Utilization of Dissolved Nitrogen by Heterotrophic Bacterioplankton: a Comparison of Three Ecosystems

The contributions of different organic and inorganic nitrogen and organic carbon sources to heterotrophic bacterioplankton in batch cultures of oceanic, estuarine, and eutrophic riverine environments were compared. The importance of the studied compounds was surprisingly similar among the three ecosystems. Dissolved combined amino acids (DCAA) were most significant, sustaining from 10 to 45% of the bacterial carbon demands and from 42 to 112% of the bacterial nitrogen demands. Dissolved free amino acids (DFAA) supplied 2 to 7% of the carbon and 6 to 24% of the nitrogen incorporated into the bacterial biomass, while dissolved DNA (D-DNA) sustained less than 5 and 12% of the carbon and nitrogen requirements, respectively. Ammonium was the second most important source of nitrogen, meeting from 13 to 45% of the bacterial demand in the oceanic and estuarine cultures and up to 270% of the demand in riverine cultures. Nitrate was taken up in the oceanic cultures (uptake equaled up to 46% of the nitrogen demand) but was released in the two others. Assimilation of DCAA, DFAA, and D-DNA combined supplied 43% of the carbon demand of the bacteria in the oceanic cultures, while approximately 25% of the carbon requirements were met by the three substrates at the two other sites. Assimilation of nitrogen from DCAA, DFAA, D-DNA, NH₄⁺, and NO₃^-, on the other hand, exceeded production of particulate organic nitrogen in one culture at 27 h and in all cultures over the entire incubation period (50 h). These results suggest that the studied nutrient sources may fully support the nitrogen needs but only partially support the carbon needs of microbial communities of geographically different ecosystems. Furthermore, a comparison of the initial concentrations of the different substrates indicated that relative pool sizes of the substrates seemed to influence which substrates were primarily being utilized by the bacteria.     

Occurrence and Bacterial Cycling of d Amino Acid Isomers in an Estuarine Environment

Abundance of d isomers of amino acids has been used in studies of organic matter diagenesis to determine the contribution of bacterial biomass to the organic matter, especially in marine sediments. However, fluxes of d amino acids in pelagic waters are poorly known. Here we present seasonal changes (March–September) in concentrations of dominant d amino acids in the pool of dissolved free and combined (hydrolysable) amino acids (DFAA and DCAA) in the shallow Roskilde Fjord, Denmark. The amino acid dynamics are related to pelagic bacterial density and activity and abundance of viruses. d␣isomers made up 3.6 and 7.9% of the DFAA and DCAA (average values), respectively, and had similar seasonal variations in concentrations. In batch cultures (0.7- and 0.2-m filtered water in a 1:9 mixture) microbial activity reduced l+d DCAA concentrations in seven of ten sampling dates, while DCAA were released at the remaining three sampling times. NH₄⁺ balance (uptake or release) in the cultures correlated significantly with variations in concentrations of d-DCAA, but not with the total DCAA pools. Abundance of viruses did not correlate with density or production of bacteria in the fjord, but covaried with mineralization of total C, DCAA and PO₄³⁻ in the batch cultures. The content of d amino acids in bacterial biomass in the cultures varied from 6.7 to 12.5% and correlated with the d isomer concentration in the fjord, except for d-Ala. In an additional six-day batch culture study, DCAA and d-DCAA were assimilated by the bacteria during the initial 36 h, but were released between 36 and 42 h simultaneous with a decline in the bacterial density. Our results demonstrate that peptidoglycan components contribute to natural amino acid pools and are assimilated by bacterial assemblages. This cell wall “cannibalism” ensures an efficient recycling of nutrients within the microbial community. Significant positive correlations between viral abundance and bacterial mineralization of organic matter in the fjord indicated that viral lysis contributed to this nutrient recycling.     

Net release of dissolved organic matter by the scleractinian coral Acropora pulchra

The net production of dissolved organic matter (DOM) and dissolved combined and free amino acids (DCAA and DFAA, respectively) by the hermatypic coral Acropora pulchra was measured in the submerged condition, and the production rates were normalized to the coral surface area, tissue biomass, and net photosynthetic rates by zooxanthellae. When normalized to the unit surface area, the production rates of dissolved organic carbon and nitrogen (DOC and DON, respectively) were 37 and 4.4 nmol cm^− 2 h^− 1, respectively. Comparing with the photosynthetic rate by zooxanthellae, which was measured by ¹³C-tracer accumulation in the soft tissue of the coral colony, the release rate of DOC corresponded to 5.4% of the daily net photosynthetic production. The tissue biomass of the coral colony was 178 µmol C cm^− 2 and 23 µmol N cm^− 2, indicating that the release of DOC and DON accounted for 0.021% h^− 1 and 0.019% h^− 1 of the tissue C and N, respectively. The C:N ratios of the released DOM (average 8.4) were not significantly different from those of the soft tissue of the coral colonies (average 7.7). While DFAA did almost not accumulate in the incubated seawater, DCAA was considerably released by the coral colonies at the rate of 2.1 nmol cm^− 2 h^− 1 on average. Calculating C and N contents of the hydrolyzable DCAA, it was revealed that about 20% and 50%–60% of the released bulk DOC and DON, respectively, were composed of DCAA.     

Seasonal changes in the dissolved free amino acid and DOC concentrations in a hypertrophic African reservoir and its inflowing rivers

Dissolved free amino acid (DFAA) concentration and composition and dissolved organic carbon (DOC) concentration were measured over 16 months at three depths in hypertrophic Hartbeespoort Dam, South Africa and in its two perenially inflowing rivers. The range of DFAA concentrations in the reservoir and both rivers were similar with dominant DFAA consisting of serine, glycine, alanine and ornithine in all three systems. The range of DOC concentrations in the rivers was 1.5–11.1 mg l^–1, the major river (Crocodile) having about twice the DOC concentration of the Magalies River. The DFAA/DOC ratios ranged between 0.02–1.1% in the Crocodile River and 0.13–3.7% in the Magalies River. DFAA and DOC concentrations were positively correlated to the Magalies River flow, but for the Crocodile River, which received domestic and industrial effluents, DOC was inversely correlated to flow. The source of DFAA in both rivers was mainly terrestrial, in contrast to the main DOC source in the Crocodile River which was the effluents. The DFAA load of the Crocodile River ranged between 0.22 and 208 kg C d^–1.DOC (5.0–24.8mg l^–1) in Hartbeespoort Dam generally decreased with depth but DFAA (15–4800 nmol l^–1) concentration showed no clear trend. The DFAA/DOC ratios varied between 0.02 and 2.9%. DFAA concentrations were correlated (r = 0.3, n = 30, p = 0.04) with bacterial numbers at 0 and 10 m only while no significant correlations were found with bacterial production, chlorophyll a concentration and phytoplankton primary and EDOC (extracellular DOC) production at any depth. The rate of bacterial utilization of DFAA was low compared with data from other lakes. Diurnal phytoplankton production of DFAA in the euphotic zone of the whole lake was calculated to vary between 268 and 30 780 t C d^–1 indicating autochthonous DFAA sources were dominant to allochthonous DFAA sources. The autochthonous production of DFAA was > 2 × gross bacterial production of the euphotic zone indicating that although DFAA concentrations were frequently < 10 g C l^–1, the rate of DFAA production exceeded bacterial requirements.     

Colloidal and dissolved organic matter in lake water: Carbohydrate and amino acid composition,and ability to support bacterial growth

Bacterial utilization of dissolved organic matter (DOM) was studied in water from a humic and a clearwater oligotrophic lake. Indigenous bacteria were inoculated into either 0.2 m natural filtered lake water, or lake water enriched fivefold with colloidal DOM >100 kD but below 0.2 m. Consumption of DOM was followed from changes in concentrations of total dissolved organic carbon (DOC), dissolved combined and free carbohydrates and amino acids (DCCHO and DFCHO, and DCAA and DFAA, respectively) and by uptake of monosaccharide and amino acid radioisotopes. DCCHO and DCAA made up 8% (humic lake) to 33–44% (clear-water lake) of the natural DOC pools, while DFCHO and DFAA contributed at most 1.7% to the DOC pools. Addition of >100 kD DOM increased the DOC concentrations by 50% (clearwater lake) to 92% (humic lake), but it only resulted in a higher bacterial production (by 63%) in the humic lake. During the incubations 13 to 37% of the DOC was assimilated by the bacteria, at estimated growth efficiencies of 4–8%. Despite the measured reduction of DOC, statistically significant changes of specific organic compounds, especially of DCCHO and DCAA, generally did not occur. Probably the presence of high molecular weight DOC interfered with the applied analytical procedures. Addition of radiotracers indicated, however, that DFAA sustained 17–58% and 29–100% of the bacterial carbon and nitrogen requirements, respectively, and that glucose met 1–3% of the bacterial carbon requirements. Thus, our experiments indicate that radiotracers, rather than measurements of concentration changes, should be used in studies of bacterial utilization of DOC in freshwaters with a high content of humic or high molecular weight organic matter.     

Contribution of Bacterial Cell Wall Components to DOM in Alkaline,Hypersaline Mono Lake,California

Soda lakes are often characterized by high densities of prokaryotes and high concentrations of dissolved organic carbon. Since bacterial cell walls are less degradable than most other cell constituents, accumulation of cell wall material may occur in these lakes and contribute to the DOM pool, but composition of DOM in soda lakes has rarely been examined. Here we report concentrations of DOM components likely originating from bacterial cell walls, including D amino acids, glucosamine (GluA) and muramic acid (MurA), in depth profiles of stratified, alkaline, hypersaline Mono Lake, CA. Concentrations of cell wall components were related to total pools of dissolved free and combined amino acids (DFAA and DCAA), and bacterial density and production. In the free pool, total DFAA ranged from 50 to 3250 nM and typically increased with depth, while GluA (5 to 140 nM) and MurA (< 0.5 nM and only detected in 2005) fluctuated with depth. In the combined pool, DCAA varied between 5000 and 15000 nM and did not show clear depth-related trends. GluA ranged from 1000 to 5000 nM and tended to increase in the hypolimnion, while MurA varied between 25 and 75 nM. Free D isomers in the DFAA pool either made up < 13% (Asp and Ser) or varied from 10 to 57% (Glu and Ala). In the combined pool, D isomers of Asp, Glu, Ser and Ala made up 24-48% of these DCAA and typically showed minor changes with depth. In 2005, lysozyme activity had highest rates in the surface and correlated negatively with most D isomers among the combined amino acids. Our observations demonstrate that the pool of dissolved combined amino compounds in the lake was about 5-fold higher than in other eutrophic lakes and that a substantial portion of these amino compounds originated from bacterial cell walls.     

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.     

Benthic decomposition of Zostera marina roots: a controlled laboratory experiment

The initial benthic decomposition of Zostera marina roots was studied in a controlled flow-through chamber experiment for 23 days. Sediment chambers without added roots served as controls. The inflowing and outflowing artificial seawater (ASW) was analyzed for O₂, ΣCO₂, urea-N, NH₄⁺ and NO₂⁻+NO₃⁻. Sediment profiles of Eh, particulate organic carbon (POC) and nitrogen, dissolved organic nitrogen (DON), dissolved free amino acids (DFAA), urea-N, NH₄⁺, DFAA and urea turnover rates, sulfate reduction and counts of total anaerobic heterotrophic bacteria and different functional groups were determined. Fluxes of O₂, ΣCO₂, urea-N and NH₄⁺ were stimulated during root decomposition compared to the unamended control. There were indications of stimulated bacterial growth based on counts of total anaerobic heterotrophic bacteria, anaerobic phosphatase utilizers, ammonifyers and sulfate reducers. Independent estimates of nitrogen and carbon incorporation into bacterial biomass during root decomposition indicate that a major fraction of the nitrogen for microbial growth was mobilized from the indigenous particulate organic nitrogen (PON) pool, whereas the energy source for bacterial growth was mainly obtained from the added eelgrass roots. Most of the nitrogen mineralized during root decomposition was incorporated into the bacterial biomass resulting in a low efflux of urea-N and inorganic nitrogen from the sediment to the water column.     

Regulation of bacterial proteolytic activity at natural concentrations in dissolved organic matter in a maritime pond]

The regulation of the bacterial exoproteolytic activity, at natural substrate concentrations, was studied during the survey of an Atlantic coastal marine pond (France). The regulation of this activity occurs at two different levels: on the one hand, at the cellular level, the ectoenzyme synthesis is regulated by hydrolysis substrates, dissolved combined amino acids (DCAA), and end products, dissolved free amino acids (DFAA), in terms of the relative amounts available to the cell, and on the other hand, at the ecosystem level, i.e. the hydrolytic activity, by the total amounts of DCAA and DFAA in situ. The DFAA acts as an inhibitor in enzymatic synthesis; in contrast, dissolved proteins induce the enzymatic synthesis and the exoproteolytic activity. These results, obtained in natural concentration conditions, confirm the functioning in situ of the ectoenzymatic activity regulation model of Chróst, until now only validated in an enriched experimental medium.     

Diel Production and Microheterotrophic Utilization of Dissolved Free Amino Acids in Waters Off Southern California

Diel patterns of dissolved free amino acid (DFAA) concentration and microheterotrophic utilization were examined in the spring and fall of 1981 in euphotic waters from the base of the mixed layer off the southern California coast. The average depths of the isotherms sampled were 19.2 m for spring and 9.0 m for fall. Total DFAA levels were generally higher in the spring than in the fall, 18 to 66 nM and 14 to 20 nM, respectively. Two daily concentration maxima and minima were observed for total DFAAs as well as for most individual DFAAs. Maxima were usually measured in the mid-dark period and in the early afternoon; minima were typically observed in early morning and late afternoon. Bacterial cell numbers reached maximal values near midnight in both seasons. These increases coincided with one of the total DFAA maxima. The second total DFAA maximum occurred in early to midafternoon, during the time of maximum photosynthetic carbon production and rapid dissolved amino acid utilization. Microbial metabolism (incorporation plus respiration) of selected ³H-amino acids was 2.7 to 4.1 times greater during the daylight hours. DFAA turnover times, based on these metabolic measurements, ranged between 11 and 36 h for the amino acids tested, and rates were 1.7 to 3.7 times faster in the daylight hours than at night. DFAA distributions were related to primary production and chlorophyll a concentrations. Amino acids were estimated to represent 9 to 45% of the total phytoplankton exudate. Microheterotrophic utilization or production of total protein amino acids was estimated as 3.6 μg of C liter⁻¹ day⁻¹ in spring and 1.9 μg of C liter⁻¹ day⁻¹ in the fall. Assimilation efficiency for dissolved amino acids averaged 65% for marine microheterotrophs.     

Diel variation in concentration,assimilation and respiration of dissolved free amino acids in relation to planktonic primary and secondary production in two eutrophic lakes

Concentration of dissolved free amino acids (DFAA) and assimilation of the 5 most abundant DFAA (glutamic acid, serine, glycine, alanine and ornithine) were measured at 3-h intervals over 27 h in two Danish, eutrophic lakes. The carbon flux of the amino acid assimilation was compared with the major routes of carbon flux, including primary production, bacterial production and zooplankton grazing. In Frederiksborg Slotssø, the mean DFAA concentration was 275 nM with distinct peaks (up to 783 nM) 3 h after sunrise. Assimilation rates of the 5 amino acids amounted on the average to 2.03 µg Cl^–1 h^–1, but high values up to 7.41 µg Cl^–1 h^–1 occurred 3 h after sunrise and at midnight. The mean turnover time of the amino acid pools was 3.2 h. In Lake Mossø, the mean DFAA concentration was 592 nM with peak of 1 161 nM at dusk. The assimilation rate averaged 0.44 µg Cl^–1 h^–1, and the mean turnover time of the amino acid pools was 39 h. In Lake Mossø, similar turnover times of glutamic acid and serine were determined from the ¹⁴C-amino acid tracer technique and Michaelis-Menten uptake kinetics, indicating that the tracer technique gave reliable values of the actual assimilation. The average respiration percentages of the assimilated amino acids were 45% in Frederiksborg Slotssø and 51% in Lake Mossø. Extracellular organic carbon (EOC) released from the phytoplankton contributed DFAA to the water. In Lake Mossø, 81% of the ambient EOC pool was <700 daltons and 9.3% of the EOC was DFAA. This corresponded to about 2.4% of the DFAA pool. Bacterial productivity, determined by means of frequency of dividing cells and ³⁵S-SO₄ dark uptake techniques gave similar results and constituted 4.5 and 3.7 µg Cl^–1 h^–1 in Frederiksborg Slotssø and Lake Mossø, respectively. The bacterial productivity suggested that DFAA were essential substrates to the bacteria, especially in Frederiksborg Slotssø. The zooplankton biomass in Frederiksborg Slotssø was six times larger than that in Lake Mossø, but cladocerans were dominant in both lakes. The zooplankton grazing probably was an important regulatory factor for the bacterial productivity.     

Distribution and characterization of dissolved and particulate organic matter in Antarctic pack ice

Distribution and composition of organic matter were investigated in Antarctic pack ice in early spring and summer. Accumulation of organic compounds was observed with dissolved organic carbon (DOC) and particulate organic carbon (POC) reaching 717 and 470 μM C, respectively and transparent exopolymeric particles (TEP) up to 3,071 μg Xanthan gum equivalent l⁻¹. POC and TEP seemed to be influenced mainly by algae. Particulate saccharides accounted for 0.2–24.1% (mean, 7.8%) of POC. Dissolved total saccharides represented 0.4–29.6% (mean, 9.7%) of DOC, while dissolved free amino acids (DFAA) accounted for only 1% of DOC. Concentrations of TEP were positively correlated with those of saccharides. Monosaccharides (d-MCHO) dominated during winter–early spring, whereas dissolved polysaccharides did in spring–summer. DFAA were strongly correlated with d-MCHO, suggesting a similar pathway of production. The accumulation of monomers in winter is thought to result from limitation of bacterial activities rather than from the nature of the substrates.     

Environmental Stress Determines the Quality of Bacterial Lysate and Its Utilization Efficiency in a Simple Microbial Loop

Heterotrophic bacteria provide the critical link in the microbial loop by converting dissolved organic matter (DOM) into particulate form. In this study, DOM was prepared from recently isolated estuarine bacterial strain Vibrio sp. (DSM14379) grown at different salinities [0.2%, 0.5%, 3%, 5%, or 10% (w/v)], washed, concentrated, and lysed by autoclaving. The corresponding lysate-containing media were designated LM_0.2, LM_0.5, LM₃, LM₅, and LM₁₀. Vibrio sp. cells grown at different salinities had similar C/N/P ratios, but different C/S ratios, different trace element composition, and different 2D gel electrophoresis protein profiles. Pseudoalteromonas sp. (DSM06238) isolated from a similar environment was able to grow on all lysates, and its biomass production was dependent on lysate type. The highest growth rate and biomass production of Pseudoalteromonas sp. at saturation lysate concentrations were observed in LM₃. The biomass production at saturation lysate concentrations was about 3-fold higher as compared to LM_0.2 and LM₁₀. The initial respiration rate, intracellular adenosine triphosphate (ATP) levels, and ³H-Leu and ³H-TdR incorporation rates were lowest in LM₃. On the other hand, in LM_0.2 or LM₁₀ lysates the situation was reversed, the growth rates and biomass production were lowest, whereas ³H-Leu and ³H-TdR incorporation, respiration rates, as well as ATP levels, were highest. These results imply uncoupling of catabolism from growth in either high- or low-salinity lysates. The results also suggest that differences in organic carbon quality generated during Vibrio sp. growth at different NaCl concentrations were propagated through the simple microbial loop, which may have important ecological implications for higher trophic levels that depend on microbial grazing.     

Fluxes of free amino acids in three Danish lakes

SUMMARY. 1. Heterotrophic assimilation rates and concentrations of dissolved free amino acids (DFAA) were followed during diel studies in the eutrophic Lake Mossø, Lake Esrom and Lake øm in spring and summer in 1982. In all three lakes, three to four fold diel variations in concentrations and assimilation rates were measured. These fluctuations appeared to be iindependent of phytoplankton and bacteria production. Pools of DFAA varied from 380 nM (Lake Mossø) to 2430 nM (Lake ørn), with serine, glycine, alanine and ornithine as dominant free amino acids.
2. When similar water samples were incubated in a natural light-dark cycle or in total darkness, different pools of DFAA were measured in light and dark.
3. Decomposition of organic matter or zooplankton activity (rather than e.g. phytoplankton exudates) appear to be responsible for the concentration changes.
4. Observed discrepancies between simultaneous concentration changes and assimilation rates are discussed in relation to the applied tracer procedure and the concentration measurements.
5. Assimilation of DFAA sustained from 6% to 25% of the bacterial carbon requirement, corresponding to 2–12% of the phytoplankton production in the lakes.     

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.     

Uncoupling of Bacterioplankton and Phytoplankton Production in Fresh Waters Is Affected by Inorganic Nutrient Limitation

Pelagic bacterial production is often positively correlated, or coupled, with primary production through utilization of autotrophically produced dissolved organic carbon. Recent studies indicate that inorganic N or P can directly limit both bacterial and phytoplanktonic growth. Our mesocosm experiments, with whole communities from mesotrophic Calder Lake, test whether this apparent bacterial-algal coupling may be the result of independent responses to limiting inorganic nutrients. In systems without N additions, numbers of bacteria but not phytoplankton increased 2- to 2.5-fold in response to P fertilization (0 to 2.0 μmol of P per liter); this resulted in uncoupled production patterns. In systems supplemented with 10 μmol of NH₄NO₃ per liter, P addition resulted in up to threefold increases in bacteria and two- to fivefold increases in total phytoplankton biomass (close coupling). P limitation of pelagic bacteria occurred independently of phytoplankton dynamics, and regressions between bacterial abundance and phytoplankton chlorophyll a were nonsignificant in all systems without added N. We describe a useful and simple coupling index which predicts that shifts in phytoplankton and bacterioplankton growth will be unrelated (Δ bacteria/Δ phytoplankton → either + ∞ or - ∞) in systems with inorganic N/P (molar) ratios of <~40. In systems with higher N/P ratios (>40), the coupling index will approach 1.0 and close coupling between bacteria and phytoplankton is predicted to occur.     

Diel,seasonal, and depth-related variability of viruses and dissolved DNA in the northern Adriatic Sea

The depth-dependent, seasonal, and diel variability of virus numbers, dissolved DNA (D-DNA), and other microbial parameters was investigated in the northern Adriatic Sea. During periods of water stratification, we found higher virus abundances and virus/bacterium ratios (VBRs) as well as a larger variability of D-DNA concentrations at the thermocline, probably as a result of higher microbial biomass. At the two investigated stations, virus densities were highest in summer and autumn (up to 9.5 × 10¹⁰ 1^–1) and lowest in winter (< 10⁹ 1^–1); D-DNA concentrations were highest in summer and lowest in winter. The VBR as well as an estimated proportion of viral DNA on total D-DNA showed a strong seasonal variability. VBR averaged 15.0 (range, 0.9–89.1), and the percentage of viral DNA in total D-DNA averaged 18.3% (range, 0.1–96.1%). An estimation of the percentage of bacteria lysed by viruses, based on 2-h sample intervals in situ, ranged from 39.6 to 212.2% d^–1 in 5 m and from 19.9 to 157.2% d^–1 in 22 m. The estimated contribution of virus-mediated bacterial DNA release to the D-DNA pool ranged from 32.9 to 161% d^–1 in 5 m and from 10.3 to 74.2% d^–1 in 22 m. Multiple regression analysis and the diel dynamics of microbial parameters indicate that viral lysis occasionally could be more important in regulating bacterial abundances than grazing by heterotrophic nanoflagellates. Correspondence to: M.G. Weinbauer     

Importance of Viral Lysis and Dissolved DNA for Bacterioplankton Activity in a P-Limited Estuary,Northern Baltic Sea

Through lysis of bacterioplankton cells, viruses mediate an important, but poorly understood, pathway of carbon and nutrients from the particulate to the dissolved form. Via this activity, nutrient-rich cell lysates may become available to noninfected cells and support significant growth. However, the nutritional value of lysates for noninfected bacteria presumably depends on the prevailing nutrient limitation. In the present study, we examined dynamics of dissolved DNA (D-DNA) and viruses along a transect in the phosphorus (P)-limited Ore Estuary, northern Baltic Sea. We found that viruses were an important mortality factor for bacterioplankton and that their activity mediated a significant recycling of carbon and especially of P. Uptake of dissolved DNA accounted for up to 70% of the bacterioplankton P demand, and about a quarter of the D-DNA pool was supplied through viral lysis of bacterial cells. Generally, the importance of viral lysates and uptake of D-DNA was highest at the estuarine and offshore stations and was positively correlated with P limitation measured as alkaline phosphatase activity. Our results highlight the importance of viral activity for the internal recycling of principal nutrients and pinpoints D-DNA as a particularly relevant compound in microbial P dynamics.     

Extracellular dissolved organic carbon released from phytoplankton as a source of carbon for heterotrophic bacteria in lakes of different humic content

The quantitative importance of photosynthetically produced dissolved organic carbon (PDOC) released from phytoplankton as a source of carbon for pelagic, heterotrophic bacteria was investigated in four temperate Swedish lakes, of which two had low (≈20 mg Pt 1⁻¹), and two moderately high (60–80 mg Pt 1⁻¹) humic content. The bacterial assimilation of PDOC was estimated with the ¹⁴C method, and the total production of the heterotrophic bacteria was estimated with the [3H]thymidine incorporation method. The release of PDOC from natural communities of phytoplankton was not restricted to periods of photosynthesis, but often continued during periods of darkness. Heterotrophic bacteria often assimilated the labile components of the PDOC at high rates (up to 73% of the released PDOC was assimilated during the incubation in our experiments). The contribution of PDOC to bacterial production exhibited large within-lake seasonal variations, but PDOC was at certain times, both in humic and non-humic lakes, a quantitatively very important carbon source for the heterotrophic bacteria. Under periods of comparatively low primary production, heterotrophic bacteria in humic lakes appear to utilize allochthonous, humic substances as a substrate.