Nitrogen metabolism by heterotrophic bacterial assemblages in Antarctic coastal waters

Field studies to examine the in situ assimilation and production of ammonium (NH₄ ⁺) by bacterial assemblages were conducted in the northern Gerlache Strait region of the Antarctic Peninsula. Short term incubations of surface waters containing ¹⁵N-NH₄ ⁺ as a tracer showed the bacterial population taking up 0.041–0.128 g-atoms Nl^–1d^–1, which was 8–25% of total NH₄ ⁺ uptake rates. The large bacterial uptake of NH₄ ⁺ occurred even at low bacterial abundance during a rich phytoplankton bloom. Estimates of bacterial production using ³H-leucine and -adenine were l.0gCl^–1 d^–1 before the bloom and 16.2 g Cl^–1 d^–1 at the bloom peak. After converting bacterial carbon production to an estimate of nitrogen demand, NH₄ ⁺ was found to supply 35–60% of bacterial nitrogen requirements. Bacterial nitrogen demand was also supported by dissolved organic nitrogen, generally in the form of amino acids. It was estimated, however, that 20–50% of the total amino acids taken up were mineralized to NH₄ ⁺. Bacterial production of NH₄ ⁺ was occurring simultaneously to its uptake and contributed 27–55% of total regenerated NH₄ ⁺ in surface waters. Using a variety of ¹⁵N-labelled amino acids it was found that the bacteria metabolized each amino acid differently. With their large mineralization of amino acids and their relatively low sinking rates, bacteria appear to be responsible for a large portion of organic matter recycling in the upper surface waters of the coastal Antarctic ecosystem.     

Seasonal changes in sediment phosphorus forms in relation to sedimentation and benthic bacterial biomass in Lake Erken

Surficial sediment and sedimenting material were sampled during spring and summer 1991 in Lake Erken. Sediment was analyzed for redox potential, P concentrations and bacterial biomass. Sedimentation and chlorophyll a concentrations of sedimenting matter were determined. Additionally, different phosphorus forms in surficial sediment were quantified using sequential fractionation. The resulting dataset was used to study the effects of sedimentation events following phytoplankton blooms and benthic bacterial biomass on the size of the various phosphorus pools in the sediment.Sedimentation of spring diatoms caused a rapid increase in the NH₄Cl- and NaOH-extractable P (NH₄Cl–P and NaOH–rP) in the sediment. During sedimentation, NaOH–rP and NH₄Cl–P increased within 3 days from 422 ± 17 g g^–1 DW to 537 ± 8.0 g g^–1 DW and from 113 ± 13 g g^–1 DW to 186 ± 26 g g^–1 DW, respectively. The NaOH–nrP (non-reactive P) fraction made up about 17% of Tot-P in sediment samples, whereas NaOH–rP and HCl–P made up 25% each. All P forms showed considerable seasonal variation. Significant relationships were found between bacterial biomass and the NaOH–nrP and NH₄Cl–P fractions in the sediment, respectively. Also regressions of NaOH–nrP and NH₄Cl–P versus the chlorophyll a concentration of sedimenting matter were highly significant. These regressions lend support to the conjecture that NaOH–nrP is a conservative measure of bacterial poly-P.     

Estuarine Microbial Food Web Patterns in a Lake Erie Coastal Wetland

Composition and distribution of planktonic protists were examined relative to microbial food web dynamics (growth, grazing, and nitrogen cycling rates) at the Old Woman Creek (OWC) National Estuarine Research Reserve during an episodic storm event in July 2003. More than 150 protistan taxa were identified based on morphology. Species richness and microbial biomass measured via microscopy and flow cytometry increased along a stream–lake (Lake Erie) transect and peaked at the confluence. Water column ammonium (NH₄⁺) uptake (0.06 to 1.82 M N h^–1) and regeneration (0.04 to 0.55 M N h^–1) rates, measured using ¹⁵NH₄⁺ isotope dilution, followed the same pattern. Large light/dark NH₄⁺ uptake differences were observed in the hypereutrophic OWC interior, but not at the phosphorus-limited Lake Erie site, reflecting the microbial community structural shift from net autotrophic to net heterotrophic. Despite this shift, microbial grazers (mostly choreotrich ciliates, taxon-specific growth rates up to 2.9 d^–1) controlled nanophytoplankton and bacteria at all sites by consuming 76 to 110% and 56 to 97% of their daily production, respectively, in dilution experiments. Overall, distribution patterns and dynamics of microbial communities in OWC resemble those in marine estuaries, where plankton productivity increases along the river–sea gradient and reaches its maximum at the confluence.     

Weak Coupling between Heterotrophic Nanoflagellates and Bacteria in a Eutrophic Freshwater Environment

In a study on the dynamics and trophic role of the heterotrophic nanoflagellate (HNAN) assemblage in the microbial food web of a eutrophic oxbow lake abundances, biomass, and production rates of HNAN and their potential prey organisms, namely heterotrophic bacteria and autotrophic picoplankton, were monitored for a period of 2 years. No coupling between HNAN abundance and biomass and the abundance and biomass of their picoplanktonic prey was observed for the investigation period. The ratio of heterotrophic bacterial to HNAN abundance ranged from 2.2 x 103 to 8.6 x 103 (mean: 4.2 x 103 +/- 1.8 x 103). HNAN carbon consumption could account for only 10% to 40% of bacterial secondary production. The lack of coupling between HNAN and their potential prey and the low HNAN abundance relative to bacterial abundance suggested (a) that HNAN grazing was an insignificant factor in the regulation of bacterial abundance and (b) that HNAN abundance was regulated by predation rather than by prey abundance. This hypothesis was supported by the fact that HNAN growth rates were high (in the range of 0.45 d-1 to 1.00 d-1 during spring and summer, yearly mean: 0.52 d-1), and only weakly correlated with prey abundance and biomass. The results indicated strong top-down control of HNAN and consequently a weak coupling of HNAN and picoplankton in the investigated eutrophic freshwater environment.     

Dynamics and trophic roles of heterotrophic protists in the plankton of a freshwater tidal estuary

Freshwater tidal estuaries comprise the most upstream reaches of estuaries and are often characterised by the presence of dense bacterial and algal populations which provide a large food source for bacterivorous and algivorous protists. In 1996, the protistan community in the freshwater tidal reaches of the Schelde estuary was monitored to evaluate whether these high food levels are reflected in a similarly high heterotrophic protistan biomass. Protistan distribution patterns were compared to those of metazoan zooplankton to evaluate the possible role of top-down regulation of protists by metazoans. Apart from the algivorous sarcodine Asterocaelum, which reached high densities in summer, heterotrophic protistan biomass was dominated by ciliates and, second in importance, heterotrophic nanoflagellates (HNAN). HNAN abundance was low (annual average 2490 cells ml^–1) and did not display large seasonal variation. It is hypothesised that HNAN were top-down controlled by oligotrich ciliates throughout the year and by rotifers in summer. Ciliate abundance was generally relatively high (annual average 65 cells ml^–1) and peaked in winter (maximum 450 cells ml^–1). The decline of ciliate populations in summer was ascribed to grazing by rotifers, which developed dense populations in that season. In winter, ciliate populations were probably regulated `internally' by carnivorous ciliates (haptorids and Suctoria). Our observations suggest that, in this type of productive ecosystems, the microbial food web is mainly top-down controlled rather than regulated by food availability.     

Heterotrophic bacterial activity and primary production in a hypertrophic African lake

The relationship between heterotrophic bacteria and phytoplankton in the epilimnion (0–10 m) of hypertrophic Hartbeespoort Dam, South Africa, was examined by statistically analyzing three years of parallel measurements of heterotrophic bacterial activity (glucose uptake) and phytoplankton particulate and dissolved organic carbon production. Algal biomass ranged between 4.0 and 921.1 mg Chl a m^-3 at the surface. Primary production varied between 69.5 and 3010.0 mg C m^-2h^-1 while algal production of dissolved organic carbon (EDOC) ranged from 2.5 to 219.2 mg C m^-2h^-1. Bacterial numbers reached a summer peak of 44.23 × 10⁶ cells ml^-1 in the first year and showed no depth variation. The maximum rate of glucose uptake, V_max, reached a peak of 5.52 g C l^-1h^-1. V_max, maximum glucose concentration (K_t + S_n) and glucose turnover time (T_t) were usually highest at the surface and decreased with depth concomitant with algal production. At the surface, V_max was correlated to EDOC (r = 0.59, n = 67, p < 0.001) and primary production (r = 0.71, n = 70, p < 0.001). At 5 and 10 m, V_max was correlated to integral euphotic zone (~ 4 m) algal production and bacterial numbers. Glucose turnover time was inversely related to integral algal production (r = -0.72, n = 70, p < 0.001) and less strongly to bacterial numbers. The data indicated that although bacterial numbers and biomass were low relative to algal biomass in this hypertrophic lake, the heterotrophic bacteria attained high rates of metabolic activity as a result of enhanced algal production of available organic carbon.     

Heterotrophic bacterial production in Lake Xolotlán (Managua) during 1988–1989

The biomass and production (thymidine incorporation) of heterotrophic bacterioplankton has been assessed from July, 1988, to October, 1989. in Lake Xolotlán, Nicaraqua. Bacterial abundance was high, 2–3.10¹⁰ cells.l^–1, and bacterial biomass averaged ca. 0.75 mg C.l^–1, or roughly 20% of the partivculate organic carbon. Bactrial production averaged between 3.5–5 g C.l^–1.h^–1 and on a areal basis was 650–959 mg C.m^–2.d^–1 or 13–20% ofthe primary production. Although bacterial production (volumetric basis) was typical for eutrophic lakeks, the bacterial specific growth rate was low, the bacteial population doubling time was ca. 1 week, perhaps indicating that there was a low grazing pressure on the bacteria.     

Seasonal variability of apoplastic NH4 + and pH in an intensively managed grassland

The stomatal compensation point of ammonia (_s) is a major factor controlling the exchange of atmospheric ammonia (NH₃) with vegetation. It is known to depend on the supply of nitrogen and to vary among plant species, but its seasonal variation has not yet been reported for grassland. In this study, we present the temporal variation of apoplastic NH₄ ⁺ concentration ([NH₄ ⁺]_apo) and pH (pH_apo) measured in leaves of Lolium perenne L. in a grassland, through two periods of cutting / fertilisation, followed by a livestock grazing period. The total free NH₄ ⁺ concentration measured in foliage ([NH₄ ⁺]_fol), and soil mineral NH₄ ⁺ and NO₃ ^– concentration are also presented. The value of [NH₄ ⁺]_apo varied from less than 0.01 mM to a maximum of 0.5 mM occurring just after fertilisation, whereas the apoplastic pH ranged from pH 6 to 6.5 for most of the time and increased up to pH 7.8, 9 days after the second fertilisation, when grazing started. [NH₄ ⁺]_fol varied between 20 and 50 g N-NH₄ ⁺ g^–1 f.w. The compensation point at 20°C, ranged from 0.02 g NH₃ m^–3 between the fertilisations to 10 g NH₃ m^–3 just after the second fertilisation. The reasons for these seasonal changes are discussed, with respect to plant metabolism and the concentration of ammonium and nitrate in the soil.     

Nutrient enrichment and nutrient regeneration stimulate bacterioplankton growth

Bacterial abundance results from predatory losses of individuals and replacement of losses through growth. Growth depends on sustained input of organic substrates and mineral nutrients. In this work we tested the hypothesis that bacterial growth in two oligotrophic Canadian shield lakes was limited by nitrogen (N) or phosphorus (P). We also determined whether consumer-regenerated resources contributed substantially to net bacterial growth. Two types of dilution assays were conducted to determine the response of bacteria to nutrient enrichment: diluted whole water (DWW, 1:9 whole/filtered with 0.2 m of filtered lake water) and diluted fractionated water (DFW, 1.0 m prefiltered then diluted as above). Replicate bottles in each dilution assay received either N (50 m), P (10 m), or both N and P enrichments. Controls received no nutrients. Resource-saturated growth rates and grazing rates were estimated from a standard dilution-growth approach. Bacterial growth was stimulated by addition of P alone and in combination with N. Consumers regenerated sufficient resources to support up to half the bacterial growth rate, but the benefit derived from consumers was minor when compared to mortality.     

Protozoan Grazing, Bacterial Activity, and Mineralization in Two-Stage Continuous Cultures

In two-stage continuous cultures, at bacterial concentrations, biovolumes, and growth rates similar to values found in Lake Vechten, ingestion rates of heterotrophic nanoflagellates (HNAN) increased from 2.3 bacteria HNAN⁻¹ · h⁻¹ at a growth rate of 0.15 day⁻¹ to 9.2 bacteria · HNAN⁻¹ · h⁻¹ at a growth rate of 0.65 day⁻¹. On a yeast extract medium with a C/N/P ratio of 100:15:1.2 (Redfield ratio), a mixed bacterial population showed a yield of 18% (C/C) and a specific carbon content of 211 fg of C · μm⁻³. The HNAN carbon content and yield were estimated at 127 fg of C · μm⁻³ and 47% (C/C). Although P was not growth limiting, HNAN accelerated the mineralization of PO₄-P from dissolved organic matter by 600%. The major mechanism of P remineralization appeared to be direct consumption of bacteria by HNAN. N mineralization was performed mainly (70%) by bacteria but was increased 30% by HNAN. HNAN did not enhance the decomposition of the relatively mineral-rich dissolved organic matter. An accelerated decomposition of organic carbon by protozoa may be restricted to mineral-poor substrates and may be explained mainly by protozoan nutrient regeneration. Growth and grazing in the cultures were compared with methods for in situ estimates. Thymidine incorporation by actively growing bacteria yielded an empirical conversion factor of 1.1 × 10¹⁸ bacteria per mol of thymidine incorporated into DNA. However, nongrowing bacteria also showed considerable incorporation. Protozoan grazing was found to be accurately measured by uptake of fluorescently labeled bacteria, whereas artificial fluorescent microspheres were not ingested, and selective prokaryotic inhibitors blocked not only bacterial growth but also protozoan grazing.     

Flagellate nutritional versatility as a key to survival in two contrasting Antarctic saline lakes

1. Seasonal patterns of grazing and photosynthesis were investigated in two saline Antarctic lakes (Highway and Ace) in the Vestfold Hills (68°S). The phototrophic nanoflagellate (PNAN) community was dominated by Pyramimonas gelidicola and two morphological forms of a cryptophyte species that occurred throughout the year. Both species were mixotrophic on bacteria, and in Highway Lake they also exploited dissolved organic carbon as determined by the uptake of fluorescently labelled dextrans. 2. Clearance rates ranged between 0.02 and 0.21 nL h^?1 cell^?1 in Ace Lake and 0.004–1.05 nL h^?1 cell^?1 in Highway Lake. On occasion cryptophyte grazing equalled that of the heterotrophic nanoflagellates (HNAN). 3. Photosynthetic rates showed similar trends in both lakes, but there were differences in chlorophyll a specific rates and photosynthetic efficiency, probably related to the meromictic characteristic of Ace Lake. Primary production was measurable in winter and peaked in summer following the maxima of mixotroph grazing. 4. The HNAN community of Highway Lake achieved clearance rates of 0.02–1.80 nL h^?1 cell^?1 and removing between 50 and 693 ng bacterial carbon L^?1 day^?1, with highest impact in winter when HNAN were most abundant. The HNAN also ingested fluorescently labelled dextrans showing a preference for 4 and 500 kDa molecules. The more diverse HNAN community of Ace Lake had lower clearance rates (0.04–0.37 nL h^?1 cell^?1) and exerted a lower grazing pressure on bacterioplankton. In Highway Lake, where the HNAN community was dominated by the choanoflagellate Diaphanoeca grandis, there was a significant correlation between mean cell volume and clearance rate. 5. The major feature was that the microbial plankton functioned throughout the year by employing nutritional versatility.     

MIXOTROPHY IN THE ANTARCTIC PHYTOFLAGELLATE,PYRAMIMONAS GELIDICOLA (CHLOROPHYTA: PRASINOPHYCEAE)1

Grazing by the planktonic phytoflagellate, Pyramimonas gelidicola McFadden (Chlorophyta: Prasinophyta), and heterotrophic nanoflagellates (HNAN) in meromictic saline Ace Lake in the Vestfold Hills, eastern Antarctica was investigated in the austral summers of 1997 and 1999. Up to 47% of the P. gelidicola population ingested fluorescently labeled prey (FLP). Ingestion rates varied with depth. In January 1997 and November 1999, maximum P. gelidicola ingestion rates of 6.95 and 0.79 FLP·cell^?1·h^?1, respectively, were measured at the chemocline (6–8 m) where a deep chl maximum composed of phototrophic nanoflagellates (PNAN DCM), predominantly P. gelidicola, persisted all year. During the summers of 1997 and 1999, the grazing P. gelidicola community removed between 0.4% and approximately 16% of in situ bacterial biomass, equivalent to between 4% and>100% of in situ bacterial production. Because of their higher abundance, the community clearance rates of HNAN in Ace Lake generally exceeded those of P. gelidicola, but HNAN removed approximately only 3%–4% of bacterial biomass, equivalent to between 28% and 32% of bacterial production. Pyramimonas gelidicola growth rates were highest at the PNAN DCM concomitant with the highest ingestion rates. It is estimated that during the summer P. gelidicola can derive up to 30% of their daily carbon requirements from bacterivory at the PNAN DCM. This study confirms mixotrophy as an important strategy by which planktonic organisms can survive in extreme, polar, lacustrine ecosystems.     

Bacterial Abundance and the Activity of Microbiological Processes in the Bay of Tugur of the Sea of Okhotsk

This paper presents the results of investigation of the total abundance and the biomass of bacterioplankton, the abundance of heterotrophic bacteria, and the activity of microbiological processes involved in the carbon cycle in the water of the Bay of Tugur of the Sea of Okhotsk. In different regions of the bay, the total abundance of bacterioplankton was found to vary from 0.51 × 10⁶ to 2.54 × 10⁶ cells/ml; the bacterioplankton biomass, from 8.5 to 46.5 g C/l; the abundance of heterotrophic bacteria, from 0.06 × 10³ to 2.12 × 10³ cells/ml; the bacterial assimilation of CO₂, glucose, acetate, and protein hydrolysate, from 0.8 to 6.3, from 0.11 to 1.88, from 0.07 to 0.56, and from 0.01 to 0.22 mg C/(m³ day), respectively; the degradation of organic matter ranged from 28 to 221 mg C/(m³ day); and the intensity of methane oxidation, from 0.0005 to 0.17 l CH₄/l. The spatial pattern and the functional characteristics of bacterioplankton in the Bay of Tugur were found to be dependent on the tidal dynamics.     

Effect of culture medium ions on chromate reduction by resting cells of Agrobacterium radiobacter

The influence of some ions in pre-growth culture medium on chromate reduction by resting cells of Agrobacterium radiobacter strain EPS-916 was investigated. The reduction was dependent on the Fe²⁺ content of the culture medium: the higher the iron content, the lower the reduction rate. The cells showed maximum chromate reduction when pre-grown in the presence of 0.243 m Mg²⁺, 20 m Ca²⁺ and 3.6 m Mn²⁺. Chromate reduction was not affected by the addition of MgCl₂, CdCl₂, ZnCl₂, MnCl₂, Na₂SO₄ (1000 m), and Na₂MoO₄ (100 m) to the activity assays. However, activity was inhibited by the presence of Na₂SO₄ (10 mm), Na₂MoO₄ (200 m) and ferric citrate.     

Conversion of phenylpyruvate to l-phenylalanine by immobilized Clostridium butyricum-alanine dehydrogenase-Micrococcus luteus under hydrogen high pressure

Summary Alanine was the best amino donor among various amino acids and NH₄Cl for the phenylalanine production of Micrococus luteus. l-Alanine was regenerated at the rate of 9.2 moles/min/g dry cells from NH₄Cl and pyruvate by immobilized Clostridium butyricum-alanine dehydrogenase. l-Phenylalanine was continuously produced from hydrogen, NH₄Cl and phenylpyruvate by coupling immobilized C. butyricum, alanine dehydrogenase and M. luteus. The rate of phenylalanine production was 1.74 moles/min/g dry cells.     

Development of ice biota in a temperate sea area (Gulf of Bothnia)

A study of sea ice biota was carried out in the Gulf of Bothnia (northern Baltic Sea) during the winter of 1989–1990. Samples (ice cores) were taken at a coastal station at regular time intervals during the ice season. Chlorophyll a concentration, algal species distribution, bacterial numbers, and primary and bacterial production were measured. Colonization of the ice began in January when daylight was low. As the available light increased, the algae started to grow exponentially. The vertical chlorophyll a distribution changed and algal species composition and biomass changed during the season. During the initial and middle phase of colonization, ice-specific diatoms, Nitzschia frigida and Navicula pelagica, dominated the algal biomass. Nutrients (PO₄ ^3– and NO_3j) were found to be depleted during the time of algal exponential growth. The maximum algal biomass exceeded 800 g C 1^–1. The primary production supplied food for heterotrophic organisms. The presence of heterotrophic organisms of different trophic levels (bacteria, flagellates, ciliates and rotifers) indicated an active microbial food web.     

Photoproduction of ammonium by immobilized mutant strains of Anabaena variabilis

Summary Ethylenediamine (EDA) is toxic to the cyanobacterium Anabaena variabilis and inhibits nitrogenase activity. The inhibition of nitrogenase was prevented by pretreatment of cells with l-methionine-d,l-sulphoximine (MSX). Mutant strains of Anabaena variabilis (ED81, ED92), resistant to EDA, had low levels of glutamine synthetase (GS) biosynthetic activity compared with the wild type strain. ED92 had a low level of GS protein whereas ED81 had a similar level to that of the parent strain as estimated using antibodies against GS. Both strains fixed N₂ and liberated NH₄ ⁺ into the media. Following immobilization of the mutant strains, sustained photoproduction of NH₄ ⁺ was obtained in air-lift reactors at rates of up to 50 mol NH₄ ⁺ mg chl a^–1 h^–1, which were comparable to the rates obtained when immobilized cyanobacteria were treated with MSX.Abbreviations EDA 1,2-diaminoethane (ethylenediamine) - GS glutamine synthetase - MSX l-methionine-d,l-sulphoximine     

Isolation and characterization of a toxic intracellular protein from Vibrio parahaemolyticus

A toxic factor released from disrupted cells of Vibrio parahaemolyticus was partially purified by gel filtration after precipitation with (NH₄)₂SO₄ at 40% saturation. The factor, which was a thermostable protein of 63 kDa, lysed human erythrocytes at a concentration of 0.15 g ml^-1. Its LD₅₀ by intravenous injection into mice was 6.4 g. Fluid accumulated in suckling mice force-fed with the toxic material (1 to 25 g). Haemolytic activity, which occurred maximall at 37°C and pH 7.0 was enhanced by Ca²⁺, Cu²⁺ and Zn²⁺, each at 1 mm. Anti-toxic-factor serum agglutinated V. parahaemolyticus cells. The factor may play a role in the pathogenesis of V. parahaemolyticus infections and in the host's defence mechanisms against infection by the microorganism.     

Seasonal change in the proportion of bacterial and phytoplankton production along a salinity gradient in a shallow estuary

We intended to evaluate the relative contribution of primary production versus allochthonous carbon in the production of bacterial biomass in a mesotrophic estuary. Different spatial and temporal ranges were observed in the values of bacterioplankton biomass (31–273 g C l^–1) and production (0.1–16.0 g C l^–1 h^–1, 1.5–36.8 mg C m^–2 h^–1) as well as in phytoplankton abundance (50–1700 g C l^–1) and primary production (0.1–512.9 g C l^–1 h^–1, 1.5–512.9 mg C m^–2 h^–1). Bacterial specific growth rate (0.10–1.68 d^–1) during the year did not fluctuate as much as phytoplankton specific growth rate (0.02–0.74 d^–1). Along the salinity gradient and towards the inner estuary, bacterio- and phytoplankton biomass and production increased steadily both in the warm and cold seasons. The maximum geographical increase observed in these variables was 12 times more for the bacterial community and 8 times more for the phytoplankton community. The warm to cold season ratios of the biological variables varied geographically and according to these variables. The increase at the warm season achieved its maximum in the biomass production, particularly in the marine zone and at high tide (20 and 112 times higher in bacterial and phytoplankton production, respectively). The seasonal variation in specific growth rate was most noticeable in phytoplankton, with seasonal ratios of 3–26. The bacterial community of the marine zone responded positively – generating seasonal ratios of 1–13 in bacterial specific growth rate – to the strong warm season increment in phytoplankton growth rate in this zone. In the brackish water zone where even during the warm season allochthonous carbon accounted for 41% (on average) of the bacterial carbon demand, the seasonal ratio of bacterial specific growth rate varied from about 1 to 2. During the warm season, an average of 21% of the primary production was potentially sufficient to support the whole bacterial production. During the cold months, however, the total primary production would be either required or even insufficient to support bacterial production. The estuary turned then into a mostly heterotrophic system. However, the calculated annual production of biomass by bacterio- and phytoplankton in the whole ecosystem showed that auto- and heterotrophic production was balanced in this estuary.     

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.