺ (ΔNO₃⁻/ΔNH₄⁺). These reactions are preferable to the application of anammox processes for wastewater treatment.     

Kinetic Studies of Bacterial Sulfate Reduction in Freshwater Sediments by High-Pressure Liquid Chromatography and Microdistillation

Indirect photometric chromatography and microdistillation enabled a simultaneous measurement of sulfate depletion and sulfide production in the top 3 cm of freshwater sediments to be made. The simultaneous measurement of sulfate depletion and sulfide production rates provided added insight into microbial sulfur metabolism. The lower sulfate reduction rates, as derived from the production of acid-volatile ³⁵S²⁻ only, were explained by a conversion of this pool to an undistillable fraction under acidic conditions during incubation. A mathematical model was applied to calculate sulfate reduction from sulfate gradients at the sediment-water interface. To avoid disturbance of these gradients, the sample volume was reduced to 0.2 g (wet weight) of sediment. Sulfate diffusion coefficients in the model were determined (D_s = 0.3 × 10⁻⁵ cm² s⁻¹ at 6°C). The results of the model were compared with those of radioactive sulfate turnover experiments by assessing the actual turnover rate constants (2 to 5 day⁻¹) and pool sizes of sulfate at different sediment depths.     

Consumption of Methane and CO2 by Methanotrophic Microbial Mats from Gas Seeps of the Anoxic Black Sea

The deep anoxic shelf of the northwestern Black Sea has numerous gas seeps, which are populated by methanotrophic microbial mats in and above the seafloor. Above the seafloor, the mats can form tall reef-like structures composed of porous carbonate and microbial biomass. Here, we investigated the spatial patterns of CH₄ and CO₂ assimilation in relation to the distribution of ANME groups and their associated bacteria in mat samples obtained from the surface of a large reef structure. A combination of different methods, including radiotracer incubation, beta microimaging, secondary ion mass spectrometry, and catalyzed reporter deposition fluorescence in situ hybridization, was applied to sections of mat obtained from the large reef structure to locate hot spots of methanotrophy and to identify the responsible microbial consortia. In addition, CO₂ reduction to methane was investigated in the presence or absence of methane, sulfate, and hydrogen. The mat had an average δ¹³C carbon isotopic signature of −67.1‰, indicating that methane was the main carbon source. Regions dominated by ANME-1 had isotope signatures that were significantly heavier (−66.4‰ ± 3.9 ‰ [mean ± standard deviation; n = 7]) than those of the more central regions dominated by ANME-2 (−72.9‰ ± 2.2 ‰; n = 7). Incorporation of ¹⁴C from radiolabeled CH₄ or CO₂ revealed one hot spot for methanotrophy and CO₂ fixation close to the surface of the mat and a low assimilation efficiency (1 to 2% of methane oxidized). Replicate incubations of the mat with ¹⁴CH₄ or ¹⁴CO₂ revealed that there was interconversion of CH₄ and CO_2. The level of CO₂ reduction was about 10% of the level of anaerobic oxidation of methane. However, since considerable methane formation was observed only in the presence of methane and sulfate, the process appeared to be a rereaction of anaerobic oxidation of methane rather than net methanogenesis.     

Kinetic Parameters of the Conversion of Methane Precursors to Methane in a Hypereutrophic Lake Sediment

The kinetic parameters K_m, V_max, T_t (turnover time), and v (natural velocity) were determined for H₂ and acetate conversion to methane by Wintergreen Lake sediment, using short-term (a few hours) methods and incubation temperatures of 10 to 14°C. Estimates of the Michaelis-Menten constant, K_m, for both the consumption of hydrogen and the conversion of hydrogen to methane by sediment microflora averaged about 0.024 μmol g⁻¹ of dry sediment. The maximal velocity, V_max, averaged 4.8 μmol of H₂ g⁻¹ h⁻¹ for hydrogen consumption and 0.64 μmol of CH₄ g⁻¹ h⁻¹ for the conversion of hydrogen to methane during the winter. Estimated natural rates of hydrogen consumption and hydrogen conversion to methane could be calculated from the Michaelis-Menten equation and estimates of K_m, V_max, and the in situ dissolved-hydrogen concentration. These results indicate that methane may not be the only fate of hydrogen in the sediment. Among several potential hydrogen donors tested, only formate stimulated the rate of sediment methanogenesis. Formate conversion to methane was so rapid that an accurate estimate of kinetic parameters was not possible. Kinetic experiments using [2-¹⁴C]acetate and sediments collected in the summer indicated that acetate was being converted to methane at or near the maximal rate. A minimum natural rate of acetate conversion to methane was estimated to be about 110 nmol of CH₄ g⁻¹ h⁻¹, which was 66% of the V_max (163 nmol of CH₄ g⁻¹ h⁻¹). A 15-min preincubation of sediment with 5.0 × 10⁻³ atm of hydrogen had a pronounced effect on the kinetic parameters for the conversion of acetate to methane. The acetate pool size, expressed as the term K_m + S_n (S_n is in situ substrate concentration), decreased by 37% and T_t decreased by 43%. The V_max remained relatively constant. A preincubation with hydrogen also caused a 37% decrease in the amount of labeled carbon dioxide produced from the metabolism of [U-¹⁴C]valine by sediment heterotrophs.     

Coal-Packed Methane Biofilter for Mitigation of Green House Gas Emissions from Coal Mine Ventilation Air

Methane emitted by coal mine ventilation air (MVA) is a significant greenhouse gas. A mitigation strategy is the oxidation of methane to carbon dioxide, which is approximately twenty-one times less effective at global warming than methane on a mass-basis. The low non-combustible methane concentrations at high MVA flow rates call for a catalytic strategy of oxidation. A laboratory-scale coal-packed biofilter was designed and partially removed methane from humidified air at flow rates between 0.2 and 2.4 L min⁻¹ at 30°C with nutrient solution added every three days. Methane oxidation was catalysed by a complex community of naturally-occurring microorganisms, with the most abundant member being identified by 16S rRNA gene sequence as belonging to the methanotrophic genus Methylocystis. Additional inoculation with a laboratory-grown culture of Methylosinus sporium, as investigated in a parallel run, only enhanced methane consumption during the initial 12 weeks. The greatest level of methane removal of 27.2±0.66 g methane m⁻³ empty bed h⁻¹ was attained for the non-inoculated system, which was equivalent to removing 19.7±2.9% methane from an inlet concentration of 1% v/v at an inlet gas flow rate of 1.6 L min⁻¹ (2.4 min empty bed residence time). These results show that low-cost coal packing holds promising potential as a suitable growth surface and contains methanotrophic microorganisms for the catalytic oxidative removal of methane.     

Effect of Temperature on Anaerobic Ethanol Oxidation and Methanogenesis in Acidic Peat from a Northern Wetland

The effects of temperature on rates and pathways of CH₄ production and on the abundance and structure of the archaeal community were investigated in acidic peat from a mire in northern Scandinavia (68°N). We monitored the production of CH₄ and CO₂ over time and measured the turnover of Fe(II), ethanol, and organic acids. All experiments were performed with and without specific inhibitors (2-bromoethanesulfonate [BES] for methanogenesis and CH₃F for acetoclastic methanogenesis). The optimum temperature for methanogenesis was 25°C (2.3 μmol CH₄ · g [dry weight]⁻¹ · day⁻¹), but the activity was relatively high even at 4°C (0.25 μmol CH₄ · g [dry weight]⁻¹ · day⁻¹). The theoretical lower limit for methanogenesis was calculated to be at −5°C. The optimum temperature for growth as revealed by real-time PCR was 25°C for both archaea and bacteria. The population structure of archaea was studied by terminal restriction fragment length polymorphism analysis and remained constant over a wide temperature range. Hydrogenotrophic methanogenesis accounted for about 80% of the total methanogenesis. Most 16S rRNA gene sequences that were affiliated with methanogens and all McrA sequences clustered with the exclusively hydrogenotrophic order Methanobacteriales, correlating with the prevalence of hydrogenotrophic methanogenesis. Fe reduction occurred parallel to methanogenesis and was inhibited by BES, suggesting that methanogens were involved in Fe reduction. Based upon the observed balance of substrates and thermodynamic calculations, we concluded that the ethanol pool was oxidized to acetate by the following two processes: syntrophic oxidation with methanogenesis (i) as an H₂ sink and (ii) as a reductant for Fe(III). Acetate accumulated, but a considerable fraction was converted to butyrate, making volatile fatty acids important end products of anaerobic metabolism.     

Methane Production in Minnesota Peatlands

Rates of methane production in Minnesota peats were studied. Surface (10- to 25-cm) peats produced an average of 228 nmol of CH₄ per g (dry weight) per h at 25°C and ambient pH. Methanogenesis rates generally decreased with depth in ombrotrophic peats, but on occasion were observed to rise within deeper layers of certain fen peats. Methane production was temperature dependent, increasing with increasing temperature (4 to 30°C), except in peats from deeper layers. Maximal methanogenesis from these deeper regions occurred at 12°C. Methane production rates were also pH dependent. Two peats with pHs of 3.8 and 4.3 had an optimum rate of methane production at pH 6.0. The addition to peat of glucose and H₂-CO₂ stimulated methanogenesis, whereas the addition of acetate inhibited methanogenesis. Cysteine-sulfide, nitrogen-phosphorus-trace metals, and vitamins-yeast extract affected methane production very little. Various gases were found to be trapped or dissolved (or both) within peatland waters. Dissolved methane increased linearly to a depth of 210 cm. The accumulation of metabolic end products produced within peat bogs appears to be an important mechanism limiting carbon turnover in peatland environments.     

Effects of Abiotic Factors on Acetylene Reduction by Cyanobacteria Epiphytic on Moss at a Subantarctic Island

Acetylene reduction (AR) rates by cyanobacteria epiphytic on a moss at Marion Island (46°54′ S, 37°45′ E) increased from −5°C to a maximum at 25 to 27°C. Q₁₀ values between 0 and 25°C were between 2.3 and 2.9, depending on photosynthetic photon flux density. AR rates declined sharply at temperatures above the optimum and were lower at 35°C than at 0°C. Photosynthetic photon flux density at low levels markedly influenced AR, and half of the maximum rate occurred at 84 μmol m⁻² s⁻¹, saturation occurring at ca. 1,000 μmol m⁻² s⁻¹. Higher photosynthetic photon flux density levels decreased AR rates. AR increased up to the highest sample moisture content investigated (3,405%), and the pH optimum was between 5.9 and 6.2. The addition of P, Co, and Mo, individually or together, depressed AR.     

Estimation of Sediment Denitrification Rates at In Situ Nitrate Concentrations

The denitrification rates in a marine sediment, estimated by using ¹⁵N-nitrate, V_max, K_m, and sediment nitrate concentrations, were 12.5 and 2.0 nmol of N₂-N cm⁻³ day⁻¹ at 0 to 1 and 1 to 3 cm, respectively, at 12°C. The total rate was 165 nmol of N₂-N m⁻² day⁻¹.     

Cyclic variations in nitrogen uptake rate in soybean plants

Uptake of NO₃⁻ by nonnodulated soybean plants (Glycine max L. Merr. cv Ransom) growing in flowing hydroponic culture at 22 and 14°C root temperatures was measured daily during a 31-day growth period. Ion chromatography was used to determine removal of NO₃⁻ from solution during each 24-hour period. At both root-zone temperatures, rate of NO₃⁻ uptake per plant oscillated with a periodicity of 3 to 5 days. The rate of NO₃⁻ uptake per plant was consistently lower at 14°C than 22°C. The lower rate of NO₃⁻ uptake at 14°C during the initial 5 to 10 days was caused by reduced uptake rates per gram root dry weight, but with time uptake rates per gram root became equal at 14 and 22°C. Thereafter, the continued reduction in rate of NO₃⁻ uptake per plant at 14°C was attributable to slower root growth.     

Method for Measuring Rates of NH4+ Turnover in Anoxic Marine Sediments, Using a 15N-NH4+ Dilution Technique

A method is described for the determination of the net and total rates of NH₄⁺ production and NH₄⁺ incorporation at different depths in an anoxic marine sediment. ¹⁵N-NH₄⁺ was added to the sediment NH₄⁺ pool, and the ¹⁵N content was assayed after 0, 2, and 5 days of incubation. The pool size changed during incubation; this change in pool size is incorporated into a model which predicts the dynamics of ¹⁵N-NH₄⁺ dilution. A simple microdiffusion of NH₃ was followed by an emission spectrometry analysis of ¹⁵N content. This procedure avoided all problems of cross-contamination. The model was tested and rates were measured in four sediment cores, at seven different depths. The high correlation coefficients (mean, 0.96 for the 0- to 2-, 2- to 4-, 4- to 6-, and 6- to 8-cm sediment fractions) indicated that the model was correct and that the measured rates were valid. The immediate distribution of ¹⁵N-NH₄⁺ between interstitial and exchangeable NH₄⁺ pools indicated that it was the combined pool that was turning over. In the 0- to 2-cm fraction at 17°C the net rate of NH₄⁺ production was 274 (standard deviation, 31) nmol cm⁻³ day⁻¹, and the mean total rate of NH₄⁺ production was 309 (standard deviation, 39) nmol cm⁻³ day⁻¹; both rates decreased to <1% of these values in the 12- to 14-cm fractions.     

Burrows of the Semi-Terrestrial Crab Ucides cordatus Enhance CO2 Release in a North Brazilian Mangrove Forest

Ucides cordatus is an abundant mangrove crab in Brazil constructing burrows of up to 2 m depth. Sediment around burrows may oxidize during low tides. This increase in sediment-air contact area may enhance carbon degradation processes. We hypothesized that 1) the sediment CO₂ efflux rate is greater with burrows than without and 2) the reduction potential in radial profiles in the sediment surrounding the burrows decreases gradually, until approximating non-bioturbated conditions. Sampling was conducted during the North Brazilian wet season at neap tides. CO₂ efflux rates of inhabited burrows and plain sediment were measured with a CO₂/H₂O gas analyzer connected to a respiration chamber. Sediment redox potential, pH and temperature were measured in the sediment surrounding the burrows at horizontal distances of 2, 5, 8 and 15 cm at four sediment depths (1, 10, 30 and 50 cm) and rH values were calculated. Sediment cores (50 cm length) were taken to measure the same parameters for plain sediment. CO₂ efflux rates of plain sediment and individual crab burrows with entrance diameters of 7 cm were 0.7–1.3 µmol m⁻² s⁻¹ and 0.2–0.4 µmol burrows⁻¹ s⁻¹, respectively. CO₂ released from a Rhizophora mangle dominated forest with an average of 1.7 U. cordatus burrows⁻¹ m⁻² yielded 1.0–1.7 µmol m⁻² s⁻¹, depending on the month and burrow entrance diameter. Laboratory experiments revealed that 20–60% of the CO₂ released by burrows originated from crab respiration. Temporal changes in the reduction potential in the sediment surrounding the burrows did not influence the CO₂ release from burrows. More oxidized conditions of plain sediment over time may explain the increase in CO₂ release until the end of the wet season. CO₂ released by U. cordatus and their burrows may be a significant pathway of CO₂ export from mangrove sediments and should be considered in mangrove carbon budget estimates.