Assessing Phytoplankton and Bacterioplankton Production During Early Spring in Lake Erken, Sweden

The spring development of both phytoplankton and bacterioplankton was investigated between 18 April and 7 May 1983 in mesotrophic Lake Erken, Sweden. By using the lake as a batch culture, our aim was to estimate, via different methods, the production of phytoplankton and bacterioplankton in the lake and to compare these production estimates with the actual increase in phytoplankton and bacterioplankton biomass. The average water temperature was 3.5°C. Of the phytoplankton biomass, >90% was the diatom Stephanodiscus hantzchii var. pusillus, by the peak of the bloom. The ¹⁴C and O₂ methods of estimating primary production gave equivalent results (r = 0.999) with a photosynthetic quotient of 1.63. The theoretical photosynthetic quotient predicted from the C/NO₃ N assimilation ratio was 1.57. The total integrated incorporation of [¹⁴C]bicarbonate into particulate material (>1 μm) was similar to the increase in phytoplankton carbon determined from cell counts. Bacterioplankton increased from 0.5 × 10⁹ to 1.52 × 10⁹ cells liter⁻¹ (~0.5 μg of C liter⁻¹ day⁻¹). Estimates of bacterioplankton production from rates of [³H]thymidine incorporation were ca. 1.2 to 1.7 μg of C liter⁻¹ day⁻¹. Bacterial respiration, measured by a high-precision Winkler technique, was estimated as 4.8 μg of C liter⁻¹ day⁻¹, indicating a bacterial growth yield of 25%. The bulk of the bacterioplankton production was accounted for by algal extracellular products. Gross bacterioplankton production (production plus respiration) was 20% of gross primary production, per square meter of surface area. We found no indication that bacterioplankton production was underestimated by the [³H]thymidine incorporation method.     

Annual Cycle of Bacterial Secondary Production in Five Aquatic Habitats of the Okefenokee Swamp Ecosystem

Rates of bacterial secondary production by free-living bacterioplankton in the Okefenokee Swamp are high and comparable to reported values for a wide variety of marine and freshwater ecosystems. Bacterial production in the water column of five aquatic habitats of the Okefenokee Swamp was substantial despite the acidic (pH 3.7), low-nutrient, peat-accumulating character of the environment. Incorporation of [³H]thymidine into cold-trichloroacetic acid-insoluble material ranged from 0.03 to 2.93 nmol liter⁻¹ day⁻¹) and corresponded to rates of bacterial secondary production of 3.4 to 342.2 μg of carbon liter⁻¹ day⁻¹ (mean, 87.8 μg of carbon liter⁻¹ day⁻¹). Bacterial production was strongly seasonal and appeared to be coupled to annual changes in temperature and primary production. Bacterial doubling times ranged from 5 h to 15 days and were fastest during the warm months of the year, when the biomass of aquatic macrophytes was high, and slowest during the winter, when the plant biomass was reduced. The high rates of bacterial turnover in Okefenokee waters suggest that bacterial growth is an important mechanism in the transformation of dissolved organic carbon into the nutrient-rich bacterial biomass which is utilized by microconsumers.     

Rates of nitrification and carbon uptake in the Rhône River plume (northwestern Mediterranean Sea)

Nitrification rates were measured along a salinity gradient in the Rhône River estuary, using specific inhibitors (allylthiourea and chlorate) coupled with the measurement of change in nitrite concentration and inorganic carbon uptake by nitrifiers. Rates of ammonium and nitrite oxidation were similar up to 15 practical salinity units (from 1 to 2 mol N oxidized liter^-1 day^-1). For higher salinities, nitrite and ammonium oxidation rates were 0.14 and 0.23 mol N oxidized liter^-1 day^-1, respectively. Ammonium oxidizers assimilated 19–150 × 10–3 mol C liter^-1 day^-1, while nitrite oxidizers fixed 4.8–72.6 × 10–3 mol C liter^-1 day^-1. The amounts of nitrogen oxidized and C incorporated demonstrated a linear correlation (r ² > 0.99). The ratio of N oxidized to C incorporated ranged between 14.3 to 12.3 for ammonium oxidizers, and between 31.6 and 29 for nitrite oxidizers, the lower values being measured in seawater. Offprint requests to: M. Bianchi.     

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.     

amoA Gene Abundances and Nitrification Potential Rates Suggest that Benthic Ammonia-Oxidizing Bacteria and Not Archaea Dominate N Cycling in the Colne Estuary,United Kingdom

Nitrification, mediated by ammonia-oxidizing bacteria (AOB) and ammonia-oxidizing archaea (AOA), is important in global nitrogen cycling. In estuaries where gradients of salinity and ammonia concentrations occur, there may be differential selections for ammonia-oxidizer populations. The aim of this study was to examine the activity, abundance, and diversity of AOA and AOB in surface oxic sediments of a highly nutrified estuary that exhibits gradients of salinity and ammonium. AOB and AOA communities were investigated by measuring ammonia monooxygenase (amoA) gene abundance and nitrification potentials both spatially and temporally. Nitrification potentials differed along the estuary and over time, with the greatest nitrification potentials occurring mid-estuary (8.2 μmol N grams dry weight [gdw]⁻¹ day⁻¹ in June, increasing to 37.4 μmol N gdw⁻¹ day⁻¹ in January). At the estuary head, the nitrification potential was 4.3 μmol N gdw⁻¹ day⁻¹ in June, increasing to 11.7 μmol N gdw⁻¹ day⁻¹ in January. At the estuary head and mouth, nitrification potentials fluctuated throughout the year. AOB amoA gene abundances were significantly greater (by 100-fold) than those of AOA both spatially and temporally. Nitrosomonas spp. were detected along the estuary by denaturing gradient gel electrophoresis (DGGE) band sequence analysis. In conclusion, AOB dominated over AOA in the estuarine sediments, with the ratio of AOB/AOA amoA gene abundance increasing from the upper (freshwater) to lower (marine) regions of the Colne estuary. These findings suggest that in this nutrified estuary, AOB (possibly Nitrosomonas spp.) were of major significance in nitrification.     

Rapid Methane Oxidation in a Landfill Cover Soil

Methane oxidation rates observed in a topsoil covering a retired landfill are the highest reported (45 g m⁻² day⁻¹) for any environment. This microbial community had the capacity to rapidly oxidize CH₄ at concentrations ranging from <1 ppm (microliters per liter) (first-order rate constant [k] = −0.54 h⁻¹) to >10⁴ ppm (k = −2.37 h⁻¹). The physiological characteristics of a methanotroph isolated from the soil (characteristics determined in aqueous medium) and the natural population, however, were similar to those of other natural populations and cultures: the Q₁₀ and optimum temperature were 1.9 and 31°C, respectively, the apparent half-saturation constant was 2.5 to 9.3 μM, and 19 to 69% of oxidized CH₄ was assimilated into biomass. The CH₄ oxidation rate of this soil under waterlogged (41% [wt/vol] H₂O) conditions, 6.1 mg liter⁻¹ day⁻¹, was near rates reported for lake sediment and much lower than the rate of 116 mg liter⁻¹ day⁻¹ in the same soil under moist (11% H₂O) conditions. Since there are no large physiological differences between this microbial community and other CH₄ oxidizers, we attribute the high CH₄ oxidation rate in moist soil to enhanced CH₄ transport to the microorganisms; gas-phase molecular diffusion is 10⁴-fold faster than aqueous diffusion. These high CH₄ oxidation rates in moist soil have implications that are important in global climate change. Soil CH₄ oxidation could become a negative feedback to atmospheric CH₄ increases (and warming) in areas that are presently waterlogged but are projected to undergo a reduction in summer soil moisture.     

Initial Effects of the Mount St. Helens Eruption on Nitrogen Cycle and Related Chemical Processes in Ryan Lake

Ryan Lake, a 1.6-hectare basin lake near the periphery of the tree blowdown area in the blast zone 19 km north of Mount St. Helens, was studied from August to October 1980 to determine the microbial and chemical response of the lake to the eruption. Nutrient enrichment through the addition of fresh volcanic material and the organic debris from the surrounding conifer forest stimulated intense microbial activity. Concentrations of such nutrients as phosphorus, sulfur, manganese, iron, and dissolved organic carbon were markedly elevated. Nitrogen cycle activity was especially important to the lake ecosystem in regulating biogeochemical cycling owing to the limiting abundance of nitrogen compounds. Nitrogen fixation, both aerobic and anaerobic, was active from aerobic benthic and planktonic cyanobacteria with rates up to 210 nmol of N₂ cm⁻¹ h⁻¹ and 667 nmol of N₂ liter⁻¹ h⁻¹, respectively, and from anaerobic bacteria with rates reaching 220 nmol of N₂ liter⁻¹ h⁻¹. Nitrification was limited to the aerobic epilimnion and littoral zones where rates were 43 and 261 nmol of NO₂ liter⁻¹ day⁻¹, respectively. Potential denitrification rates were as high as 30 μmol of N₂O liter⁻¹ day⁻¹ in the anaerobic hypolimnion. Total bacterial numbers ranged from 1 × 10⁶ to 3 × 10⁸ ml⁻¹ with the number of viable sulfur-metal-oxidizing bacteria reaching 2 × 10⁶ ml⁻¹ in the hypolimnion. A general scenario for the microbial cycling of nitrogen, carbon, sulfur, and metals is presented for volcanically impacted lakes. The important role of nitrogen as these lakes recover from the cataclysmic eruption and proceed back towards their prior status as oligotrophic alpine lakes is emphasized.     

Estimating Bacterioplankton Production by Measuring [3H]thymidine Incorporation in a Eutrophic Swedish Lake

Bacterioplankton abundance, [³H]thymidine incorporation, ¹⁴CO₂ uptake in the dark, and fractionated primary production were measured on several occasions between June and August 1982 in eutrophic Lake Norrviken, Sweden. Bacterioplankton abundance and carbon biomass ranged from 0.5 × 10⁹ to 2.4 × 10⁹ cells liter⁻¹ and 7 to 47 μg of C liter⁻¹, respectively. The average bacterial cell volume was 0.185 μm³. [³H]thymidine incorporation into cold-trichloroacetic acid-insoluble material ranged from 12 × 10⁻¹² to 200 × 10⁻¹² mol liter⁻¹ h⁻¹. Bacterial carbon production rates were estimated to be 0.2 to 7.1 μg of C liter⁻¹ h⁻¹. Bacterial production estimates from [³H]thymidine incorporation and ¹⁴CO₂ uptake in the dark agreed when activity was high but diverged when activity was low and when blue-green algae (cyanobacteria) dominated the phytoplankton. Size fractionation indicated negligible uptake of [³H]thymidine in the >3-μm fraction during a chrysophycean bloom in early June. We found that >50% of the ³H activity was in the >3-μm fraction in late August; this phenomenon was most likely due to Microcystis spp., their associated bacteria, or both. Over 60% of the ¹⁴CO₂ uptake in the dark was attributed to algae on each sampling occasion. Algal exudate was an important carbon source for planktonic bacteria. Bacterial production was roughly 50% of primary production.     

Contributions of Autotrophic and Heterotrophic Nitrifiers to Soil NO and N2O Emissions

Soil emission of gaseous N oxides during nitrification of ammonium represents loss of an available plant nutrient and has an important impact on the chemistry of the atmosphere. We used selective inhibitors and a glucose amendment in a factorial design to determine the relative contributions of autotrophic ammonium oxidizers, autotrophic nitrite oxidizers, and heterotrophic nitrifiers to nitric oxide (NO) and nitrous oxide (N₂O) emissions from aerobically incubated soil following the addition of 160 mg of N as ammonium sulfate kg⁻¹. Without added C, peak NO emissions of 4 μg of N kg⁻¹ h⁻¹ were increased to 15 μg of N kg⁻¹ h⁻¹ by the addition of sodium chlorate, a nitrite oxidation inhibitor, but were reduced to 0.01 μg of N kg⁻¹ h⁻¹ in the presence of nitrapyrin [2-chloro-6-(trichloromethyl)-pyridine], an inhibitor of autotrophic ammonium oxidation. Carbon-amended soils had somewhat higher NO emission rates from these three treatments (6, 18, and 0.1 μg of N kg⁻¹ h⁻¹ after treatment with glucose, sodium chlorate, or nitrapyrin, respectively) until the glucose was exhausted but lower rates during the remainder of the incubation. Nitrous oxide emission levels exhibited trends similar to those observed for NO but were about 20 times lower. Periodic soil chemical analyses showed no increase in the nitrate concentration of soil treated with sodium chlorate until after the period of peak NO and N₂O emissions; the nitrate concentration of soil treated with nitrapyrin remained unchanged throughout the incubation. These results suggest that chemoautotrophic ammonium-oxidizing bacteria are the predominant source of NO and N₂O produced during nitrification in soil.     

Fungal Growth, Production, and Sporulation during Leaf Decomposition in Two Streams

I examined the activity of fungi associated with yellow poplar (Liriodendron tulipifera) and white oak (Quercus alba) leaves in two streams that differed in pH and alkalinity (a hardwater stream [pH 8.0] and a softwater stream [pH 6.7]) and contained low concentrations of dissolved nitrogen (<35 μg liter⁻¹) and phosphorus (<3 μg liter⁻¹). The leaves of each species decomposed faster in the hardwater stream (decomposition rates, 0.010 and 0.007 day⁻¹ for yellow poplar and oak, respectively) than in the softwater stream (decomposition rates, 0.005 and 0.004 day⁻¹ for yellow poplar and oak, respectively). However, within each stream, the rates of decomposition of the leaves of the two species were not significantly different. During the decomposition of leaves, the fungal biomasses determined from ergosterol concentrations, the production rates determined from rates of incorporation of [¹⁴C]acetate into ergosterol, and the sporulation rates associated with leaves were dynamic, typically increasing to maxima and then declining. The maximum rates of fungal production and sporulation associated with yellow poplar leaves were greater than the corresponding rates associated with white oak leaves in the hardwater stream but not in the softwater stream. The maximum rates of fungal production associated with the leaves of the two species were higher in the hardwater stream (5.8 mg g⁻¹ day⁻¹ on yellow poplar leaves and 3.1 mg g⁻¹ day⁻¹ on oak leaves) than in the softwater stream (1.6 mg g⁻¹ day⁻¹ on yellow poplar leaves and 0.9 mg g⁻¹ day⁻¹ on oak leaves), suggesting that effects of water chemistry other than the N and P concentrations, such as pH or alkalinity, may be important in regulating fungal activity in streams. In contrast, the amount of fungal biomass (as determined from ergosterol concentrations) on yellow poplar leaves was greater in the softwater stream (12.8% of detrital mass) than in the hardwater stream (9.6% of detrital mass). This appeared to be due to the decreased amount of fungal biomass that was converted to conidia and released from the leaf detritus in the softwater stream.     

Primary and Bacterial Secondary Production in a Southwestern Reservoir

Rates of primary and bacterial secondary production in Lake Arlington, Texas, were determined. The lake is a warm (annual temperature range, 7 to 32°C), shallow, monomictic reservoir with limited macrophyte development in the littoral zone. Samples were collected from six depths within the photic zone from a site located over the deepest portion of the lake. Primary production and bacterial production were calculated from NaH¹⁴CO₃ and [methyl-³H]thymidine incorporation, respectively. Peak instantaneous production ranged between 14.8 and 220.5 μg of C liter⁻¹ h⁻¹. There were two distinct periods of high rates of production. From May through July, production near the metalimnion exceeded 100 μg of C liter⁻¹ h⁻¹. During holomixis, production throughout the water column was in excess of 100 μg of C liter⁻¹ h⁻¹ and above 150 μg of C liter⁻¹ h⁻¹ near the surface. Annual areal primary production was 588 g of C m⁻². Bacterial production was markedly seasonal. Growth rates during late fall through spring were typically around 0.002 h⁻¹, and production rates were typically 5 μg of C liter⁻¹ h⁻¹. Growth rates were higher during warmer parts of the year and reached 0.03 h⁻¹ by August. The maximum instantaneous rate of bacterial production was approximately 45 μg of C liter⁻¹ h⁻¹. Annual areal bacterial production was 125 g of C m⁻². Temporal and spatial distributions of bacterial numbers and activities coincided with temporal and spatial distributions of primary production. Areal primary and bacterial secondary production were highly correlated (r = 0.77, n = 15, P < 0.002).     

Effects of Toxic Substances on Natural Bacterial Assemblages Determined by Means of [3H]Thymidine Incorporation

The effects of 3,5-dichlorophenol, 2,4-dinitrophenol, and potassium dichromate on natural bacterial assemblages were examined by means of [³H]thymidine incorporation into trichloroacetic acid-insoluble material. Results from a large number of coastal marine and freshwater samples suggest the following. (i) The effects of the three toxicants included reductions in the bacterial cell number as well as changes in rates of [³H]thymidine incorporation and in [³H]thymidine incorporation per cell. The concentrations that inhibited [³H]thymidine incorporation by 50% ranged from 3 to 11 mg liter⁻¹ for 3,5-dichlorophenol, 6 to 10 mg liter⁻¹ for 2,4-dinitrophenol, and 21 to 123 mg liter⁻¹ for potassium dichromate, with a tendency to higher values in bacterial assemblages from more eutrophic environments. (ii) The effects of 3,5-dichlorophenol and potassium dichromate determined by [³H]leucine incorporation into bacterial protein were similar or larger than those obtained from [³H]thymidine incorporation. (iii) Two to four hours of exposure to the toxicants was necessary before stable maximum effects were found in [³H]thymidine incorporation. (iv) Storage of natural environmental samples should be avoided, since tests with water stored for 1 to 3 days sometimes produced results different from results obtained from in situ tests. (v) The effects of 3,5-dichlorophenol, 2,4-dinitrophenol, and potassium dichromate on natural bacterial assemblages were relatively constant during periods with different growth rates in the assemblages, during various periods of the year, and between samples from freshwater and marine localities. With some precautions, [³H]thymidine incorporation can be used as a quick and sensitive method for determining the effects of toxicants on aquatic bacterial assemblages from natural environmental samples.     

Ammonium Removal by the Oxygen-Limited Autotrophic Nitrification-Denitrification System

The present lab-scale research reveals the potential of implementation of an oxygen-limited autotrophic nitrification-denitrification (OLAND) system with normal nitrifying sludge as the biocatalyst for the removal of nitrogen from nitrogen-rich wastewater in one step. In a sequential batch reactor, synthetic wastewater containing 1 g of NH₄⁺-N liter⁻¹ and minerals was treated. Oxygen supply to the reactor was double-controlled with a pH controller and a timer. At a volumetric loading rate (B_v) of 0.13 g of NH₄⁺-N liter⁻¹ day⁻¹, about 22% of the fed NH₄⁺-N was converted to NO₂⁻-N or NO₃⁻-N, 38% remained as NH₄⁺-N, and the other 40% was removed mainly as N₂. The specific removal rate of nitrogen was on the order of 50 mg of N liter⁻¹ day⁻¹, corresponding to 16 mg of N g of volatile suspended solids⁻¹ day⁻¹. The microorganisms which catalyzed the OLAND process are assumed to be normal nitrifiers dominated by ammonium oxidizers. The loss of nitrogen in the OLAND system is presumed to occur via the oxidation of NH₄⁺ to N₂ with NO₂⁻ as the electron acceptor. Hydroxylamine stimulated the removal of NH₄⁺ and NO₂⁻. Hydroxylamine oxidoreductase (HAO) or an HAO-related enzyme might be responsible for the loss of nitrogen.     

Nitrogen, Carbon, and Sulfur Metabolism in Natural Thioploca Samples

Filamentous sulfur bacteria of the genus Thioploca occur as dense mats on the continental shelf off the coast of Chile and Peru. Since little is known about their nitrogen, sulfur, and carbon metabolism, this study was undertaken to investigate their (eco)physiology. Thioploca is able to store internally high concentrations of sulfur globules and nitrate. It has been previously hypothesized that these large vacuolated bacteria can oxidize sulfide by reducing their internally stored nitrate. We examined this nitrate reduction by incubation experiments of washed Thioploca sheaths with trichomes in combination with ¹⁵N compounds and mass spectrometry and found that these Thioploca samples produce ammonium at a rate of 1 nmol min⁻¹ mg of protein⁻¹. Controls showed no significant activity. Sulfate was shown to be the end product of sulfide oxidation and was observed at a rate of 2 to 3 nmol min⁻¹ mg of protein⁻¹. The ammonium and sulfate production rates were not influenced by the addition of sulfide, suggesting that sulfide is first oxidized to elemental sulfur, and in a second independent step elemental sulfur is oxidized to sulfate. The average sulfide oxidation rate measured was 5 nmol min⁻¹ mg of protein⁻¹ and could be increased to 10.7 nmol min⁻¹ mg of protein⁻¹ after the trichomes were starved for 45 h. Incorporation of ¹⁴CO₂ was at a rate of 0.4 to 0.8 nmol min⁻¹ mg of protein⁻¹, which is half the rate calculated from sulfide oxidation. [2-¹⁴C]acetate incorporation was 0.4 nmol min⁻¹ mg of protein⁻¹, which is equal to the CO₂ fixation rate, and no ¹⁴CO₂ production was detected. These results suggest that Thioploca species are facultative chemolithoautotrophs capable of mixotrophic growth. Microautoradiography confirmed that Thioploca cells assimilated the majority of the radiocarbon from [2-¹⁴C]acetate, with only a minor contribution by epibiontic bacteria present in the samples.     

High-Rate Nitrification at Low pH in Suspended- and Attached-Biomass Reactors

This article reports on high-rate nitrification at low pH in biofilm and suspended-biomass reactors by known chemolithotrophic bacteria. In the biofilm reactor, at low pH (4.3 ± 0.1) and low bulk ammonium concentrations (9.3 ± 3.3 mg·liter⁻¹), a very high nitrification rate of 5.6 g of N oxidized·liter⁻¹·day⁻¹ was achieved. The specific nitrification rate (0.55 g of N·g of biomass⁻¹·day⁻¹) was similar to values reported for nitrifying reactors at optimal pH. In the suspended-biomass reactor, the average pH was significantly lower than that in the biofilm reactor (pH 3.8 ± 0.3), and values as low as pH 3.2 were found. In addition, measurements in the suspended-biomass reactor, using isotope-labeled ammonium (¹⁵N), showed that in spite of the very low pH, biomass growth occurred with a yield of 0.1 g of biomass·g of N oxidized⁻¹. Fluorescence in situ hybridization using existing rRNA-targeted oligonucleotide probes showed that the nitrifying bacteria were from the monophyletic genus Nitrosomonas, suggesting that autotrophic nitrification at low pH is more widespread than previously thought. The results presented in this paper clearly show that autotrophic nitrifying bacteria have the ability to nitrify at a high rate at low pH and in the presence of only a negligible free ammonia concentration, suggesting the presence of an efficient ammonium uptake system and the means to cope with low pH.     

Impact of Nitrogen and Phosphorus on [14C]Lignocellulose Decomposition by Stream Wood Microflora

Nutritional and physical factors affecting the decomposition of [¹⁴C]lignocellulose prepared from Douglas fir (Pseudotsuga menziesii) were examined by incubating the labeled substrate with homogenized surface wood scrapings obtained from a Douglas fir log in a Pacific Northwest stream. Incubations were conducted in distilled water, in stream water collected from four different sources, or in a defined mineral salts solution with or without supplemental N (KNO₃). Decomposition rates of [¹⁴C]lignocellulose, as measured by ¹⁴CO₂ evolution, were greater in each of the four filter-sterilized sources of stream water than in distilled water alone. Decomposition experiments conducted in stream water media with the addition of defined mineral salts demonstrated that [¹⁴C]cellulose decomposition was stimulated 50% by the addition of either KNO₃ or KH₂PO₄/K₂HPO₄ and further enhanced (167%) by a combination of both. In contrast, [¹⁴C]lignin decomposition was stimulated (65%) only by the addition of both N and P. Decomposition of [¹⁴C]lignocellulose was greatest when supplemental KNO₃ was supplied in concentrations of at least 10.0 mg of N liter⁻¹ but not increased further by higher concentrations. The decomposition of [¹⁴C]lignocellulose increased as the incubation temperature was raised and NO₃⁻¹-N supplementation further increased these rates between three-and sevenfold over the range of temperatures examined (5 to 22°C). Accumulation of NH₄⁺ (2 to 4 mg of N liter⁻¹) was always observed in culture filtrates of incubations which had been supplemented with KNO₃, the quantity being independent of NO₃⁻ concentrations ≥ 10 mg of N liter⁻¹. The role of supplemental NO₃⁻ in the decomposition of [¹⁴C]lignocellulose is discussed in relation to wood decomposition and the low concentrations of N found in stream ecosystems of the Pacific Northwest.     

Ammonium Uptake by Phytoplankton Regulates Nitrification in the Sunlit Ocean

Nitrification, the microbial oxidation of ammonium to nitrate, is a central part of the nitrogen cycle. In the ocean’s surface layer, the process alters the distribution of inorganic nitrogen species available to phytoplankton and produces nitrous oxide. A widely held idea among oceanographers is that nitrification is inhibited by light in the ocean. However, recent evidence that the primary organisms involved in nitrification, the ammonia-oxidizing archaea (AOA), are present and active throughout the surface ocean has challenged this idea. Here we show, through field experiments coupling molecular genetic and biogeochemical approaches, that competition for ammonium with phytoplankton is the strongest regulator of nitrification in the photic zone. During multiday experiments at high irradiance a single ecotype of AOA remained active in the presence of rapidly growing phytoplankton. Over the course of this three day experiment, variability in the intensity of competition with phytoplankton caused nitrification rates to decline from those typical of the lower photic zone (60 nmol L⁻¹ d⁻¹) to those in well-lit layers (<1 nmol L⁻¹ d⁻¹). During another set of experiments, nitrification rates exhibited a diel periodicity throughout much of the photic zone, with the highest rates occurring at night when competition with phytoplankton is lowest. Together, the results of our experiments indicate that nitrification rates in the photic zone are more strongly regulated by competition with phytoplankton for ammonium than they are by light itself. This finding advances our ability to model the impact of nitrification on estimates of new primary production, and emphasizes the need to more strongly consider the effects of organismal interactions on nutrient standing stocks and biogeochemical cycling in the surface of the ocean.     

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.     

Bacterial production in a mesohumic lake estimated from [14C]leucine incorporation rate

Incorporation of [¹⁴C]leucine into proteins of bacteria was studied in a temperate mesohumic lake. The maximum incorporation of [¹⁴C] leucine was reached at a concentration of 30 nm determined in dilution cultures. Growth experiments were used to estimate factors for converting leucine incorporation to bacterial cell numbers or biomass. The initially high conversion factors calculated by the derivative method decreased to lower values after the bacteria started to grow. Average conversion factors were 7.09 × 10¹⁶ cells mol^–1 and 7.71 × 10¹⁵ m³ mol^–1, if the high initial values were excluded. Using the cumulative method, the average conversion factor was 5.38 × 10¹⁵ m^–3 mol^–1 I . The empirically measured factor converting bacterial biomass to carbon was 0.36 pg C m^–3 or 33.1 fg C cell^–1. Bacterial production was highest during the growing season, ranging between 1.8 and 13.2 g C liter^–1 day^–1, and lowest in winter, at 0.2–2.9 g C liter^–1 day^–1. Bacterial production showed clear response to changes in the phytoplankton production, which indicates that photosynthetically produced dissolved compounds were used by bacteria. In the epilimnion bacterial production was, on average, 19–33% of primary production. Assuming 50% growth efficiency for bacteria, the allochthonous organic carbon could have also been an additional energy and carbon source for bacteria, especially in autumn and winter. In winter, a strong relationship was found between temperature and bacterial production. The measuring of [¹⁴C]leucine incorporation proved to be a simple and useful method for estimating bacterial production in humic water. However, an appropriate amount of [¹⁴C]leucine has to be used to ensure the maximum uptake of label and to minimize isotope dilution.     

Evidence for the Cooccurrence of Nitrite-Dependent Anaerobic Ammonium and Methane Oxidation Processes in a Flooded Paddy Field

Anaerobic ammonium oxidation (anammox) and nitrite-dependent anaerobic methane oxidation (n-damo) are two of the most recent discoveries in the microbial nitrogen cycle. In the present study, we provide direct evidence for the cooccurrence of the anammox and n-damo processes in a flooded paddy field in southeastern China. Stable isotope experiments showed that the potential anammox rates ranged from 5.6 to 22.7 nmol N₂ g⁻¹ (dry weight) day⁻¹ and the potential n-damo rates varied from 0.2 to 2.1 nmol CO₂ g⁻¹ (dry weight) day⁻¹ in different layers of soil cores. Quantitative PCR showed that the abundance of anammox bacteria ranged from 1.0 × 10⁵ to 2.0 × 10⁶ copies g⁻¹ (dry weight) in different layers of soil cores and the abundance of n-damo bacteria varied from 3.8 × 10⁵ to 6.1 × 10⁶ copies g⁻¹ (dry weight). Phylogenetic analyses of the recovered 16S rRNA gene sequences showed that anammox bacteria affiliated with “Candidatus Brocadia” and “Candidatus Kuenenia” and n-damo bacteria related to “Candidatus Methylomirabilis oxyfera” were present in the soil cores. It is estimated that a total loss of 50.7 g N m⁻² per year could be linked to the anammox process, which is at intermediate levels for the nitrogen flux ranges of aerobic ammonium oxidation and denitrification reported in wetland soils. In addition, it is estimated that a total of 0.14 g CH₄ m⁻² per year could be oxidized via the n-damo process, while this rate is at the lower end of the aerobic methane oxidation rates reported in wetland soils.