Community Metabolism in Tropical Undrainable Rural Fish Ponds

The process of community metabolism including primary production and community respiration in eighteen undrainable rural fish ponds in India was quantified using the diurnal oxygen curve method. A considerable portion of the gross primary production that was in the moderate ranges, was consumed for the community respiration. Negative, and low positive values of net primary production indicated the significance of the allochthonous supply in these organic enriched systems. The low values of sediment respiration signified the reduced condition of stagnating sediment acting as energy trap.     

Oxygen and nitrate respiration in a reed swamp sediment from a eutrophic lake

Seasonal measurements of the oxygen and nitrate uptake by a reed swamp sediment were carried out in a shallow, eutrophic Danish lake, Arreskov Sø. The oxidation of organic carbon in the sediment by aerobic and nitrate respiration was 290 and 188 g C m⁻² yr⁻¹ respectively. During winter, nitrate respiration amounted to 94% of the total carbon oxidation, whereas it was zero during summer. On an annual basis nitrate respiration constituted 39% of total respiration. Sediment nitrate uptake was correlated to nitrate concentration. In consequence of this the nitrate uptake rates varied during the year from zero in summer to 55 mg N m⁻² d⁻¹ in spring.
Oxygen uptake rates varied from 30 to 250 mg O₂ m⁻² h⁻¹ during the year, with a maximum uptake in August. The oxygen uptake per year was calculated to 860 g O₂ m⁻². The oxygen uptake rate was correlated to lake temperature and Kjeldahl nitrogen content of the sediment. The oxygen uptake rate, however, showed no correlation with loss on ignition of the sediment. A Q₁₀-value of 2.2 was found for lake measurements in the temperature interval of 5–15°C. The corresponding O₁₀-value in the laboratory was 2.6. A high microbial biomass indicated by the maximum content of Kjeldahl nitrogen and the lowest ratio of loss on ignition on Kjeldahl nitrogen appeared in late August, when the maximum oxygen uptake occurred. The oxygen uptake rate increased during the time interval from sampling to the start of the experiments.     

主养草鱼高密度池塘溶氧收支平衡的研究

采用原位生态学的方法测定广东省中山市9口主养草鱼高密度池塘中浮游植物光合作用产氧量、水柱呼吸耗氧量、底泥呼吸耗氧量和鱼呼吸耗氧量, 并用数学模型计算增氧机增氧量及用差减法计算大气扩散作用引起的得氧或失氧, 对高密度养殖池塘中溶氧收支平衡状况进行了研究。结果显示: 在水深为1.5-2.0 m的主养草鱼高密度池塘中, 光合作用产氧量随着水深的增加而显著降低(P0.05), 底层出现负值呈现氧债现象。水呼吸耗氧量在表层、中层和底层之间没有显著差异(P0.05)。表层水光合作用产氧量显著大于水呼吸耗氧量(P0.05), 而中层和底层水光合作用产氧量却显著小于水呼吸耗氧量(P0.05)。在主养草鱼高密度池塘溶氧的收入中, 浮游植物光合作用产氧量、增氧机增氧量和大气扩散溶入氧量分别占总溶氧来源的44.7%、42.3%和13.0%, 机械增氧作用已接近光合作用, 成为溶氧来源的主要贡献者; 在池塘溶氧的支出中, 水呼吸、鱼呼吸和底泥呼吸耗氧量分别占总耗氧量的45.9%、45.0%和9.1%, 鱼呼吸耗氧与水呼吸耗氧相当, 成为水体中氧气的主要消耗者。结果表明在草鱼高密度养殖过程中, 合理使用机械增氧是池塘溶氧管理的有效措施。       

Nitrogen cycling and retention in fish-cum-livestock ponds

Synthesizing the results of 18 PhD studies carried out at the Fisheries Research Institute in Szarvas, Hungary, on fish-cum-pig and fish-cum-duck ponds, the nitrogen cycles in these organically loaded ecosystems were constructed. Data for 13 water and sediment nitrogen components, including is solved and particulate matter, combined and freely dissolved amino acids and dissolved and ‘absorbed’ sediment total ammonia concentrations, were quantitatively analysed. All known nitrogen transfer rates were determined simultaneously or estimated through mechanisms of nitrogen balance: nitrogen fixation, ammonification, ammonia regeneration, amino acid uptake, ammonia uptake, nitrification, nitrate uptake, nitrate respiration and denitrification. Based on management parameters as well as compartment and transfer rate measurements, the nitrogen balance and budgets were also calculated for the experimental ponds and for a fish-cum-duck commercial farm. The farm nitrogen budget was determined by analysing 20 years of nitrogen input-output data. The analysis shows the high assimilative capacity of the fish-cum-livestock ecosystems for nutrients, indicating the possible buffer function of the system in highly utilized agricultural areas.     

Heterotrophic bacterial activity in Roskilde Fjord sediment during an autumn sedimentation peak

Total oxygen uptake, bacterial oxygen uptake, total bacterial biomass and active bacterial biomass were determined at the sediment-water interface at two stations in the brackish Roskilde Fjord between September and December in 1986 before, during and after sedimentation of a phytoplankton bloom. Bacterial oxygen consumption was separated from total oxygen consumption by addition of cycloheximide. The fractional and the absolute bacterial oxygen uptake were greatest at the most eutrophic station, where total oxygen uptake was 870–1740 mg O₂ m^–2 d^–1 and the bacterial oxygen uptake was 232–870 mg O₂ m^–2 d^–1. At the less eutrophic station, total oxygen uptake was 725–1740 mg O₂ m^–2 d^–1. and bacterial oxygen uptake was 200–550 mg O₂ m^–2 d^–1.Active bacterial biomass was separated from total bacterial biomass by addition of the terminal electron acceptor INT-formazan. The active bacterial biomass was 70–120 µg C mg^–1 ww of sediment at the most eutrophic station and 50–90 µg C g^–1 ww of sediment at the other station. Differences in capacity of bacterial oxygen uptake between the two stations correlated to the active bacterial biomass. The non-temperature dependent bacterial oxygen uptake correlated with the sedimentation rate.     

Effects of small scale fluid motion on bacterial growth and respiration

1. Laboratory experiments were conducted to investigate the effect of small‐scale turbulent motion on the growth and respiration of bacteria in an oscillating grid apparatus. The experiments were performed under a range of energy dissipation levels similar to those occurring in freshwater systems. 2. The results showed that small‐scale turbulent motion does have an effect on bacterial growth and respiration. A higher gradient in the dissolved oxygen time series, higher 5‐day biochemical oxygen demand values, increased bacterial abundance, increased bacterial specific respiration, higher bacterial growth rate and increased nutrient uptake were all observed when the energy dissipation rate in the water column was increased. 3. This has implications for traditional laboratory procedures that are used to characterise bacterial metabolic rates under stagnant fluid‐flow conditions, such as biochemical oxygen demand (BOD), which would be influenced by the effects of the small‐scale fluid motion inherent in aquatic environments. According to our results, BOD values in natural systems experiencing fluid motion would be higher than traditional bottle‐derived rates.     

Biological vs. physical mixing effects on benthic food web dynamics

Biological particle mixing (bioturbation) and solute transfer (bio-irrigation) contribute extensively to ecosystem functioning in sediments where physical mixing is low. Macrobenthos transports oxygen and organic matter deeper into the sediment, thereby likely providing favourable niches to lower trophic levels (i.e., smaller benthic animals such as meiofauna and bacteria) and thus stimulating mineralisation. Whether this biological transport facilitates fresh organic matter assimilation by the metazoan lower part of the food web through niche establishment (i.e., ecosystem engineering) or rather deprives them from food sources, is so far unclear. We investigated the effects of the ecosystem engineers Lanice conchilega (bio-irrigator) and Abra alba (bioturbator) compared to abiotic physical mixing events on survival and food uptake of nematodes after a simulated phytoplankton bloom. The (13)C labelled diatom Skeletonema costatum was added to 4 treatments: (1) microcosms containing the bioturbator, (2) microcosms containing the bio-irrigator, (3) control microcosms and (4) microcosms with abiotic manual surface mixing. Nematode survival and subsurface peaks in nematode density profiles were most pronounced in the bio-irrigator treatment. However, nematode specific uptake (Δδ(13)C) of the added diatoms was highest in the physical mixing treatment, where macrobenthos was absent and the diatom (13)C was homogenised. Overall, nematodes fed preferentially on bulk sedimentary organic material rather than the added diatoms. The total C budget (μg C m(-2)), which included TO(13)C remaining in the sediment, respiration, nematode and macrobenthic uptake, highlighted the limited assimilation by the metazoan benthos and the major role of bacterial respiration. In summary, bioturbation and especially bio-irrigation facilitated the lower trophic levels mainly over the long-term through niche establishment. Since the freshly added diatoms represented only a limited food source for nematodes, the macrobenthic effect was more pronounced in niche establishment than the negative structuring effects such as competition.     

Respiration in eutrophic lakes: the contribution of bacterioplankton and bacterial growth yield

The contribution of bacterioplankton to total plankton respirationwas measured in two eutrophic Danish lakes and in experimentalenclosures treated with planktivorous fish and nutrients. Bacterialrespiration was calculated from measured oxygen uptake ratesin particles passing a 1.0-µm pore size filter, the rateswere then corrected for the size distribution of glucose uptake.During summer the respiration of the planktonic bacteria contributed{small tilde}50% of the community respiration in the two lakes.Prolific phytoplankton growth induced by biomanipulation andnutrient addition created situations where the contributionof the bacteria decreased to 20%. High bacterial contributionsto community respiration were found when the phytoplankton biomassdecreased. Simultaneous measurements of bacterial respirationand production (by means of [³H]thymidine incorporation) allowedan estimation of bacterial growth yield, which ranged from 9to 66%. In the two lakes the growth yield was constant witha mean of 29 ± 5% (±SD, RQ = 1). The variabilityof the growth yield was larger in the enclosures. The wide range(9–66%) was mainly caused by changes in bacterial netproduction without concomitant changes in respiration. The discussionincludes an evaluation of the oxygen uptake method in size fractionatedsamples and the availability of labile organic substrates asa factor controlling bacterial growth yield. Present address: Institute of Biology and Chemistry, Universityof Roskilde, P. Box 260, DK-4000 Roskilde, Denmark     

Bacterial communities and greenhouse gas emissions of shallow ponds in the High Arctic

Permafrost thawing in lowland Arctic tundra results in a polygonal patterned landscape and the formation of numerous small ponds. These ponds emit biologically mediated carbon dioxide (CO₂) and methane (CH₄). Their greenhouse gas (GHG) emissions are variable, for reasons that are not well understood. Emissions are related to a balance between GHG producers and consumers, as well as to physical properties of the water column controlling gas exchange rates with the atmosphere. Here, we investigated the bacterial diversity of polygonal and runnel ponds, two geomorphologically distinct pond types commonly found in continuous permafrost regions. Using a combination of 16S rRNA Sanger sequencing and high-throughput amplicon sequencing, we found that bacterial communities in overlying waters were clearly dominated by carbon degraders and were similar in both pond types, despite their variable physical and chemical properties. However, surface sediment communities in the two pond types were significantly different. Polygonal pond sediment was colonized by carbon degraders (46–29 %), cyanobacteria (20–27 %) that take up CO₂ and produce oxygen, and methanotrophs (11–20 %) that consume CH₄ and require oxygen. In contrast, cyanobacteria were effectively absent from the surface sediment of runnel ponds, which in addition to carbon degraders (65–81 %), were colonized by purple non-sulfur bacteria (5–21 %), and by fewer methanotrophs (1–5 %). The link between the methanotrophic community and the type of ponds could potentially be used to improve upscale estimates of GHG emissions based on landscape morphology in such remote regions.     

The effects of abscisic acid on growth and respiratory characteristics of potato tuber tissue discs

Incubation of potato tuber tissue discs on B5 medium supplemented with 1-naphtyl-acetic acid (NAA) led to callus formation, irrespective of the presence of kinetin; without NAA no callus formation occurred. Incubation in the presence of abscisic acid (ABA) reduced the increases in fresh weight and dry weight both in callus-forming and in non-callus-forming tissue. Mitochondrial respiration was lowered by ABA as well. The induction of the alternative, CN-resistant pathway was inhibited by the presence of ABA, especially in mitochondria from non-callus-forming tissue.The in vivo respiration of the callus-forming tissue was higher than that of the non-callus-forming tissue. Total respiration, cytochrome pathway activity and the capacity of the alternative pathway were all lowered in callus-forming tissue by treatment with ABA. The in vivo activity of the alternative pathway was low in all tissue types, especially after ABA-treatment. The slight stimulation by hydroxamates of the oxygen uptake of callus-forming tissue incubated on medium with NAA and ABA indicates the involvement of a hydroxamate-activated peroxidase in the oxygen uptake of this tissue; this peroxidase seemed not to participate in the oxygen uptake of the other tissues types.In non-callus-forming tissue the oxygen uptake of ABA-treated tissue was very low and almost completely resistant to the combined addition of inhibitors of both the cytochrome and the alternative pathway, indicating that the in vivo activity of the mitochondria in the oxygen uptake of the tissue was very low. The possible causes for this ABA-effect are discussed. In non-callus-forming tissue the treatment with ABA creates a situation which is comparable with that observed in intact potato tubers. This situation is characterized by a tissue respiration lower than that of the isolated mitochondria and an alternative pathway capacity that is low or absent.     

Exchange between interstitial and surface water: Implications for stream metabolism and nutrient cycling

Metabolism of a Sonoran Desert stream was investigated by both enclosure and whole system oxygen techniques. We used recirculating chambers to estimate surface sediment metabolism and measured deep sediment respiration in isolated sediment cores. Metabolism of the stream ecosystem was determined for a 30-m reach as dark and light oxygen change with and without black plastic sheeting that darkened the stream and prevented diffusion. Average ecosystem respiration for two dates in August (440 mg O₂ m^-2 h^-1) exceeded respiration of either the surface sediment community (155 Mg O₂ m^-2 h^-1) or the hyporheic community (170 mg O₂ m^-2 h^-1) alone. Deep sediments show substantial oxygen and nitrate uptake when isolated. In the stream, this low nitrate interstitial water is exchanged with surface water. Metabolism of the isolated surface community suggests a highly productive and autotrophic system, yet gross production is balanced or exceeded by community respiration when ecosystem boundaries include the hyporheic zone. Thus, despite high rates of gross primary production (600–1200 mg O₂ m^-2 h^-1), desert streams may be heterotrophic (P_G < R) during summer.     

Oxygen regulation of nitrate uptake in denitrifying Pseudomonas aeruginosa.

Oxygen had an immediate and reversible inhibitory effect on nitrate respiration by denitrifying cultures of Pseudomonas aeruginosa. Inhibition of nitrate utilization by oxygen appeared to be at the level of nitrate uptake, since nitrate reduction to nitrite in cell extracts was not affected by oxygen. The degree of oxygen inhibition was dependent on the concentration of oxygen, and increasing nitrate concentrations could not overcome the inhibition. The inhibitory effect of oxygen was maximal at approximately 0.2% oxygen saturation. The inhibition appeared to be specific for nitrate uptake. Nitrite uptake was not affected by these low levels of aeration, and nitrite reduction was only partially inhibited in the presence of oxygen. The regulation of nitrate respiration at the level of transport by oxygen may represent a major mechanism by which the entire denitrification pathway is regulated in P. aeruginosa.     

Oxygen regulation of nitrate uptake in denitrifying Pseudomonas aeruginosa

Oxygen had an immediate and reversible inhibitory effect on nitrate respiration by denitrifying cultures of Pseudomonas aeruginosa. Inhibition of nitrate utilization by oxygen appeared to be at the level of nitrate uptake, since nitrate reduction to nitrite in cell extracts was not affected by oxygen. The degree of oxygen inhibition was dependent on the concentration of oxygen, and increasing nitrate concentrations could not overcome the inhibition. The inhibitory effect of oxygen was maximal at approximately 0.2% oxygen saturation. The inhibition appeared to be specific for nitrate uptake. Nitrite uptake was not affected by these low levels of aeration, and nitrite reduction was only partially inhibited in the presence of oxygen. The regulation of nitrate respiration at the level of transport by oxygen may represent a major mechanism by which the entire denitrification pathway is regulated in P. aeruginosa.     

New and fast method to quantify respiration rates of bacterial and plankton communities in freshwater ecosystems by using optical oxygen sensor spots

A new method of respiration rate measurement based on oxygen luminescence quenching in sensor spots was evaluated for the first time for aquatic bacterial communities. The commonly used Winkler and Clark electrode methods to quantify oxygen concentration both require long incubation times, and the latter additionally causes signal drift due to oxygen consumption at the cathode. The sensor spots proved to be advantageous over those methods in terms of precise and quick oxygen measurements in natural bacterial communities, guaranteeing a respiration rate estimate during a time interval short enough to neglect variations in organism composition, abundance, and activity. Furthermore, no signal drift occurs during measurements, and respiration rate measurements are reliable even at low temperatures and low oxygen consumption rates. Both a natural bacterioplankton sample and a bacterial isolate from a eutrophic river were evaluated in order to optimize the new method for aquatic microorganisms. A minimum abundance of 2.2 x 10(6) respiring cells ml(-1) of a bacterial isolate was sufficient to obtain a distinct oxygen depletion signal within 20 min at 20 degrees C with the new oxygen sensor spot method. Thus, a culture of a bacterial isolate from a eutrophic river (OW 144; 20 x 10(6) respiring bacteria ml(-1)) decreased the oxygen saturation about 8% within 20 min. The natural bacterioplankton sample respired 2.8% from initially 94% oxygen-saturated water in 30 min. During the growth season in 2005, the planktonic community of a eutrophic river consumed between 0.7 and 15.6 micromol O(2) liter(-1) h(-1). The contribution of bacterial respiration to the total plankton community oxygen consumption varied seasonally between 11 and 100%.     

Oxygen uptake by roots of Rumex species at different temperatures: the relative importance of diffusive resistance and enzyme kinetics

Abstract. Oxygen uptake characteristics of the roots of three Rumex species were compared, and related to kinetics of the respiratory system and to root anatomy. The observed differences could not be explained by differences in fundamental characteristics of the oxygen uptake system: with all three species, cytochrome-mediated respiration contributed 70% and cyanide-insensitive (alternative) respiration 30% of the total respiration rate, and apparent K_m values of cytochrome oxidase were lower than those obtained for the alternative oxidase in all cases. However, differences in critical oxygen pressure for respiration (COPR) and in apparent K_m for oxygen, were strongly correlated with differences in root porosity and root diameter. K_m(O₂) values at high and low temperatures were determined, and from Arrhenius plots of oxygen uptake rates between 11 and 32°C, the role of diffusional impedance could be estimated. Root respiration of Rumex maritimus and R. crispus , both with high root porosity, but differing in root diameter, had a low K_m for oxygen (3–7 mmol m⁻³). In contrast with this were the responses of R. thvrsiflorus , which has thin roots but low root porosity: a high K_m (10-20 mmol m⁻³) was found at all temperatures. The role of diffusional impedance as a function of temperature in oxygen uptake rate by the three species is discussed and related to the differential resistance of the species towards flooding.     

Effects of bioturbation by tube-dwelling chironomid larvae on oxygen uptake and denitrification in eutrophic lake sediments

1. Oxygen uptake and denitrification were determined in two bioturbated sediments from a eutrophic lake in southern Sweden. In laboratory mesocosms, an organic profundal sediment was incubated with Chironomus plumosus L. and a sandy littoral sediment with an organic-rich top layer was incubated with Polypedilum sp. Both species of chironomid are sediment tube-dwelling. 2. Oxygen consumption, expressed per gram of larval dry weight, was enhanced to the same extent by the larvae in both sediments. Measurements of the respiration rate of individual larvae revealed that the respiration per gram dry weight of the smaller Polypedilum sp. was more than three times higher than that of C. plumosus. 3. Denitrification was measured using the ‘nitrogen isotope pairing’ technique. In the organic sediment, denitrification of nitrate from the water phase (dw) and denitrification of nitrate from coupled nitrification (dn) were each correlated with the biomass of C. plumosus. In the sandy sediment, dw was correlated with the biomass of Polypedilum sp., while dn did not show any correlation with Polypedilum sp. 4. Oxygen uptake in the organic sediment was increased by a factor of 2.5, dw 5-fold and dn 2.5-fold at a biomass of 10 g m^–2 dry weight of C. plumosus. The same biomass of Polypedilum sp. in the sandy sediment resulted in a 2-fold stimulation of oxygen uptake and a 3-fold stimulation of dw, while dn was not affected. These differences in stimulation between oxygen uptake and denitrification by the larvae in the sediments suggest that the stimulation pattern cannot be explained by simple extension of the sediment surface. The burrows evidently reduce the distance between the nitrate source in the water column and the denitrifiers in the anoxic zones. 5. This study indicates that bioturbation by macrofauna elements can have a great impact on denitrification in lake sediments, and that different organisms can influence nitrogen turnover in specific ways.     

Temporal variations in microbenthic metabolism and inorganic nitrogen fluxes in sandy and muddy sediments of a tidally dominated bay in the northern Wadden Sea

Factors controlling seasonal variations in benthic metabolism (O₂ flux) and dissolved inorganic nitrogen (DIN) fluxes were examined during a 12–14 month period at three intertidal Wadden Sea stations. Since the flux measurements were made as small-scale laboratory core incubations, the results are primarily related to the microbenthic community (microalgae, bacteria, micro-, meio- and small macrofauna) and cannot be considered representative of the total benthic community in the Wadden Sea. Furthermore, it has to be emphasized that light intensity during day-time simulations were constant and saturating at all times. Benthic primary production and oxygen uptake appeared to be temperature dependent with a ‘seasonal Q₁₀’ of 1.7–1.8 and 2.7–4.3, respectively. Inundation had no effect on oxygen fluxes as evidenced by similar sediment respiration with and without water cover. A stronger temperature dependence of primary production in muddy than in sandy sediment indicated that the overall control in the latter may be complex due to factors like macrofaunal grazing and nutrient availability. Benthic respiration may not be controlled by temperature alone, as sedimentary organic matter content correlated significantly with both temperature and benthic respiration. Annual gross primary production in high intertidal sandy sediment was 10 and 50% higher than in low intertidal sandy and muddy sediments, respectively. Since annual benthic community respiration was 2 times higher in muddy than sandy sediments, the annual net primary production was about 0 in the former and 17–19 mol C m^?2 yr^?1 in the latter. However, heterotrophic contribution by larger faunal components as well as removal of organic carbon by waves and tidal currents, which are not included here, may balance the budget at the sandy stations. There was no or only weak relationships between (light and dark) DIN exchange and factors like temperature, sedimentary organic content, and oxygen fluxes. Factors related to nutrient fluxes, such as denitrification and nutrient concentration in the overlying water, may have hampered any such relationships. In fact, DIN fluxes at all three stations appeared to be strongly controlled by DIN concentrations in the overlying water. On an annual basis, the sediment appeared to be a net sink for DIN.     

The physiological basis of seed dormancy in Avena fatua VI. Respiration and the stimulation of germination by ethanol

Ethanol induced germination in several partly after-ripened dormant lines of Avena fatua L. The dose-response curves for the stimulation of germination and for oxygen uptake were similar, indicating that ethanol may stimulate germination by promoting oxygen uptake. A time-sequence study showed that ethanol stimulated oxygen uptake by as much as 70% prior to the first visible signs of germination. A similar methanol treatment failed to induce germination or significantly elevate oxygen uptake, indicating that the promotive effects of ethanol are not common to all alcohols. The stimulation of both germination and oxygen uptake by ethanol was not inhibited significantly by salicylhydroxamic acid, an inhibitor of alternative respiration. Thus, stimulation of germination and oxygen uptake by ethanol does not require the operation of the alternative pathway of respiration. Similarly, the stimulation of germination and oxygen uptake by ethanol were not inhibited by sodium azide, an inhibitor of cytochrome-mediated respiration. However, both germination and oxygen uptake were prevented when salicylhydroxamic acid and sodium azide were administered together. Thus, stimulation of these events by ethanol requires only the operation of one or other of these pathways of respiration; a specific requirement for the operation of the alternative pathway of respiration does not exist. The function of ethanol as a promoter of respiration is discussed with reference to dormancy and involvement of the Krebs cycle.     

Characterization of nitrifying, denitrifying, and overall bacterial communities in permeable marine sediments of the northeastern Gulf of Mexico

Sandy or permeable sediment deposits cover the majority of the shallow ocean seafloor, and yet the associated bacterial communities remain poorly described. The objective of this study was to expand the characterization of bacterial community diversity in permeable sediment impacted by advective pore water exchange and to assess effects of spatial, temporal, hydrodynamic, and geochemical gradients. Terminal restriction fragment length polymorphism (TRFLP) was used to analyze nearly 100 sediment samples collected from two northeastern Gulf of Mexico subtidal sites that primarily differed in their hydrodynamic conditions. Communities were described across multiple taxonomic levels using universal bacterial small subunit (SSU) rRNA targets (RNA- and DNA-based) and functional markers for nitrification (amoA) and denitrification (nosZ). Clonal analysis of SSU rRNA targets identified several taxa not previously detected in sandy sediments (i.e., Acidobacteria, Actinobacteria, Chloroflexi, Cyanobacteria, and Firmicutes). Sequence diversity was high among the overall bacterial and denitrifying communities, with members of the Alphaproteobacteria predominant in both. Diversity of bacterial nitrifiers (amoA) remained comparatively low and did not covary with the other gene targets. TRFLP fingerprinting revealed changes in sequence diversity from the family to species level across sediment depth and study site. The high diversity of facultative denitrifiers was consistent with the high permeability, deeper oxygen penetration, and high rates of aerobic respiration determined in these sediments. The high relative abundance of Gammaproteobacteria in RNA clone libraries suggests that this group may be poised to respond to short-term periodic pulses of growth substrates, and this observation warrants further investigation.     

Loss and recovery of nitrogenase in Abuts incana nodules exposed to low oxygen and low temperature

The possibility that respiration limits oxygen access to nitrogenase was tested by artificially upsetting the balance between oxygen consumption (respiration) and oxygen influx (diffusion). Argon treatment of the nodulated root system on intact plants stopped in vivo nitrogenase activity almost completely. Upon return to air, nitrogenase activity was very low and recovered gradually to full activity after about 5 h. In vitro measurements on nodule homogenates indicated that active nitrogenase was lost upon the shift from low (argon) to normal (air) oxygen. Maintenance of nodulated root systems at low temperature (2°C) inhibited both respiration and in vivo nitrogenase activity. Upon return to normal temperature (22°C), oxygen uptake recovered very rapidly, but nitrogenase activity recovered only gradually to full activity after about 5 to 6 h. Again, loss of active nitrogenase could, at least partly, explain the reduced in vivo nitrogenase activity. The effects from a temporarily impaired balance between oxygen consumption and oxygen influx thus point to the importance of respiration for limiting oxygen access to nitrogenase.