Influence of pH on Terminal Carbon Metabolism in Anoxic Sediments from a Mildly Acidic Lake

The carbon and electron flow pathways and the bacterial populations responsible for transformation of H₂-CO₂, formate, methanol, methylamine, acetate, glycine, ethanol, and lactate were examined in sediments collected from Knaack Lake, Wis. The sediments were 60% organic matter (pH 6.2) and did not display detectable sulfate-reducing activity, but they contained the following average concentration (in micromoles per liter of sediment) of metabolites and end products: sulfide, 10; methane, 1,540; CO₂, 3,950; formate, 25; acetate, 157; ethanol, 174; and lactate, 138. Methane was produced predominately from acetate, and only 4% of the total CH₄ was derived from CO₂. Methanogenesis was limited by low environmental temperature and sulfide levels and more importantly by low pH. Increasing in vitro pH to neutral values enhanced total methane production rates and the percentage of CO₂ transformed to methane but did not alter the amount of ¹⁴CO₂ produced from [2-¹⁴C]acetate (~24%). Analysis of both carbon transformation parameters with ¹⁴C-labeled tracers and bacterial trophic group enumerations indicated that methanogenesis from acetate and both heterolactic- and acetic acid-producing fermentations were important to the anaerobic digestion process.     

Nitrogen Turnover in Drying Sediments of an Amazon Floodplain Lake

In the Amazon floodplain large areas are subject to annual cycles of drying and rewetting. The turnover of nitrogen in the periodically drying sediments is an important regulator of floodplain fertility. In the present study the transition of a lake sediment from flooded to dry conditions was studied with respect to microbial nitrogen turnover. Soil nitrogen pools, as well as the activity and abundance of denitrifying and nitrifying bacteria, were investigated during one dry season. During the first weeks after drying, most of the inorganic nitrogen vanished from the sediment. The process was inhibited by a nitrification inhibitor, showing that coupled nitrification–denitrification was responsible for the nitrogen loss. Assimilation by plants or microbes, as well as leaching, were not important mechanisms of nitrogen loss. During a period of only 10 days, 59% of the total denitrification and 94% of the total N₂O emission during the dry period occurred. Cell numbers of denitrifiers were not correlated with activities. Denitrification was not correlated with other sediment variables but was regulated by the patchy distribution of reduced and oxidized zones in the uppermost centimeters of the sediment. This heterogeneity was probably introduced by the bioturbation of small insects, which was restricted to a rather short time period shortly after drying.     

Effect of Temperature on Heterotrophic Glucose Uptake, Mineralization, and Turnover Rates in Lake Sediments

The V_max and turnover rates (TR) of [U-¹⁴C]glucose uptake and mineralization of Lake Kinneret (Israel) sediment are temperature dependent. The following activation energies were determined: glucose uptake, ~15,000 cal (62,760 J); TR of glucose uptake, ~10,000 cal (41,840 J); glucose mineralization, 7,500 to 15,000 cal (31,380 to 62,760 J); and TR of glucose mineralization, ~15,000 cal. Q₁₀ values varied as follows: glucose uptake, ~2.3; TR of glucose uptake, ~1.8; and glucose mineralization, ~2.5. K + S_n values increased slightly with temperature and might reflect an increased K with increased temperatures. Glucose respiration/uptake ratios were low (9.5 to 12%) and were apparently not greatly influenced by the presence or absence of oxygen or by different assay temperatures. Aerobic or anaerobic sediments assayed under either aerobic or anaerobic conditions did not exhibit greatly different V_max, TR, or K + S_n values.     

Effect of Temperature on Blue-Green Algae (Cyanobacteria) in Lake Mendota

The temperature optimum for photosynthesis of natural populations of blue-green algae (cyanobacteria) from Lake Mendota was determined during the period of June to November 1976. In the spring, when temperatures ranged from 0 to 20°C, there were insignificant amounts of blue-green algae in the lake (less than 1% of the biomass). During the summer and fall, when the dominant phytoplankton was blue-green algae, the optimum temperature for photosynthesis was usually between 20 and 30°C, whereas the environmental temperatures during this period ranged from 24°C in August to 12°C in November. In general, the optimum temperature for photosynthesis was higher than the environmental temperature. More importantly, significant photosynthesis also occurred at low temperature in these samples, which suggests that the low temperature alone is not responsible for the absence of blue-green algae in Lake Mendota during the spring. Temperature optima for growth and photosynthesis of laboratory cultures of the three dominant blue-green algae in Lake Mendota were determined. The responses of the two parameters to changes in temperature were similar; thus, photosynthesis appears to be a valid index of growth. However, there was little photosynthesis by laboratory cultures at low temperatures, in contrast to the natural samples. Evidence for an interaction between temperature and low light intensities in their effect on photosynthesis of natural samples is presented.     

Overwintering of Microcystis in Lake Mendota

SUMMARY. Microcystis , an important bloom-forming blue-green alga in Lake Mendota, Wisconsin, was found to accumulate at the lake bottom and overwinter there. Microcystis collected from winter sediments were photosynthetically active, showing temperature and light optima similar to summer and autumn populations. Sediments incubated under in situ conditions in the laboratory maintained photosynthetic activity for at least four months. Such a maintenance ability may be part of an overall strategy by Microcystis populations to ensure a suitable inoculum for the following growth season.     

Zooplankton dynamics in Lake Mendota: short-term versus long-term changes

SUMMARY.

• 1 A study of zooplankton dynamics was carried out in Lake Mendota, Wisconsin, for the years 1976–80. The data were compared with those of E. A. Birge, obtained in the years 1894–96.
• 2 The zooplankton annual cycles and sizes of standing crop were similar in both studies. There was more year-to-year variation within each study than there was between the two studies separated by over 80 years.
• 3 The data are discussed in relation to the prediction of long-term change in lakes. The value of a historical data set for long-term ecological studies is emphasized.

     

Intermediary Metabolism of Organic Matter in the Sediments of a Eutrophic Lake

The rates, products, and controls of the metabolism of fermentation intermediates in the sediments of a eutrophic lake were examined. ¹⁴C-fatty acids were directly injected into sediment subcores for turnover rate measurements. The highest rates of acetate turnover were in surface sediments (0- to 2-cm depth). Methane was the dominant product of acetate metabolism at all depths. Simultaneous measurements of acetate, propionate, and lactate turnover in surface sediments gave turnover rates of 159, 20, and 3 μM/h, respectively. [2-¹⁴C]propionate and [U-¹⁴C]lactate were metabolized to [¹⁴C]acetate, ¹⁴CO₂, and ¹⁴CH₄. [¹⁴C]formate was completely converted to ¹⁴CO₂ in less than 1 min. Inhibition of methanogenesis with chloroform resulted in an immediate accumulation of volatile fatty acids and hydrogen. Hydrogen inhibited the metabolism of C₃-C₅ volatile fatty acids. The rates of fatty acid production were estimated from the rates of fatty acid accumulation in the presence of chloroform or hydrogen. The mean molar rates of production were acetate, 82%; propionate, 13%; butyrates, 2%; and valerates, 3%. A working model for carbon and electron flow is presented which illustrates that fermentation and methanogenesis are the predominate steps in carbon flow and that there is a close interaction between fermentative bacteria, acetogenic hydrogen-producing bacteria, and methanogens.     

Dark Carbon Fixation: An Important Process in Lake Sediments

Close to redox boundaries, dark carbon fixation by chemoautotrophic bacteria may be a large contributor to overall carbon fixation. Still, little is known about the relative importance of this process in lake systems, in spite the potentially high chemoautotrophic potential of lake sediments. We compared rates of dark carbon fixation, bacterial production and oxygen consumption in sediments from four Swedish boreal and seven tropical Brazilian lakes. Rates were highly variable and dark carbon fixation amounted up to 80% of the total heterotrophic bacterial production. The results indicate that non-photosynthetic carbon fixation can represent a substantial contribution to bacterial biomass production, especially in sediments with low organic matter content.     

Anaerobic metabolism of immediate methane precursors in Lake Mendota.

Lake Mendota sediments and the immediate overlying water column were studied to better understand the metabolism of the methanogenic precursors H2/CO2 and acetate in nature. The pool size of acetate (3.5 microns M) was very small, and the acetate turnover time (0.22h) was very rapid. The dissolved inorganic carbon pool was shown to be large (6.4 to 8.3 mM), and the turnover time was slow (111 H.). CO2 was shown to account for 41 +/- 5.5% of the methane produced in sediment. Acetate and H2/CO2 were simultaneously converted to CH4. The addition of H2 to sediments resulted in an increase specific activity of CH4 from H(14)CO3- and a decrease in specific activity of CH4 from [2-14C]acetate. Acetate addition resulted in a decrease in specific activity of CH4 from H(14)CO3-. The metabolism of H(14)CO3- or [2-14C]acetate to 14CH4 was not inhibited by addition of acetate or H2. After greater than 99% of added [2-14C]acetate had been turned over, 42% of the label was recovered as 14CH4 20% was recovered as 14CO2 and 38% was incorporated into sediment. Inhibitor studies of [2-14C]acetate metabolism in sediments demonstrated that CHCl3 completely inhibited CH4 formation, but not CO2 production. Air and nitrate addition inhibited CH4 formation and stimulated CO2 production, whereas fluoroacetate addition totally inhibited acetate metabolism. The oxidation of [2-14C]acetate to 14CO2 was shown to decrease with time when sediment was incubated before the addition of label, suggesting depletion of low levels of an endogenous sediment electron acceptor. Acetate metabolism varied seasonally and was related to the concentration of sulfate in the lake and interstitial water. Methanogenesis occurred in the sediment and in the water immediately overlying the sediment during period of lake stratification and several centimeters below the sediment-water interface during lake turnovers. These data indicate that methanogenesis in Lake Mendota sediments was limited by "immediate" methane precursor availability (i.e., acetate and H2), by competition for these substrates by nonmethanogens, and by seasonal variations which altered sediment and water chemistry.     

Anaerobic metabolism of immediate methane precursors in Lake Mendota.

Lake Mendota sediments and the immediate overlying water column were studied to better understand the metabolism of the methanogenic precursors H2/CO2 and acetate in nature. The pool size of acetate (3.5 microns M) was very small, and the acetate turnover time (0.22h) was very rapid. The dissolved inorganic carbon pool was shown to be large (6.4 to 8.3 mM), and the turnover time was slow (111 H.). CO2 was shown to account for 41 +/- 5.5% of the methane produced in sediment. Acetate and H2/CO2 were simultaneously converted to CH4. The addition of H2 to sediments resulted in an increase specific activity of CH4 from H(14)CO3- and a decrease in specific activity of CH4 from [2-14C]acetate. Acetate addition resulted in a decrease in specific activity of CH4 from H(14)CO3-. The metabolism of H(14)CO3- or [2-14C]acetate to 14CH4 was not inhibited by addition of acetate or H2. After greater than 99% of added [2-14C]acetate had been turned over, 42% of the label was recovered as 14CH4 20% was recovered as 14CO2 and 38% was incorporated into sediment. Inhibitor studies of [2-14C]acetate metabolism in sediments demonstrated that CHCl3 completely inhibited CH4 formation, but not CO2 production. Air and nitrate addition inhibited CH4 formation and stimulated CO2 production, whereas fluoroacetate addition totally inhibited acetate metabolism. The oxidation of [2-14C]acetate to 14CO2 was shown to decrease with time when sediment was incubated before the addition of label, suggesting depletion of low levels of an endogenous sediment electron acceptor. Acetate metabolism varied seasonally and was related to the concentration of sulfate in the lake and interstitial water. Methanogenesis occurred in the sediment and in the water immediately overlying the sediment during period of lake stratification and several centimeters below the sediment-water interface during lake turnovers. These data indicate that methanogenesis in Lake Mendota sediments was limited by "immediate" methane precursor availability (i.e., acetate and H2), by competition for these substrates by nonmethanogens, and by seasonal variations which altered sediment and water chemistry.     

Methane, Carbon Dioxide, and Hydrogen Sulfide Production from the Terminal Methiol Group of Methionine by Anaerobic Lake Sediments

A significant portion of the sulfide in lake sediments may be derived from sulfur-containing amino acids. Methionine degradation in Lake Mendota (Wisconsin) sediments was studied with gas chromatographic and radiotracer techniques. Temperature optimum and inhibitor studies showed that this process was biological. Methane thiol and dimethyl sulfide were produced in sediments when 1-μmol/ml unlabeled methionine was added. When chloroform (an inhibitor of one-carbon metabolism) was added to the sediments, methane thiol, carbon disulfide, and n-propane thiol were produced, even when no methionine was added. When ³⁵S-labeled methionine was added to the sediments in tracer quantities (1.75 nmol/ml), labeled hydrogen sulfide was produced, and a roughly equal amount of label was incorporated into insoluble material. Methane and carbon dioxide were produced from [methyl-¹⁴C]methionine. Evidence is given favoring methane thiol as an intermediate in the formation of methane, carbon dioxide, and hydrogen sulfide from the terminal methiol group of methionine. Methionine may be an important source of sulfide in lake sediments.     

Association of hydrogen metabolism with methanogenesis in Lake Mendota sediments.

Lake Mendota sediments were studied to determine the role of H2 in sediment methanogenesis. H2 was generally not detectable in sediment. The addition of H2 to sediment significantly increased methanogenensis. The amount of methane produced was proportional to the concentration of hydrogen added. H2 addition stimulated the reduction of CO2 to methane, but did not significantly stimulate the conversion of methanol or the methyl position of acetate to methane. Various organic compounds also stimulated sediment methanogenesis. Formate, ethanol, and glucose were shown to serve as electron donors for CO2 reduction to methane. The addition of formate to sediment resulted in H2 evolution. H2 was not deith the phenomenon of interspecies hydrogen transfer. The results indicate that hydrogen is an important intermediate and a rate-limiting factor in sediment methanogenesis.     

Association of hydrogen metabolism with methanogenesis in Lake Mendota sediments.

Lake Mendota sediments were studied to determine the role of H2 in sediment methanogenesis. H2 was generally not detectable in sediment. The addition of H2 to sediment significantly increased methanogenensis. The amount of methane produced was proportional to the concentration of hydrogen added. H2 addition stimulated the reduction of CO2 to methane, but did not significantly stimulate the conversion of methanol or the methyl position of acetate to methane. Various organic compounds also stimulated sediment methanogenesis. Formate, ethanol, and glucose were shown to serve as electron donors for CO2 reduction to methane. The addition of formate to sediment resulted in H2 evolution. H2 was not deith the phenomenon of interspecies hydrogen transfer. The results indicate that hydrogen is an important intermediate and a rate-limiting factor in sediment methanogenesis.     

Assessing Biomass and Production of Bacteria in Eutrophic Lake Mendota, Wisconsin

Estimates were made of the biomass and production of heterotrophic bacteria in the epilimnion of Lake Mendota, Wis. Cell counts were done with epifluorescence microscopy and varied from 3 × 10⁵ bacteria per ml in winter to 3 × 10⁶ bacteria per ml in summer. Cell volumes were measured in scanning electron micrographs. The average cell volume was 0.159 μm³. Annual variations and depth distribution were studied. Production was estimated from the frequency of dividing cells and from dark radioactive sulfate uptake. Annual productivity and daily average productivity were very close with both methods: 107 to 205 g of C per m² per year for sulfate and 89 to 117 g of C per m² per year for frequency of dividing cells. Zooplankton feeding removed 2 to 10% of the bacterial net production annually. When compared with biomass changes and losses due to zooplankton feeding, production values were very high. Therefore, it was suggested that other loss factors have to be more important than zooplankton feeding in controlling the bacterial population. Bacterial heterotrophic production was about 50% of gross primary production.     

The Modulating Role of Dissolved Organic Matter on Spatial Patterns of Microbial Metabolism in Lake Erie Sediments

To evaluate the role of dissolved organic matter (DOM) on microbial community metabolism, we established extracellular enzyme activity (EEA) and substrate-induced respiration (SIR) profiles of sediment samples collected from littoral and profundal regions of the western, central, and eastern basins of Lake Erie. Lake Erie is spatially structured such that the central and western basins receive relatively major inputs of allochthonous DOM in comparison to the eastern basin. Overall, spatial patterns of EEA and SIR profiles suggest both greater metabolic diversity and activity in the littoral regions of the central and western basins. In contrast, the eastern basin demonstrated much less structuring between littoral and profundal areas. To evaluate whether the observed spatial patterns are the result of microbial community adaptations to local DOM availability, we performed three experimental treatments by inoculating sediment samples with polyvinylpyrrolidone, which sequesters large polyphenols, or with either vanillin or catechol, two small phenolic compounds. Our results revealed that esterase and glycosidase EEA from the eastern basin were induced by small phenolics and inhibited by large polyphenols. In contrast, the addition of small phenolics decreased esterase and glycosidase activities from the central basin, while polyphenols had a negligible effect. These results suggest that the source and composition of DOM play a significant role in the local adaptation of microbial communities, determining large-scale spatial patterns of microbial functional diversity in Lake Erie sediments.     

Decomposition of Blue-Green Algal (Cyanobacterial) Blooms in Lake Mendota, Wisconsin

Decomposition of natural populations of Lake Mendota phytoplankton dominated by blue-green algae (cyanobacteria) was monitored by using oxygen uptake and disappearance of chlorophyll, algal volume (fluorescence microscopy), particulate protein, particulate organic carbon, and photosynthetic ability (¹⁴CO₂ up-take). In some experiments, decomposition of ¹⁴C-labeled axenic cultures of Anabaena sp. was also measured. In addition to decomposition, mineralization of inorganic nitrogen and phosphorus were followed in some experiments. Decomposition could be described as a first-order process, and the rate of decomposition was similar to that found by others using pure cultures of eucaryotic algae. Nitrogen and phosphorus never limited the decomposition process, even when the lake water was severely limited in soluble forms of these nutrients. This suggests that the bacteria responsible for decomposition can obtain all of their key nutrients for growth from the blue-green algal cells. Filtration of lake water through plankton netting that removed up to 90% of the algal biomass usually did not cause a similar decrease in oxygen demand, suggesting that most of the particulate organic matter used for respiration of the decomposing bacteria was in a small-particle fraction. Short-term oxygen demand correlated well with the particulate chlorophyll concentration of the sample, and a relationship was derived that could be used to predict community respiration of the lake from chlorophyll concentration. Kinetic analysis showed that not all analyzed components disappeared at the same rate during the decomposition process. The relative rates of decrease of the measured parameters were as follows: photosynthetic ability > algal volume > particulate chlorophyll > particulate protein. Decomposition of ¹⁴C-labeled Anabaena occurred at similar rates with aerobic epilimnetic water and with anaerobic sediment, but was considerably slower with anaerobic hypolimnetic water. Of the various genera present in the lake, Aphanizomenon and Anabaena were more sensitive to decomposition than was Microcystis. In addition to providing a general picture of the decomposition process, the present work relates to other work on sedimentation to provide a detailed picture of the fate of blue-green algal biomass in a eutrophic lake ecosystem.     

Glucose Metabolism in Sediments of a Eutrophic Lake: Tracer Analysis of Uptake and Product Formation

The uptake of glucose and the formation of end products from glucose catabolism have been measured for sediments of eutrophic Wintergreen Lake with a combination of tritiated and ¹⁴C-labeled tracers. Time course analyses of the loss of [³H]glucose from sediments were used to establish rate constants for glucose uptake at natural substrate concentrations. Turnover times from these analyses were about 1 min for littoral and profundal sediments. No seasonal or site differences were noted in turnover times. Time course analyses of [U-¹⁴C]glucose uptake and ¹⁴C-labeled end product formation indicated that glucose mass flow could not be calculated from end product formation since the specific activity of added [¹⁴C]glucose was significantly diluted by pools of intracellular glucose and glucose metabolites. Mass flow could only be accurately estimated by use of rates of uptake from tracer studies. Intermediate fermentation end products included acetate (71%), propionate (15%), lactate (9%), and only minor amounts of butyrates or valerates. Addition of H₂ to sediments resulted in greater production of lactate (28%) and decreased formation of acetate (50%), but did not affect glucose turnover. Depth profiles of glucose uptake indicated that rates of uptake decreased with depth over the 0- to 18-cm interval and that glucose uptake accounted for 30 to 40% of methanogenesis in profundal sediments.     

Influence of Three Contrasting Detrital Carbon Sources on Planktonic Bacterial Metabolism in a Mesotrophic Lake

Abstract Lakes receive organic carbon from a diversity of sources which vary in their contribution to planktonic microbial food webs. We conducted a mesocosm study to test the effects of three different detrital carbon sources (algae, aquatic macrophytes, terrestrial leaves) on several measures of microbial metabolism in a small meso-eutrophic lake (DOC ≈ 5 mg/L). Small DOC additions (ΔC < 1 mg/L) affected bacterial numbers, growth, and pathways of carbon acquisition. Macrophyte and leaf detritus significantly increased TDP and color, but bacterial densities initially (+12 h) were unaffected. After 168 h, densities in systems amended with terrestrial detritus were 60% less than in controls, while production rates in mesocosms with macrophyte detritus were 4-fold greater. Detritus treatments resulted in greater per-cell production rates either through stable cell numbers and greater growth rates (macrophyte-C) or lower densities with stable production rates (terrestrial-C). After only 12 h, rates of leucine aminopeptidase (LAPase) activity were 2.5× greater in macrophyte-C systems than in controls, but LAPase and β-N-acetylglucosamindase activities in systems amended with terrestrial-C were only 50% of rates in controls. After 168 h, β-xylosidase rates were significantly greater in communities with terrestrial and phytoplankton detritus. Microbial utilization of >20% of 102 carbon sources tested were affected by at least one detritus addition. Macrophyte-C had positive (6% of substrates) and negative (14%) effects on substrate use; terrestrial detritus had mainly positive effects. An ordination based on carbon-use profiles (+12 h) revealed a cluster of macrophyte-amended communities with greater use of psicose, lactulose, and succinamic acid; controls and algal-detritus systems were more effective in metabolizing two common sugars and cellobiose. After 168 h, communities receiving terrestrial detritus were most tightly clustered, exhibiting greater use of raffinose, pyroglutamic acid, and sebacic acid. Results suggest that pelagic bacterial communities respond to changes in organic carbon source rapidly and by different routes, including shifts in per-cell production rates and variations in degradation of a variety of compounds comprising the DOC pool. Received: 5 June 1998; Accepted: 24 August 1998     

Changes in photosynthetic rate and pigment content of blue-green algae in Lake Mendota.

Blue-green algal blooms were present in Lake Mendota (Dane County, Wis.) from June to November 1976. Concentrations of total algal biomass and of particular algal species were monitored and compared with the pigment contents (chlorophyll a and phycocyanin) and photosynthetic rate of the algal populations. The specific photosynthetic rate (micrograms of C fixed per microgram of chlorophyll a per hour) was a good measure of the physiological state of the algae because this quantity increased just before each population increase and decreased before algal densities diminished. Since the quantity of light in the epilimnion which was available for photosynthesis by algal cells decreased in summer when the high algal densities attenuated incoming radiation, we investigated the possibility that the organisms would utilize lower light intensities more efficiently by increasing their pigment contents. Although some evidence of enhanced utilization of low light levels was found in the period from July to October, this result was not due to increasing chlorophyll and phycocyanin contents. There was a decrease in the phycocyanin content of the algae during this period, perhaps related to the availability of inorganic nitrogen.