Volumetric and aerial rates of heterotrophic bacterial production in epi- and hypolimnia: the role of nutrients and system morphometry

Epilimnetic and hypolimnetic bacterial production (BP) were measured once in summer, by the incorporation of [³H] - Leucine in each of 14 Quebec (Canada) lakes varying in nutrient content and morphometry. The epilimnetic and hypolimnetic BP were evaluated at two scales: the common per unit volume and areal (m^–2) scale. The per unit volume scale epilimnetic BP was best predicted by total phosphorus (TP, r ²=0.63), and by water residence time (WRT r ²=0.57), with WRT serving as a surrogate for the nutrient and organic matter supply from the catchments. Total phosphorus and lake mean depth (Z _m) together explained 79% of the variation in epilimnetic BP (l^–1). In contrast, hypolimnetic BP (l^–1) was neither linked to nutrients (TP or TN) or dissolved organic carbon (DOC) but only to measures of lake morphometry and best of all to hypolimnetic thickness (Zh; r ²=0.74). With increased Zh, there is an increased dilution of settling organic particles and their nutrients, resulting in a decrease in BP per litre. Conversely, when BP is expressed in areal units (m^–2), hypolimnetic production increases with increasing hypolimnetic thickness. Water column thickness is a master variable, which together with Chl a (abundance of particles) determines hypolimnetic BP at the whole system scale even though the trophic status is the best single indicator of epilimnetic BP on a volumetric scale. Conclusions drawn invariably change with the scale of investigation. Moreover, it is clear that lake morphometry has a major impact on BP. A comparison of whole water column integrated BP with literature derived estimates of the equivalent sediment production (m^–2) below suggests that if the estimated sediment rates are not complete technique artefacts, they are likely to be an order of magnitude higher than the water column rates (m^–2) at the maximum depth sampling sites. The relative importance of the sediments could be expected to rise with a decline in the maximum depth of lakes, characterized by progressively thinner hypolimnia. The present findings point to both a primarily allocthonous fuelling of sediment production and an uncoupling of water and sediment BP.     

Effects of post-induction feed strategies on secretory production of recombinant streptokinase in Escherichia coli

The effects of post-induction nutrient feed rates, on recombinant streptokinase production in fed-batch processes, were investigated using various feed profiles. Recombinant streptokinase was produced using a secretory expression system and was induced by a temperature up-shift, using a temperature-sensitive λ_PL promoter. The specific growth rates decreased sharply upon induction of recombinant protein expression, thus necessitating a variable feed strategy in the post-induction phase. The various feed profiles employed in the post-induction phase included constant feed rates, linearly increasing feed rate and exponentially varying feed rates. Significant differences were obtained in the specific activity of streptokinase produced in these fed-batch processes. A maximum activity per unit biomass of 4.96 × 10⁶ and 4.43 × 10⁶ IU/g DCW was achieved for exponentially decreasing feed and linearly increasing feed, respectively. The decrease in specific growth rate during the post-induction phase was also less pronounced in these cases in comparison to other fed-batch experiments. It was observed that streptokinase productivity is governed by the nutrient feed rate per unit biomass at a critical juncture after induction. The highest activity per unit biomass was obtained when the nutrient feed rate per unit biomass was around 0.7–0.8 g glucose (g DCW)⁻¹ h⁻¹, between 2 and 4 h after induction.     

Penetration of solar ultraviolet radiation into New Zealand lakes: influence of dissolved organic carbon and catchment vegetation

Eleven lakes in the South Island of New Zealand were sampled in summer 1996. Water column profiles of ultraviolet radiation (UVR) at four wavelengths and photosynthetically available radiation (PAR) were obtained, along with analyses of dissolved organic carbon (DOC) concentration, total suspended solids (TSS), and catchment vegetation, including forest and natural grassland. Downward attenuation coefficients (K _d) and lake water transparency (1/K _d) for UVR were examined in relation to these variables. Consistent with other regions of the world, DOC concentration and variables related to DOC were the best predictors of UVR penetration. With our data set, we calculated ratios of water column integrals (RI) of UVR/PAR irradiance, using equations from the literature. At DOC concentrations below 4 g m⁻³, a progressive increase in RI shows that lakes become increasingly transparent to UVR. We also normalized chromophoric dissolved organic matter (CDOM) absorption of UVR at 380 nm (a ₃₈₀) to DOC concentration and found that the UVR-absorbing capacity per unit DOC increases with increasing percentage of forest in the catchment area. This indicates that not only DOC concentration but also DOC type or composition is important in determining the transparency of lake water to UVR, and that qualitative differences in DOC are dictated by the type and amount of vegetation present in the lake's catchment area. Received: September 18, 2000 / Accepted: December 14, 2000     

Use of hot wire anemometry to measure velocity of the limb during human movement

Hot film anemometry, x-configuration probes were used in two experiments to evaluate their effectiveness at measurement of limb velocity. Data from tests with a probe attached to the end of a pendulum establish that the hot films measure velocity in the swing phase within 0.098 ms⁻¹. The kinetic energy per unit mass of the pendulum was predicted within ±0.005 m² s⁻², from the measured velocity. In gait experiments with one human subject at speeds greater than 0.25 ms⁻¹, the hot film anemometer and a video system predicted speeds within 0.083 ms⁻¹. The hot film data are electronic signals that are easily stored and processed. The results from these experiments demonstrate that hot film anemometry is an effective and efficient method for direct measurement and analysis of the limb velocity.     

Characterization of β-adrenergic receptor subtypes in androgen-induced mouse kidney hypertrophy using a new high-affinity ligand, [125I]iodocyanopindolol

In fully developed androgen-induced hypertrophy of female mouse kidney, β-adrenergic receptors per unit membrane protein were increased approx. 2.5-fold, as measured by the binding of [¹²⁵I]iodocyanopindolol, with no change in apparent dissociation constants (K_d range 20–25 pM). Membrane protein relative to total kidney protein, Na⁺/K⁺-dependent ATPase (EC 3.6.1.3) and 5′-nucleotidase (EC 3.1.3.5) activities and cholesterol content per unit membrane protein did not differ significantly in preparations from control and treated animals. The binding of iodocyanopindolol to kidney membranes was characterized with respect to association and dissociation kinetics, and also in regard to the less-specific contributions of other major catecholamine or indolamine receptors, using mixtures of the corresponding specific competitors. β1-selective drugs, practolol and metoprolol, and β2-selective agents, IPS-339 and zinterol, were competed with iodocyanopindolol to assess the receptor type specificity, and the ensuing binding profiles were dissected by a nonlinear regression analysis as described by Munson, P.J. and Rodbard, D. (Anal. Biochem. (1982) 107, 220–239). Most of the androgen-induced β-adrenergic receptors had the binding properties corresponding to β2-subtype. No consistent increase in the density of β1-adrenergic recepotors could be shown.     

Identification, isolation, and N-terminal sequencing of style glycoproteins associated with self-incompatibility in Nicotiana alata.

S-Gene-associated glycoproteins (S-glycoproteins) from styles of Nicotiana alata, identified by non-equilibrium two-dimensional electrophoresis, were purified by cation exchange fast protein liquid chromatography with yields of 0.5 to 8 micrograms of protein per style, depending on the S-genotype of the plant. The method relies on the highly basic nature of the S-glycoproteins. The elution profiles of the different S-glycoproteins from the fast protein liquid chromatography column were characteristic of each S-glycoprotein, and could be used to establish the S-genotype of plants in outbreeding populations. In all cases, the S-genotype predicted from the style protein profile corresponded to that predicted from DNA gel blot analysis using S-allele-specific DNA probes and to that established by conventional breeding tests. Amino-terminal sequences of five purified S-glycoproteins showed a high degree of homology with the previously published sequences of N. alata and Lycopersicon esculentum S-glycoproteins.     

Factors influencing nitrogen processing in lakes: an experimental approach

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Spatial and temporal photosynthetic compartments during summer in Antarctic Lake Kitiesh

Summary Four autotrophic compartments were recognised in Lake Kitiesh, King George Island (Southern Shetland) at the beginning of the summer in 1987: snow microalgae, ice bubble communities, phytoplankton in the water column and benthic communities of moss with epiphytes. Chlorophyll a concentration and pigment absorption spectra were obtained in these four compartments before and/or after the thawing of the ice cover. During the ice free period, carbon fixation and biomass was measured in the phytoplankton and in the benthic moss Campyliadelphus polygamus. From these measurements we conclude that the benthic moss is the most significant autotrophic component in this lake in terms of biomass, chlorophyll a content and primary productivity. The integral assimilation number (The ratio of carbon fixation per unit area to biomass per unit area) values were similar for both phytoplankton and the moss, ranging from 3.6 to 5.4 mg C (mg Chl a)^–1h^–1in phytoplankton and from 4.0 to 6.4 mgC (mg Chl a)^–1h^–1 in the benthic moss. This approach allows comparisons of carbon fixation efficiency of the chlorophyll a under a unit area between compartments in their different light environments.     

Gel-filtration analysis of soluble adenylate cyclase from bovine corpus luteum.

1. Sepharose 6B gel-filtration analysis of soluble adenylate cyclase from bovine corpus luteum is described. Both zonal and frontal techniques of analysis were used. 2. Under conditions of zonal analysis recoveries of activity were low. It was concluded that dissociation of two or more components of the adenylate cyclase complex was occurring on the column and that the maintenance of the complex was essential for the high-activity state of the catalytic unit. Two peaks of adenylate cyclase activity, of approximate mol. wts. 45,000 and 160,000 were detected. 3. The theory of frontal analysis (or steady-state gel filtration), applied to the study of the interacting components of the adenylate cyclase complex is discussed, and activity profiles are predicted. Activity profiles obtained experimentally be frontal analysis compared well with the theoretically predicted profile and provide evidence that dissociation of a high-activity complex, with concomitant loss of activity, does occur. Recoveries of activity under conditions of frontal analysis were higher than with zonal analysis. 4. The effects of concentration and removal of detergent on the activity of the soluble enzyme are discussed.     

Compartments of oxygen consumption in a tidal mesotrophic estuary (Ria de Aveiro,Portugal)

Oxygen consumption rates were determined, in parallel with primary production and bacterial biomass production, as an approach to the analysis of carbon cycling in the estuarine community of the Ria de Aveiro. The water column of the marine zone was the major contributor (64–99 %) to the total aerobic carbon remineralisation in which O₂ uptake rates averaged from 80 to 127 mg O₂.m^–2.h^–1, respectively at low tide and high tide. The planktonic consumption of O₂ varied from 0.010 to 0.041 mg O₂.L^–1.h^–1 with the highest values in the brackish zone. Small water column depths in this zone, however, reduced the integrated average consumption of the plankton, per unit of surface area, to 57 (LT) and 66 % (HT) of that observed in the marine zone. Benthic O₂ consumption rates, 5.1 to 22.0 mg O₂.m^–2.h^–1, were two to four times higher in the brackish zone when compared to the rates in the marine zone. It represented 1–31 % of the total surface integrated values in different areas and at different tides. From the ratios of the primary production and bacterial biomass production, on a per surface unit basis, it is concluded that, in late autumn, the Ria de Aveiro was mostly a heterotrophic system with a feeble recovery of primary production at HT in the marine zone and at LT in the brackish water zone.     

The myosin filament: V. Intermediate voltage electron microscopy and optical diffraction studies of the substructure

Using a 200 kV electron microscope (JEM 200 A), thick (up to 0.4 μm) crosssections of the myosin filaments of vertebrate striated muscle were studied. It was found that: (a) with increasing section thickness the cross-sectional profiles of the shaft of the filament were increasingly more triangular and in sections 0.4 μm thick each apex of the triangle was clearly blunted. This unique cross-sectional profile is predicted by the model proposed by Pepe (1966,1967) in which 12 parallel structural units are packed to form a triangular profile with a structural unit missing at each apex of the triangle. (b) With increasing section thickness the substructure of the myosin filament was enhanced, with the best substructure visible in sections 0.2 μm to 0.3 μm thick. This strongly supports parallel alignment of structural units in the shaft of the filament as proposed by Pepe (1966,1967). (c) The substructure spacing, determined by optical diffraction from electron micrographs of cross-sections of individual myosin filaments or groups of filaments is about 4 nm. (d) The different optical diffraction patterns observed from individual myosin filaments can be explained if the projection of each structural unit in the plane of the section has an elongated profile. With a substructure spacing of 4 nm an elongated cross-sectional profile could be produced by having two myosin molecules per structural unit. Models drawn with two myosin molecules per structural unit in the model proposed by Pepe (1966,1967) gave optical diffraction patterns similar to those observed from individual filaments. (e) The different optical diffraction patterns observed from individual myosin filaments can be explained if the elongated profiles in each structural unit are similarly oriented but with the orientation changing along the length of the filament. The change in orientation per unit length of the filament must be small enough to maintain an elongated profile for the projection of the structural unit in the plane of the sections 0.3 μm thick. All of these observations and conclusions strongly support the model for the myosin filament proposed by Pepe (1966,1967).     

Leaf expansion,net CO2 uptake,Rubisco activity,and efficiency of long-term biomass gain for the common desert subshrub Encelia farinosa

Encelia farinosa is one of the most abundant and highly studied species of the Sonoran Desert, yet characteristics of its leaf development and long-term photosynthetic capacity are relatively unknown. The net CO₂ uptake rate and the Rubisco activity per unit leaf area for E. farinosa in a glasshouse increased in parallel for about 18 days after leaf emergence (leaf area was then 5 cm²), after which both were constant, suggesting that Rubisco levels controlled net CO₂ uptake. Instantaneous net CO₂ uptake rates at noon for well-watered E. farinosa in the glasshouse at different temperatures and light levels correctly predicted differences in daily net CO₂ uptake at four seasonally diverse times for transplanted plants under irrigated conditions in the field but overpredicted the daily means by 13%. After this correction, seasonally adjusted net CO₂ uptake per unit leaf area multiplied by the estimated monthly leaf area predicted that 42% of the net carbon gain was incorporated into plant dry weight over a 17-month period. The ecological success of E. farinosa apparently reflects an inherently high daily net CO₂ uptake and retention of a substantial fraction of its leaf carbon gain.     

Comparison of In Vitro and In Vivo Labeling of Virus-Induced L-Cell Interferon

Preparations of NDV_uv-induced L-cell interferon were labeled in vitro with ¹²⁵I and ³H gas, or in vivo through incorporation of amino acids-³H during synthesis. Prior to purification, more than 90% of the interferon titer was lost during in vitro labeling by either procedure, whereas 34% of the initial activity of in vivo-labeled material was preserved during preparatory handling. Purification by carboxymethyl-Sephadex chromatography and electrophoresis in polyacrylamide gels was about 100-fold, and electrophoretic profiles revealed close concordance between isotopes and interferon titers in all instances. Noninterferon proteins from control cells, although less extensively labeled with tritium during synthesis than proteins from interferon-producing cells and released in lesser amounts, also contained components of identical electrophoretic mobility and distribution in acrylamide gels as interferon. The highest specific activity (6 x 10⁶ U/mg protein) but lowest cpm per interferon unit ratio (0.3) were exhibited by in vivo-labeled interferon. The advantage of better isotope incorporation through in vitro labeling techniques was largely offset by extensive losses in interferon activity.     

Effects of 12 weeks of aerobic exercise plus dietary restriction on body composition, resting energy expenditure and aerobic fitness in mildly obese middle-aged women

This study investigated the effects of 12 weeks of aerobic exercise plus voluntary food restriction on the body composition, resting metabolic rate (RMR) and aerobic fitness of mildly obese middle-aged women. The subjects were randomly assigned to exercise/diet (n = 17) or control (n = 15) groups. The exercise/diet group participated in an aerobic training programme, 45–60 min · day ^–1 at 50%–60% of maximal oxygen uptake (VO_2max), 3–4 days · week^–1, and also adopted a self-regulated energy deficit relative to predicted energy requirements (–1.05 MJ · day ^–1 to –1.14 MJ · day ^–1 ). After the regimen had been followed for 12 weeks, the body mass of the subjects had decreased by an average of 4.5 kg, due mainly to fat loss, with little change of fat free mass (m _ff). The absolute RMR did not change, but the experimental group showed significant increases in the RMR per unit of body mass (10%) and the RMR per unit of m _ff (4%). The increase in RMR/m _ff was not correlated with any increase in VO_2max/m _ff. The resting heat production per unit of essential body mass increased by an average of 21%, but the resting heat production rate per unit of fat tissue mass remained unchanged. We concluded that aerobic exercise enhances the effect of moderate dietary restriction by augmenting the metabolic activity of lean tissue.     

Pulse response in an immobilized-enzyme column: Elution profiles in reversible and consecutive reactions

A pulse of substrate solution was applied to an immobilized-enzyme column, in which the substrate was then converted by reversible or consecutive reactions. Immobilized glucose isomerase was used for the reversible reaction, and immobilized invertase and glucose oxidase for the consecutive reactions. The elution profiles of substrate and product were determined experimentally. These profiles were in good agreement with the ones predicted theoretically. The effect of some parameters on the elution profiles for reversible and consecutive reactions is discussed.     

Light absorption by marine macrophytes

Tissues of 338 marine macrophytes comprising 103 species, collected from the Atlantic, Mediterranean, South China, and Caribbean Seas, and encompassing a broad range in thallus form and pigmentation, were examined to quantify the importance of phylogenetic differences, spectral variability, and plant form and pigment content to account for differences in the absorption of light by marine macrophytes. Phylogenetic differences accounted for 2.5% of the variance in absorption observed, non-phylogenetic spectral differences being much larger (26%). Differences among individual specimens were much larger (72%), absorption at 675 nm increasing non-linearly as chlorophyll a density^1/2, indicating that light absorption increases with increasing chlorophyll a density following a law of diminishing returns, as predicted by theory. The energy return per unit tissue produced (i.e. light absorption per unit plant weight) increased linearly with increasing chlorophyll a concentration. However, the light absorbed per unit weight decreased, for a given chlorophyll a concentration, as plant thickness increased. This indicates that while increasing thickness may increase chlorphyll a density and, hence, the light absorbed by marine macrophyte thalli, this strategy represents a burden limiting potential carbon turnover and plant growth. These results indicate that the diverse repertoire of light absorption by marine macrophytes can be adequately modeled as a continuum, dependent on plant thickness and pigment content, independent of phylogenetic differences.     

The effect of fertilization on sap flux and canopy conductance in a Eucalyptus saligna experimental forest

Land devoted to plantation forestry (50 million ha) has been increasing worldwide and the genus Eucalyptus is a popular plantation species (14 million ha) for its rapid growth and ability to grow well on a wide range of sites. Fertilization is a common silvicultural tool to improve tree growth with potential effects on stand water use, but the relationship between wood growth and water use in response to fertilization remains poorly quantified. Our objectives in this study were to determine the extent, timing and longevity of fertilization effects on water use and wood growth in a non‐water limited Eucalyptus saligna experimental forest near Hilo, HI. We evaluated the short‐ and long‐term effects of fertilization on water use by measuring sap flux per unit sapwood area, canopy conductance, transpiration per unit leaf area and water‐use efficiency in control and fertilized stands. Short‐term effects were assessed by comparing sap flux before and after fertilizer application. Long‐term effects were assessed by comparing control plots and plots that had received nutrient additions for 5 years. For the short‐term response, total water use in fertilized plots increased from 265 to 487 mm yr^?1 during the 5 months following fertilization. The increase was driven by an increase in stand leaf area accompanied by an increase in sap flux per unit sapwood area. Sap flux per unit leaf area and canopy conductance did not differ during the 5 months following fertilizer additions. For the last 2 months of our short‐term measurements, fertilized trees used less water per unit carbon gain (361 compared with 751 kg H₂O kg C^?1 in control stands). Trees with 5 years of fertilization also used significantly more water than controls (401 vs. 302 mm yr^?1) because of greater leaf area in the fertilized stands. Sap flux per unit sapwood area, sap flux per unit leaf area, and canopy conductance did not differ between control and fertilized trees in the long‐term plots. In contrast to the short‐term response, the long‐term response of water use per unit wood growth was not significant. Overall, fertilization of E. saligna at our site increased stand water use by increasing leaf area. Fertilized trees grew more wood and used more water, but fertilization did not change wood growth per unit water use.     

Structure of the Photosynthetic Apparatus in Leaves of Freshwater Hydrophytes: 1. General Characteristics of the Leaf Mesophyll and a Comparison with Terrestrial Plants

The structure of photosynthetic elements was investigated in leaves of 42 boreal plant species featuring different degrees of submergence (helophytes, neustophytes, and hydatophytes). The mesophyll structure types were identified for all these species. Chlorenchyma tissues and phototrophic cells were quantitatively described by such characteristics as the sizes of cells and chloroplasts in the mesophyll and epidermis, the abundance of cells and chloroplasts in these tissues, the total surface area of cells and chloroplasts per unit leaf area, the number of plastids per cell, etc. The hydrophytes typically had thick leaves (200–350 m) with a well-developed aerenchyma; their specific density per unit area (100–200 mg/dm²) was lower than in terrestrial plants. Mesophyll cells in aquatic plants occupied a larger volume (5–20 × 10³m³) than epidermal cells (1–15 × 10³m³). The number of mesophyll cells per unit leaf area was nearly 1.5 times higher than that of epidermal cells. Chloroplasts were present in the epidermis of almost all species, including emergent leaves, but the ratio of the chloroplast total number to the number of all plastids varied depending on the degree of leaf submergence. The total number of plastids per unit leaf area (2–6 × 10⁶/cm²) and the surface of chloroplasts per unit leaf area (2–6 cm²/cm²) were lower in hydrophytes than in terrestrial plants from climatically similar habitats. The functional relations between mesophyll parameters were similar for hydrophytes and terrestrial plants (a positive correlation between the leaf weight per unit area, leaf thickness, and the number of mesophyll cells per unit leaf area), although no correlation was found in hydrophytes between the volume of mesophyll cells and the leaf thickness. Phototrophic tissues in aquatic plants contributed a larger fraction to the leaf weight than in terrestrial plants, because the mechanical tissues were less developed in hydrophytes. The CO₂assimilation rates by leaves were lower in hydrophytes than in terrestrial plants, because the total surface area of chloroplasts per unit leaf area is comparatively small in hydrophytes, which reduces the conductivity for carbon dioxide diffusion towards the carboxylation sites.     

Inflorescence characteristics,seed composition,and allometric relationships predicting seed yields in the biomass crop Cynara cardunculus

Cynara (Cynara cardunculus) is a perennial C₃ herb that has its potential as bioenergy crop. This paper aims (a) to derive empirical relationships to predict cynara seed yield per head and per unit area, avoiding laborious extraction of seeds from the complex structure of its inflorescences; (b) to determine the head‐weight distribution per unit area, the seed composition and the oil profile of cynara seeds; and (c) to estimate the range of cynara biomass, seed and oil yield in representative parts of Greece. We analyzed 16 field experiments, varying in crop age and environmental conditions in Greece. Seed yield per head (SY_head) can be accurately predicted as a linear function of dry head weight (H_w): SY_head=0.429·H_w?2.9 (r²=0.96; n=617). Based on this relationship, we developed a simple two‐parameter equation to predict seed yield per unit area (SY): SY=HN·(0.429·μ?2.9), where μ is the mean head weight (g head^?1) and HN is the total number of heads per unit area, respectively. The models were tested against current and published data (n=180 for head‐level; n=35 for unit area‐level models), and proved to be valid under diverse management and environmental conditions. Attainable cynara seed yields ranged from 190 to 480 g m^?2 yr^?1, on dry soils and on aquic soils (shallow ground water level). This variation in seed yield was sufficiently explained by the analyses of head‐weight distribution per unit area (small, medium and large heads) and variability of seed/head weight ratio at head level. Seed oil concentration (average: 23%) and crude protein concentration (average: 18.7%) were rather invariant across different seed sizes (range: 26–56 mg seed^?1) and growing environments.     

Microbial Methanogenesis and Acetate Metabolism in a Meromictic Lake

Methanogenesis and the anaerobic metabolism of acetate were examined in the sediment and water column of Knaack Lake, a small biogenic meromictic lake located in central Wisconsin. The lake was sharply stratified during the summer and was anaerobic below a depth of 3 m. Large concentrations (4,000 μmol/liter) of dissolved methane were detected in the bottom waters. A methane concentration maximum occurred at 4 m above the sediment. The production of ¹⁴CH₄ from ¹⁴C-labeled HCOOH, HCO₃⁻, and CH₃OH and [2-¹⁴C]acetate demonstrated microbial methanogenesis in the water column of the lake. The maximum rate of methanogenesis calculated from reduction of H¹⁴CO₃⁻ by endogenous electron donors in the surface sediment (depth, 22 m) was 7.6 nmol/h per 10 ml and in the water column (depth, 21 m) was 0.6 nmol/h per 10 ml. The methyl group of acetate was simultaneously metabolized to CH₄ and CO₂ in the anaerobic portions of the lake. Acetate oxidation was greatest in surface waters and decreased with water depth. Acetate was metabolized primarily to methane in the sediments and water immediately above the sediment. Sulfide inhibition studies and temperature activity profiles demonstrated that acetate metabolism was performed by several microbial populations. Sulfide additions (less than 5 μg/ml) to water from 21.5 m stimulated methanogenesis from acetate, but inhibited CO₂ production. Sulfate addition (1 mM) had no significant effect on acetate metabolism in water from 21.5 m, whereas nitrate additions (10 to 14,000 μg/liter) completely inhibited methanogenesis and stimulated CO₂ formation.