Measurements of Diel Rates of Bacterial Secondary Production in Aquatic Environments

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 [H]thymidine into cold-trichloroacetic acid-insoluble material ranged from 0.03 to 2.93 nmol liter day) and corresponded to rates of bacterial secondary production of 3.4 to 342.2 mug of carbon liter day (mean, 87.8 mug of carbon liter day). 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.

Consequences of accounting for isotopic dilution in thymidine incorporation assays

Rates of thymidine incorporation into DNA were corrected for isotope dilution by internal nucleotide pools and were compared with rates obtained from uncorrected data. Differences as large as 109% were observed between corrected and uncorrected estimates of thymidine incorporation. The degree of underestimation varied seasonally and, to a lesser extent, spatially.

Protozoan grazing and bacterial production in stratified lake vechten estimated with fluorescently labeled bacteria and by thymidine incorporation

In stratified Lake Vechten, The Netherlands, protozoan grazing was estimated on the basis of uptake of fluorescently labeled bacteria and compared with bacterial production estimated on the basis of thymidine incorporation. By using a grazer-free mixed bacterial population from the lake in continuous culture, an empirical relationship between cell production and thymidine incorporation was established. Thymidine incorporation into total cold-trichloroacetic-acid-insoluble macromolecules yielded a relatively constant empirical conversion factor of ca. 10 (range, 0.38 x 10 to 1.42 x 10) bacteria mol of thymidine at specific growth rates (mu) ranging from 0.007 to 0.116 h. Although thymidine incorporation has been assumed to measure DNA synthesis thymidine incorporation appeared to underestimate the independently measured bacterial DNA synthesis by at least 1.5- to 13-fold, even if all incorporated label was assumed to be in DNA. However, incorporation into DNA was found to be insignificant as measured by conventional acid-base hydrolysis. Methodological problems of the thymidine technique are discussed. Like the cultures, Lake Vechten bacteria showed considerable thymidine incorporation into total macromolecules, but no significant incorporation into DNA was found by acid-base hydrolysis. This applied not only to the low-oxygen hypo- and metalimnion but also to the aerobic epilimnion. Thus, the established empirical conversion factor for thymidine incorporation into total macromolecules was used to estimate bacterial production. Maximum production rates (141 x 10 bacteria liter h; mu, 0.012 h) were found in the metalimnion and were 1 order of magnitude higher than in the epi- and hypolimnion. In all three strata, the estimated bacterial production was roughly balanced by the estimated protozoan grazing. Heterotrophic nanoflagellates were the major consumers of the bacterial production and showed maximum numbers (up to 40 x 10 heterotrophic nanoflagellates liter) in the microaerobic metalimnion.

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 . h at a growth rate of 0.15 day to 9.2 bacteria . HNAN . h at a growth rate of 0.65 day. 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 . mum. The HNAN carbon content and yield were estimated at 127 fg of C . mum and 47% (C/C). Although P was not growth limiting, HNAN accelerated the mineralization of PO(4)-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 x 10 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.

Uptake and incorporation of thymidine by bacterial isolates from an upwelling environment

Thymidine uptake and incorporation by marine bacterial isolates from an upwelling environment were studied. Of 17 isolates each from upwelled and downwelled water, 1 and 6 isolates, respectively, were found to be negative for [H]thymidine incorporation at a substrate concentration of 19 muM. Strains lacking the ability to take up thymidine were not confined to one genus. The measurable rates of uptake and incorporation by the 34 isolates varied greatly. Studies carried out using starved Vibrio, Pseudomonas, and Cytophaga cells showed that these isolates transported and incorporated thymidine after periods of as long as 5 weeks of nutrient deprivation. This occurred in the absence of any other exogenously supplied nutrients. Overall, these results indicate that not all marine bacteria take up thymidine and that those that do incorporate the nucleoside may do so at very different rates. The assumption that only actively growing or dividing cells incorporate thymidine must be viewed with caution.

Spatial and Temporal Variations in Bacterial Macromolecule Labeling with [methyl-H]Thymidine in a Hypertrophic Lake

The incorporation of [methyl-H]thymidine into three macromolecular fractions, designated as DNA, RNA, and protein, by bacteria from Hartbeespoort Dam, South Africa, was measured over 1 year by acid-base hydrolysis procedures. Samples were collected at 10 m, which was at least 5 m beneath the euphotic zone. On four occasions, samples were concurrently collected at the surface. Approximately 80% of the label was incorporated into bacterial DNA in surface samples. At 10 m, total incorporation of label into bacterial macromolecules was correlated to bacterial utilization of glucose (r = 0.913, n = 13, P < 0.001). The labeling of DNA, which ranged between 0 and 78% of total macromolecule incorporation, was inversely related to glucose uptake (r = -0.823), total thymidine incorporation (r = -0.737), and euphotic zone algal production (r = -0.732, n = 13, P < 0.005). With decreased DNA labeling, increasing proportions of label were found in the RNA fraction and proteins. Enzymatic digestion followed by chromatographic separation of macromolecule fragments indicated that DNA and proteins were labeled while RNA was not. The RNA fraction may represent labeled lipids or other macromolecules or both. The data demonstrated a close coupling between phytoplankton production and heterotrophic bacterial activity in this hypertrophic lake but also confirmed the need for the routine extraction and purification of DNA during [methyl-H]thymidine studies of aquatic bacterial production.

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 degrees C. Of the phytoplankton biomass, >90% was the diatom Stephanodiscus hantzchii var. pusillus, by the peak of the bloom. The C and O(2) 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/NO(3) N assimilation ratio was 1.57. The total integrated incorporation of [C]bicarbonate into particulate material (>1 mum) was similar to the increase in phytoplankton carbon determined from cell counts. Bacterioplankton increased from 0.5 x 10 to 1.52 x 10 cells liter ( approximately 0.5 mug of C liter day). Estimates of bacterioplankton production from rates of [H]thymidine incorporation were ca. 1.2 to 1.7 mug of C liter day. Bacterial respiration, measured by a high-precision Winkler technique, was estimated as 4.8 mug of C liter day, 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 [H]thymidine incorporation method.

Determining [H]Thymidine Incorporation into Bacterioplankton DNA: Improvement of the Method by DNase Treatment

Determination of [H] thymidine incorporation into bacterial DNA versus other macromolecules is usually achieved by NaOH and hot trichloroacetic acid hydrolysis. This procedure was found not to be specific enough. An alternative method founded on DNase treatment is proposed. Under the new method, the fraction of thymidine incorporated into DNA ranged from 10 to 83%.

[3H]Leucine incorporation method as a tool to measure secondary production by periphytic bacteria associated to the roots of floating aquatic macrophyte

The present study assessed the application of [(3)H]Leucine incorporation into protein by periphytic bacteria associated with the roots of the floating aquatic macrophyte Eichornia crassipes. Basic assumptions underlying the method, such as linearity of leucine incorporation, saturation level of incorporation rates, incorporation into other macromolecules, specificity of incorporation for bacterial assemblages and [(3)H]Leucine degradation during samples storage were tested, and two procedures for extracting the incorporated leucine were compared. Both methods gave the same results, however, the hot TCA extraction method was less time consuming than the alkaline extraction method. Incorporation of [(3)H]Leucine was linear for up to 40 min. Saturation concentration of [(3)H]Leucine incorporation into protein was 1500 nM. An experiment with prokaryotic and eukaryotic inhibitors showed no significant [(3)H]Leucine incorporation into eukaryotes even in high leucine concentrations. No significant amounts of radiolabel were incorporated into other macromolecules. The maximum time of sample storage after the incubation is 15 days. The leucine incorporation method can be a reliable tool to measure bacterial production in the periphyton root-associated bacteria.

Response of water column microbial communities to sudden exposure to deltamethrin in aquatic mesocosms

Sudden exposure of an aquatic system to an insecticide can have significant effects on populations other than susceptible organisms. Although this is intuitively obvious, little is actually known about how such exposure might affect bacterial communities and their relative metabolic activity in ecosystems. Here, we assessed small sub-unit (ssu)-RNA levels in open and shaded 9 m(3) aquatic mesocosms (16 units - 2 x 2 factorial design in quadruplicate) to examine the effects of sudden addition of deltamethrin to the units. When deltamethrin was added, a cascade of bacterial then phytoplankton "blooms" occurred over time. The bacterial bloom, which most likely included organisms from the plastid/cyanobacterial phylogenetic guild, was almost immediate (within hours), whereas the phytoplankton (algal) bloom lagged by about 4 days. This sequential response can be explained by an apparent sudden release of nutrients consequent to arthropod death that triggered a series of responses in the microbial loop. Interestingly, bacterial blooms were noted in both open and shaded mesocosms, whereas the algal bloom was only seen in open units, suggesting that both deltamethrin addition (and presumptive nutrient release) and an adequate light supply was required for the phytoplankton response. Overall, this work shows that microbial activities as reflected by ssu-rRNA levels can respond dramatically via apparently indirect effects following insecticide application.

Strong indirect effects of a submersed aquatic macrophyte, Vallisneria americana, on bacterioplankton densities in a mesotrophic lake

Phytoplankton and allochthonous matter are important sources of dissolved organic carbon (DOC) for planktonic bacteria in aquatic ecosystems. But in small temperate lakes, aquatic macrophytes may also be an important source of DOC, as well as a source or sink for inorganic nutrients. We conducted micro- and mesocosm studies to investigate the possible effects of an actively growing macrophyte, Vallisneria americana, on bacterial growth and water chemistry in mesotrophic Calder Lake. A first microcosm (1 L) study conducted under high ambient NH4+ levels (NH4+ > or = 10 microM) demonstrated that macrophytes had a positive effect on bacterial densities through release of DOC and P. A second microcosm experiment, conducted under NH4+-depleted conditions (NH4+ < 10 microM), examined inter- active effects of macrophytes and their sediments on bacterial growth and water chemistry. Non-rooted macrophytes had negative effects on bacterial numbers, while rooted macrophytes had no significant effects, despite significant increases in DOC and P. A 70-L mesocosm experiment manipulated macrophytes, as well as N and P supply under surplus NH4-+conditions (NH4+ > or = 10 gmicro), and measured effects on bacterial growth, Chl a concentrations, and water chemistry. Bacterial growth and Chl a concentrations declined with macrophyte additions, while bacterial densities increased with P addition (with or without N). Results suggest that the submersed macrophyte Vallisneria exerts a strong but indirect effect on bacteria by modifying nutrient conditions and/or suppressing phytoplankton. Effects of living macrophytes differed with ambient nutrient conditions: under NH4+-surplus conditions, submersed macrophytes stimulated bacterioplankton through release of DOC or P, but in NH4-+depleted conditions, the influence of Vallisneria was negative or neutral. Effects of living macrophytes on planktonic bacteria were apparently mediated by the macrophytes use and/or release of nutrients, as well as through possible effects on phytoplankton production.

An endogenous periodicity exhibited in the activity of a natural bacterioplankton community isolated in mesocosms

In July 1996, bacterial abundance and incorporation of [3H]thymidine (3H-TdR) were determined every 4 h during a mesocosm experiment initially designed to study the effects of different intensities of ultraviolet-B (UVB) radiation on the summer planktonic community of the lower St. Lawrence Estuary. Water was obtained from the quay of the Maurice Lamontagne Institute (Mont-joli, Qué.) and incubated in experimental mesocosms (1500 L total volume, n = 8) with continuous mixing provided by a pumping system. During 72 h, different UVB intensities showed no significant effects on the bacterial incorporation of3H-TdR. This indicates that in the presence of other trophic levels and with continuous mixing, bacterioplankton responses to UVB are substantially different from those reported in axenic bacterial cultures or even whole-water incubations exposed to UVB at fixed depths. In conjunction with this observation,3H-TdR incorporation exhibited a significant periodic variation within all experimental treatments. The periodicity consisted of a 16-h cycle occurring independently of the time of the day. When the3H-TdR incorporation was normalized to cell abundance, the resulting cell-specific thymidine incorporation exhibited the same periodic oscillatory pattern. On the other hand, other factors suspected of inducing such a variability showed no consistent oscillation. In addition to suggesting an endogenously controlled activity of the studied bacterial community, the results of the present study indicate that failure of taking temporal variations of bacterial activity into account may introduce an error of almost 50% in the estimation of the daily thymidine incorporation rates. This represents a considerable error, because several studies rely on this measurement to estimate bacterial carbon production and to establish carbon budgets within different oceanic provinces.Key words: bacterioplankton, [3H]thymidine, ultraviolet-B radiation, periodicity, endogenous cycles, St. Lawrence Estuary.

Review Lecture - Picoplankton

Picoplankton consists of those organisms found in the open waters of seas and lakes which are capable of passing through a filter with 2 μm pores but not through one with 0.2 μm pores. Cells in this size range are well adapted to planktonic life in that they sink extremely slowly and are more efficient than larger forms in taking up nutrients and absorbing radiant energy. Picophytoplankton includes coccoid cyanobacteria and a variety of eukaryotic algal forms. Strains studied in the laboratory have all been found to show maximum growth at relatively low irradiances, the eukaryotic forms being more efficient than the cyanobacteria in utilizing the blue light which predominates at the bottom of the photic zone in clear oceanic waters. Oceanic strains of coccoid cyanobacteria, however, are characterized by high concentrations of phycoerythrin, which appears to function as a nitrogenous reserve as well as an accessory pigment in photosynthesis. The seasonal and spatial distribution of picophytoplankton seems explicable in terms of these physiological characteristics. Numbers of coccoid cyanobacteria have shown a striking correlation with temperature in a number of different situations. Heterotrophic bacteria are also included in the picoplankton, and a review of the information concerning them suggests that they form a highly dynamic population subsisting on dissolved organic matter liberated by living phytoplankton and zooplankton and by decomposition of dead matter. The productivity of this population in the euphotic zone approaches that of the phytoplankton. Both the picophytoplankton and the bacterioplankton are preyed on by phagotrophic flagellates. Both bacteria and flagellates are active in regeneration of mineral nutrients. Regardless of the salinity, temperature or nutrient status of the water, the numbers of heterotrophic bacteria, picophytoplankton and flagellates tend to be around 10 6 , 10 4 and 10 3 organisms per millilitre respectively. It is suggested that these populations form a basic, self-sustaining and self-regulating community in all natural waters. From present information, it seems that little of the energy which passes through this community finds its way into the larger planktonic organisms, but the role of picoplankton in recycling nutrient elements is of great importance in the marine ecosystem.

Bacterioplankton Production Determined by DNA Synthesis, Protein Synthesis, and Frequency of Dividing Cells in Tuamotu Atoll Lagoons and Surrounding Ocean

This study compares three independent methods used for estimating bacterioplankton production in waters from the lagoon (mesotrophic) and the surrounding ocean (oligotrophic) of two atolls from the Tuamotu archipelago (French Polynesia).Thymidine and leucine incorporation were calibrated in dilution cultures and gave consistent results when the first was calibrated against cell multiplication and the second against protein synthesis. This study demonstrates that determining conversion factors strongly depends on the selected calculation method (modified derivative, integrative, and cumulative). These different estimates are reconciled when the very low proportion of active cells is accounted for.Frequency of dividing-divided cells (FDDC) calibrated using the same dilution cultures led to unrealistically high estimates of bacterial production. However, highly significant correlations between FDDC and either thymidine- or leucine-specific incorporation per cell were found in lagoon waters in situ. These correlations became more positive when oceanic data were added. This suggests that the FDDC method is also potentially valid to determine bacterioplankton growth rates after cross calibration with thymidine or leucine methods. If recommended precautions are observed, the three methods tested in the present study would give reliable production estimates.

Thymidine incorporation of bacteria sequentially extracted from soil using repeated homogenization-centrifugation

Bacteria were sequentially extracted from soil into a water suspension after shaking soil with water or mixing it in a blender followed by a low-speed centrifugation. Bacteria, which were released only after several cycles of homogenization-centrifugation, had higher growth rates as judged from thymidine and leucine incorporation, whereas bacteria that were more readily released by a gentle shaking procedure had the lowest growth rate. This indicated that bacteria more tightly bound to soil particles were growing faster than those that were more easily released into the water suspension. The same pattern was found both in an agricultural and a forest soil, with contrasting pH and organic matter content, and irrespective of whether the bacteria were labeled before or after the centrifugation steps. The different growth rates of the bacteria could not be explained by different partitioning of label between different macromolecules, different cell size, different viability of the bacteria, or different dilution of the added radioactive substrate in the different homogenization-centrifugation fractions. The total amount of phospholipid fatty acids per bacterial cell was also similar in the different fractions. Different composition of the bacterial communities in the different homogenization-centrifugation fractions was indicated by a gradually altered phospholipid fatty acid pattern of the extracted bacteria, and an increased hydrophobicity of the bacteria released only after several homogenization-centrifugation treatments.

Thymidine and leucine incorporation in soil bacteria with different cell size

Thymidine and leucine incorporation into macromolecules of soil bacteria extracted by homogenization-centrifugation were measured after size-fractionation of the bacterial suspension through different sized filters (1.0, 0.8, 0.6, 0.4 μm). The specific thymidine incorporation rate was highest for the unfiltered and 1.0 μm filtered suspensions (approximately 10 × 10(-21) mol thymidine bacteria(-1) h(-1)), but decreased to 1.39 × 10(-21) mol bacteria(-1) h(-1) for bacteria passing the 0.4 μm filter. The proportion of culturable bacteria (percent colony forming units/acridine orange direct counts) also decreased with bacterial cell size from 5.0% for the unfiltered bacterial suspension to 0.8% in the 0.4 µm filtrate. A strong linear correlation (r (2) = 0.995) was found between the specific thymidine incorporation rate and the proportion of culturable bacteria. Leucine incorporation gave similar results to the thymidine incorporation. No effects of cell size on the degree of isotope dilution or unspecific labeling of other macromolecules were found either for the thymidine or the leucine incorporation technique. These data indicate that small bacteria, although more numerous than larger ones, not only constitute a smaller proportion of the soil bacterial biomass than larger bacteria, but also contribute to a lesser degree to carbon transformations in soil.

Short-term variations in specific biovolumes of different bacterial forms in aquatic ecosystems

Short-term and spatial fluctuations in specific biovolumes (volume x cell(-1)) of different morphological categories of planktonic bacteria were estimated microscopically. Samples were taken from two lakes occurring in two different climatic systems: Lake Aydat (France) and Lake Cromwell (Canada). The study was done in summer, using 24-hour cycles of sampling.Due to their large size, the specific volume of filamentous bacteria constituted, on average, the major part (>70%) of the total specific volume of all bacterial forms considered. Greatest variations in specific biovolumes were recorded for filamentous bacteria (coefficients of variation ranged from 16 to 109%). These variations were more pronounced in the oxygenated and microaerophilic strata (DOC ≈1.5 mg liter(-1)). Fluctuations in cell volume were high (coefficients of variation =12-80%) for coccal bacteria, whereas no marked fluctuations were found for the rod and vibrio bacteria (coefficients of variation =4-10%).Evidence of diel patterns of cell volume of filamentous bacteria is provided. These cells displayed their maximum size during the day until early night, indicating cell division was occurring at night. Homogeneous circadian patterns were not provided by specific volume variations of coccal, rod, and vibrio bacteria.Statistical relationships between bacterial specific biovolumes and the biotic and abiotic parameters considered are discussed.

DNA Synthesis and Tritiated Thymidine Incorporation by Heterotrophic Freshwater Bacteria in Continuous Culture

Continuous cultivation of heterotrophic freshwater bacteria was used to assess the relationship between DNA synthesis and tritiated thymidine incorporation. The bacteria were grown on a yeast extract medium with generation times of 0.25 to 3.7 days. In six different continuous cultures, each inoculated with a grazer-free mixed bacterial sample from Lake Vechten (The Netherlands), tritiated thymidine incorporation into a cold trichloroacetic acid precipitate and bacterial cell production were measured simultaneously. Empirical conversion factors were determined by division of both parameters. They ranged from 0.25 × 10 18 to 1.31 × 10 18 cells mol of tritiated thymidine -1 (mean, 0.60 × 10 18 cells mol of tritiated thymidine -1 ). In addition, DNA concentrations were measured by fluorometry with Hoechst 33258. The validity of this technique was confirmed. Down to a generation time of 0.67 day, bacterial DNA content showed little variation, with values of 3.8 to 4.9 fg of DNA cell -1 . Theoretical conversion factors, which can be derived from DNA content under several assumptions, were between 0.26 × 10 18 and 0.34 × 10 18 cells mol of thymidine -1 (mean, 0.30 × 10 18 cells mol of thymidine -1 ). Isotope dilution was considered the main factor in the observed discrepancy between the conversion factors. In all experiments, a tritiated thymidine concentration of 20 nM was used. Control experiments indicated maximum incorporation at this concentration. It was therefore concluded that the observed difference resulted from intracellular isotope dilution which cannot be detected by current techniques for isotope dilution analysis.

The phytoplanktonic ways of life

The adaptations of phytoplankton to life in suspension in water are considered with particular reference to hydromechanical factors ranging from molecular motion to ocean currents. The smallest phytoplankton, the picoplankton (0.2-2.0 μm), are the best adapted to the physico-chemical environment of the open waters of seas and lakes and, other things being equal, can out-compete the larger forms. The picophytoplankton are an autotrophic component in a microbial community, the ultraplankton, which also includes bacteria and flagellates up to about 20 μm in linear dimensions. This community is a highly dynamic and self-contained equilibrium system operating within a domain dominated by molecular diffusion. Within the photic zone it is limited, not by nutrient supply, but by its internal predator-prey relationships. It appears to be cosmopolitan, both in the sea and in freshwaters, to vary little either in time or space in species composition or in biomass concentration, and to contribute a minimum of organic carbon to higher trophic levels. There appears to be a fundamental divide in form and function between this and the microplankton, composed of organisms larger than about 20 μm. This community can only develop when nutrient levels are in excess of the concentrations required by the picoplankton. It is opportunistic, non-equilibrium in its dynamics, and highly variable in floristic composition and biomass concentration. Its life processes are dominated by turbulence. Nutrient supply is largely determined by turbulent eddy diffusion and movement of the organisms relative to the water mass. The microphytoplankton falls broadly into two types, one of which, exemplified by the diatoms, depends on turbulence to maintain it in the photic zone and the other, exemplified by the dinoflagellates and colony-forming cyanobacteria, relies on motility or buoyancy control to position it in a relatively stable water column so as to have best access to light and nutrients. The waxing and waning of microplankton populations is largely determined by hydrography and their floristic compositions by the interactions of the daily and seasonal rhythms of the organisms with the periodicities in the environment. In contrast to the ultraplankton microplankton species show distinct differences in biogeographical distribution. Throughout the discussion attention is drawn to the intimate relationships between the activities of phytoplankton and those of viruses, bacteria and zooplankton and the impossibility of getting a proper understanding of the physiology of the phytoplankton if they are considered in isolation. CONTENTS Summary 191 I. Introduction 192 II. Some reminders about the aqueous medium 193 III. The ideal phytoplankton organism 194 IV. Some aspects of the physiological ecology of the picoplankton 195 V. The ultraplanktonic community 197 VI. A divide in the phytoplankton 201 VII. Some aspects of the physiological ecology of the microphytoplankton 203 VIII. Small scale turbulence in the environment of phytoplankton 205 IX. Intermediate scale turbulence in the environment of phytoplankton 207 X. The effects of large scale rotational stirring 220 XI. Phytoplankton as part of a community 221 XII. Conclusions 222 References 223.

Correlation of nonspecific macromolecular labeling with environmental parameters during [(3)H]Thymidine incorporation in the waters of southwest florida

During routine [(3)H]thymidine incorporation measurements of environmental samples, significant amounts of radioactivity are often incorporated into macromolecules other than DNA. Although the percentage of nonspecific labeling varies both temporally and spatially, the cause(s) of these variations remain unknown. Correlations between the percent incorporated radioactivity in DNA and a variety of experimental and environmental parameters measured in the Alfia River, Crystal River, Medard Reservoir, and Bayboro Harbor were examined. The amount of radioactivity incorporated into DNA ranged from 6 to 95% ([Formula: see text]; n=121). Nonspecific labeling began immediately upon the addition of [(3)H]thymidine and was linear over time. Labeling patterns were independent of both the amount of thymidine added and cell-size fraction. A two year study of Bayboro Harbor indicated no conclusive relationship between nonspecific labeling and seasonality. The amount of radioactivity incorporated into DNA was inversely correlated with total rates of thymidine incorporation and a strong diurnal pattern was observed in the Crystal River. No consistent relationship was observed between labeling patterns and primary productivity, chlorophylla, particulate DNA, dissolved DNA, bacterial cell numbers, temperature, salinity, and dissolved organic carbon. The only relationship with dissolved inorganic nutrients (N and P) occurred in the Crystal River. In this phosphate limited river, the percent of radioactivity incorporated into DNA was positively correlated with phosphate concentrations. These results indicate that nonspecific labeling is not dependent on any one parameter but may be a function of many interacting environmental factors or a function of the specific ambient bacterial population.

Purification of DNA for bacterial productivity estimates

[methyl-3H]thymidine-labeled DNA from natural populations of aquatic bacteria was completely separated from RNA and protein by hydroxylapatite chromatography. The procedure was validated by monitoring increases in Escherichia coli cell count, A550, DNA concentration, and thymidine incorporation into DNA isolated by the proposed technique. The procedure can be used in the field and does not rely on the use of acid-base hydrolysis or volatile organic solvents.

Thymidine incorporation in saltern ponds of different salinities: Estimation of in situ growth rates of halophilic archaeobacteria and eubacteria

Incorporation of [methyl-(3)H]thymidine was measured in solar saltern ponds of different salinities. Estimated doubling times of the bacterial communities were in the range of 1.1 to 22.6 days. Even at the highest salt concentrations (NaCl saturation), relatively rapid thymidine incorporation was observed. In an attempt to differentiate between activity of halophilic archaeobacteria (theHalobacterium group) and halophilic eubacteria, taurocholate, which causes lysis of the halobacteria without affecting eubacteria, was used. At salt concentrations exceeding 250 g/liter all thymidine incorporation activity could be attributed to halobacteria. Aphidicolin, a potent inhibitor of DNA synthesis in halobacteria, completely abolished thymidine incorporation at the highest salinities, but also caused significant inhibition at salinities at which halobacteria are expected to be absent. Attempts to use nalidixic acid to selectively inhibit DNA synthesis by the eubacterial communities were unsuccessful.

Production of extracellular nucleic acids by genetically altered bacteria in aquatic-environment microcosms

The factors which affect the production of extracellular DNA by genetically altered strains of Escherichia coli, Pseudomonas aeruginosa, Pseudomonas cepacia, and Bradyrhizobium japonicum in aquatic environments were investigated. Cellular nucleic acids were labeled in vivo by incubation with [3H]thymidine or [3H]adenine, and production of extracellular DNA in marine waters, artificial seawater, or minimal salts media was determined by detecting radiolabeled macromolecules in incubation filtrates. The presence or absence of the ambient microbial community had little effect on the production of extracellular DNA. Three of four organisms produced the greatest amounts of extracellular nucleic acids when incubated in low-salinity media (2% artificial seawater) rather than high-salinity media (10 to 50% artificial seawater). The greatest production of extracellular nucleic acids by P. cepacia occurred at pH 7 and 37 degrees C, suggesting that extracellular-DNA production may be a normal physiologic function of the cell. Incubation of labeled P. cepacia cells in water from Bimini Harbor, Bahamas, resulted in labeling of macromolecules of the ambient microbial population. Collectively these results indicate that (i) extracellular-DNA production by genetically altered bacteria released into aquatic environments is more strongly influenced by physiochemical factors than biotic factors, (ii) extracellular-DNA production rates are usually greater for organisms released in freshwater than marine environments, and (iii) ambient microbial populations can readily utilize materials released by these organisms.

Measurement of bacterial growth rates in subsurface sediments using the incorporation of tritiated thymidine into DNA

Microbial growth rates in subsurface sediment from three sites were measured using incorporation of tritiated thymidine into DNA. Sampling sites included Lula, Oklahoma, Traverse City, Michigan, and Summit Lake, Wisconsin. Application of the thymidine method to subsurface sediments required (1) thymidine concentrations greater than 125 nM, (2) incubation periods of less than 4 hours, (3) addition of SDS and EDTA for optimum macromolecular extraction, and (4) DNA purification, in order to accurately measure the rate of thymidine incorporation into DNA. Macromolecule extraction recoveries, as well as the percentage of tritium label incorporated into the DNA fraction, were variable and largely dependent upon sediment composition. In general, sandy sediments yielded higher extraction recoveries and demonstrated a larger percentage of label incorporated into DNA than sediments that contained a high silt-clay component. Reported results also indicate that the acid-base hydrolysis procedure routinely used for macromolecular fractionation in water samples may not be routinely applicable to the modified sediment procedure where addition of SDS and EDTA are required for macromolecule extraction. Growth rates exhibited by subsurface communities are relatively slow, ranging from 5.1 to 10.2×10(5) cells g(-1) day(-1). These rates are 2-1,000-fold lower than growth rates measured in surface sediments. These data lend support to the supposition that subsurface microbial communities are nutritionally stressed.

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 degrees 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 NaHCO(3) and [methyl-H]thymidine incorporation, respectively. Peak instantaneous production ranged between 14.8 and 220.5 mug of C liter h. There were two distinct periods of high rates of production. From May through July, production near the metalimnion exceeded 100 mug of C liter h. During holomixis, production throughout the water column was in excess of 100 mug of C liter h and above 150 mug of C liter h near the surface. Annual areal primary production was 588 g of C m. Bacterial production was markedly seasonal. Growth rates during late fall through spring were typically around 0.002 h, and production rates were typically 5 mug of C liter h. Growth rates were higher during warmer parts of the year and reached 0.03 h by August. The maximum instantaneous rate of bacterial production was approximately 45 mug of C liter h. Annual areal bacterial production was 125 g of C m. 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).

Seasonal and Diel Variability in Dissolved DNA and in Microbial Biomass and Activity in a Subtropical Estuary

Dissolved DNA and microbial biomass and activity parameters were measured over a 15-month period at three stations along a salinity gradient in Tampa Bay, Fla. Dissolved DNA showed seasonal variation, with minimal values in December and January and maximal values in summer months (July and August). This pattern of seasonal variation followed that of particulate DNA and water temperature and did not correlate with bacterioplankton (direct counts and [H]thymidine incorporation) or phytoplankton (chlorophyll a and CO(2) fixation) biomass and activity. Microautotrophic populations showed maxima in the spring and fall, whereas microheterotrophic activity was greatest in late summer (September). Both autotrophic and heterotrophic microbial activity was greatest at the high estuarine (low salinity) station and lowest at the mouth of the bay (high salinity station), irrespective of season. Dissolved DNA carbon and phosphorus constituted 0.11 +/- 0.05% of the dissolved organic carbon and 6.6 +/- 6.5% of the dissolved organic phosphorus, respectively. Strong diel periodicity was noted in dissolved DNA and in microbial activity in Bayboro Harbor during the dry season. A noon maximum in primary productivity was followed by an 8 p.m. maximum in heterotrophic activity and a midnight maximum in dissolved DNA. This diel periodicity was less pronounced in the wet season, when microbial parameters were strongly influenced by episodic inputs of freshwater. These results suggest that seasonal and diel production of dissolved DNA is driven by primary production, either through direct DNA release by phytoplankton, or more likely, through growth of bacterioplankton on phytoplankton exudates, followed by excretion and lysis.

Production Rate of Planktonic Bacteria in the North Basin of Lake Biwa, Japan

Vertical and seasonal variations in the cell number and production rate of planktonic bacteria were investigated at a pelagic site (water depth, ca. 72 m) of the north basin of Lake Biwa during April to October 1986. The [ methyl - 3 H]thymidine uptake rate into a cold trichloroacetic acid-insoluble fraction and the frequency of dividing cells (FDCs) were measured for each sample as indices of the bacterial production rate. The seasonal data of bacterial number, thymidine uptake rate, and bacterial growth rate based on the FDCs were correlated with one another (rank correlation analysis, P < 0.05). These bacterial variables were not correlated positively with the chlorophyll a concentration. Vertically, the maxima of both bacterial number and the thymidine uptake rate were found in the euphotic zone. The direct counting of bacteria and the measurements of thymidine uptake rate combined with the size-fractionation method revealed that more than 90% of the bacterial biomass and production rate were attributed to unattached bacteria throughout the investigation period. The carbon flux estimates of bacterial production were less certain due to the variability of the conversion factor for the thymidine uptake method and that of the calibration for the FDC method, but even when the conservative range of bacterial net production rate was used (5 to 60 μg of carbon per liter per day), it can be suggested that bacterial net production in the investigated area was a significant fraction (ca. 30%) of the level of the primary production rate in the same water basin.

Zooplankton induced changes in dissolved free amino acids and in production rates of freshwater bacteria

This study examined the importance of zooplankton in the flux of dissolved free amino acids (DFAA) in the water and into bacteria. DFAA release rates were followed in laboratory grazing experiments usingDaphnia galeata andEudiaptomus graciloides as grazers, andScenedesmus acutus andSynechococcus elongatus as food sources. Except for minor initial peaks, DFAAs were released continuously during the first 2 hours and made up 6-12% (in one experiment 50%) of the calculated ingestion rates. During three diel studies in lakes, effects of removal and increase of the density of zooplankton (>200μm) on the pools of DFAA as well as on the bacterial production were followed. During two of the diel studies, higher DFAA pools were measured when 3-4 times the natural zooplankton density was present, and in one study a minor increase also occurred in the bacterial production, compared with results from experiments without zooplankton and with a natural zooplankton density. The increase in bacterial growth coincided with a decline in DFAA. During the third study, neither DFAA nor the bacterial production changed significantly when the zooplankton density was increased 3 times. Removal of zooplankton, however, caused a decline in both DFAA and bacterial production. Our data suggest a close relationship between occurrence of zooplankton and release of DFAA, but the factors regulating the amount of DFAA released and its effect on bacterial growth are not yet understood.

Bacterial productivity in ponds used for culture of penaeid prawns

The quantitative role of bacteria in the carbon cycle of ponds used for culture of penaeid prawns has been studied. Bacterial biomass was measured using epifluorescence microscopy and muramic acid determinations. Bacterial growth rates were estimated from the rate of tritiated thymidine incorporation into DNA. In the water column, bacterial numbers ranged from 8.3×10(9) 1(-1) to 2.57×10(10) 1(-1) and production ranged from 0.43 to 2.10 mg Cl(-1) d(-1). In the 0-10 mm zone in sediments, bacterial biomass was 1.4 to 5.8 g C m(-2) and production was 250 to 500 mg C m(-2) d(-1). The results suggested that most organic matter being supplied to the ponds as feed for the prawns was actually being utilized by the bacteria. When the density of meiofauna increased after chicken manure was added, bacterial biomass decreased and growth rates increased.

Diel changes in the specific growth rate and mean cell volume of natural bacterial communities in two different water masses in the Irish sea

Diel changes in the specific growth rates of natural bacterial communities as a whole and of different groups within the communities were followed over 2 days during July 1982, in stratified waters in the vicinity of a shallow sea tidal mixing front in the Irish Sea. Waters well above (4 m) and below (60 m) the thermocline were enclosed in dialysis bags and incubated in situ. The results show that there were periods of altered growth rates of the whole bacterial community and synchronous cell division of morphological groups. An increase in mean cell volume within both 4 and 60 m communities preceded an increase in specific growth rates, with a resultant decrease in the mean cell volume. Above the thermocline the whole bacterial community, as well as the rod and coccoid forms, doubled in number once a day. The doubling time of the whole bacterial community at 60 m was 2 days and slower than that at 4 m. This was due to a slower doubling time (3 days) for the coccoid forms. Rod forms at the two depths had a similar doubling time (1 day). The time of day when maximum division rate occurred was also different in the two water masses. At 4 m more coccoid forms divided during the night, whereas at 60 m more divided during the day. Conversely, at 4 m more rod forms divided during the day, whereas at 60 m more divided at night.These data indicate that the bacterial community and members of the community may be adapted to exploit the diurnal rhythms of dissolved organic carbon (DOC) release by other organisms and that portions of the bacterial community may therefore be more active at certain times of the day. The diurnal growth of the bacterial community may thus vary between different water masses and largely reflects the differences in the chemical and biological characteristics of the two water masses investigated.

Potential Importance of Fish Predation and Zooplankton Grazing on Natural Populations of Freshwater Bacteria

The rates of ingestion of natural bacterial assemblages by natural populations of zooplankton (>50 μm in size) were measured during a 19-day period in eutrophic Frederiksborg Slotssø, Denmark, as well as in experimental enclosures (containing 5.3 m 3 of lake water). The fish and nutrients of the enclosures were manipulated. In enclosures without fish, large increases in ingestion by zooplankton >140 μm in size were found (up to 3 μg of C liter −1 h −1 ), compared with values less than 0.3 μg of C liter −1 h −1 in the enclosures with fish and in the open lake. Daphnia cucullata and D. galeata dominated the community of zooplankton of >140 μm. Ingestion rates for zooplankton between 50 and 140 μm decreased after a period of about 8 days, in all enclosures and in the lake, to values below 0.1 μg of C liter −1 h −1 . On the last 2 sampling days, somewhat higher values were observed in the enclosures with fish present. The >50-μm zooplankton ingested 48 to 51% of the bacterial net secondary production in enclosures without fish, compared to 4% in the enclosures with added fish. Considering the sum of bacterial secondary production plus biomass change, 35 to 41% of the available bacteria were ingested by zooplankton of >50 μm in the enclosures without fish, compared with 4 to 6% in the enclosures with added fish and 21% in the open lake. Fish predation reduced the occurrence of zookplankton sized >50 μm and thus left a large proportion of the available bacteria to zooplankton sized <50 μm. In fact, there were 4.6 × 10 3 to 5.0 × 10 3 flagellates (4 to 8 μm in size) ml −1 in the enclosures with fish added as well as in the lake, compared with 0.5 × 10 2 to 2.3 × 10 2 ml −1 in the enclosures without fish. This link in the food chain was reduced when fish predation on zooplankton was eliminated and a direct route of dissolved organic matter, via the bacteria to the zooplankton, was established.