Bacterial Production and Growth Rate Estimation from [3H]Thymidine Incorporation for Attached and Free-Living Bacteria in Aquatic Systems

Production and specific growth rates of attached and free-living bacteria were estimated in an oligotrophic marine system, La Salvaje Beach, Vizcaya, Spain, and in a freshwater system having a higher nutrient concentration, Butron River, Vizcaya, Spain. Production was calculated from [methyl-³H]thymidine incorporation by estimating specific conversion factors (cells or micrograms of C produced per mole of thymidine incorporated) for attached and free-living bacteria, respectively, in each system. Conversion factors were not statistically different between attached and free-living bacteria: 6.812 × 10¹¹ and 8.678 × 10¹¹ μg of C mol⁻¹ for free-living and attached bacteria in the freshwater system, and 1.276 × 10¹¹ and 1.354 × 10¹¹ μg of C mol⁻¹ for free-living and attached bacteria in the marine system. Therefore, use of a unique conversion factor for the mixed bacterial population is well founded. However, conversion factors were higher in the freshwater system than in the marine system. This could be due to the different trophic conditions of the two systems. Free-living bacteria contributed the most to production in the two systems (85% in the marine system and 67% in the freshwater system) because of their greater contribution to total biomass. Specific growth rates calculated from production data and biomass data were similar for attached and free-living bacteria.     

Synergistic Interaction Between Anabaena and Zoogloea spp. in Carbon Dioxide-Limited Continuous Cultures

Flocs consisting of Anabaena and Zoogloea spp. were used as a model system for the study of planktonic phototroph-heterotroph interactions. In CO₂-limited continuous culture (3.2 μmol of NaHCO₃ liter⁻¹ h⁻¹, 1.5 μmol of glucose liter⁻¹ h⁻¹, pH 8.5, D = 0.026 h⁻¹), the biomass of the phototroph increased 8.6-fold due to association. However, direct CO₂ exchange accounted for only a 3.8-fold increase. When the glucose supply rate was increased to 7.5 μmol liter⁻¹ h⁻¹, there was a 26-fold increase in biomass. When CO₂ was supplied in excess, there was no difference due to association. In batch culture, using the same medium, the specific growth rate was 0.029 h⁻¹ for the phototroph alone and 0.047 h⁻¹ for the phototroph in association with the heterotroph. The stimulatory effect of the heterotroph was found only under CO₂-limiting conditions and was directly related to the concentration of organic matter supplied in the medium. Both the biomass and the growth rate of the Anabaena sp. were increased by association with the Zoogloea sp. Thus, dissolved organic matter may substitute for CO₂ to maximize both growth rate and biomass production by phototrophs when heterotrophic bacteria are present.     

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

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

Comparison of Extracellular Enzyme Activities and Community Composition of Attached and Free-Living Bacteria in Porous Medium Columns

Free-living and surface-associated microbial communities in sand-packed columns perfused with groundwater were compared by examination of compositional and functional characteristics. The composition of the microbial communities was assessed by bulk DNA extraction, PCR amplification of 16S ribosomal DNA fragments, separation of these fragments by denaturing gradient gel electrophoresis, and sequence analysis. Community function was assessed by measurement of β-glucosidase and aminopeptidase extracellular enzyme activities. Free-living populations in the aqueous phase exhibited a greater diversity of phylotypes than populations associated with the solid phase. The attached bacterial community displayed significantly greater β-glucosidase and aminopeptidase enzyme activities per volume of porous medium than those of the free-living community. On a per-cell basis, the attached community had a significantly higher cell-specific aminopeptidase enzyme activity (1.07 × 10⁻⁷ nmol cell⁻¹ h⁻¹) than the free-living community (5.02 × 10⁻⁸ nmol cell⁻¹ h⁻¹). Conversely, the free-living community had a significantly higher cell-specific β-glucosidase activity (1.92 × 10⁻⁶ nmol cell⁻¹ h⁻¹) than the surface-associated community (6.08 × 10⁻⁷ nmol cell⁻¹ h⁻¹). The compositional and functional differences observed between these two communities may reflect different roles for these distinct but interacting communities in the decomposition of natural organic matter or biodegradation of xenobiotics in aquifers.     

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 m³ 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 × 10³ to 5.0 × 10³ 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 × 10² to 2.3 × 10² 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.     

Dynamics of Microbial Communities on Marine Snow Aggregates: Colonization, Growth, Detachment, and Grazing Mortality of Attached Bacteria

We studied the dynamics of microbial communities attached to model aggregates (4-mm-diameter agar spheres) and the component processes of colonization, detachment, growth, and grazing mortality. Agar spheres incubated in raw seawater were rapidly colonized by bacteria, followed by flagellates and ciliates. Colonization can be described as a diffusion process, and encounter volume rates were estimated at about 0.01 and 0.1 cm³ h⁻¹ for bacteria and flagellates, respectively. After initial colonization, the abundances of flagellates and ciliates remained approximately constant at 10³ to 10⁴ and ~10² cells sphere⁻¹, respectively, whereas bacterial populations increased at a declining rate to >10⁷ cells sphere⁻¹. Attached microorganisms initially detached at high specific rates of ~10⁻² min⁻¹, but the bacteria gradually became irreversibly attached to the spheres. Bacterial growth (0 to 2 day⁻¹) was density dependent and declined hyperbolically when cell density exceeded a threshold. Bacterivorous flagellates grazed on the sphere surface at an average saturated rate of 15 bacteria flagellate⁻¹ h⁻¹. At low bacterial densities, the flagellate surface clearance rate was ~5 × 10⁻⁷ cm² min⁻¹, but it declined hyperbolically with increasing bacterial density. Using the experimentally estimated process rates and integrating the component processes in a simple model reproduces the main features of the observed microbial population dynamics. Differences between observed and predicted population dynamics suggest, however, that other factors, e.g., antagonistic interactions between bacteria, are of importance in shaping marine snow microbial communities.     

Sea Ice Microbial Communities: Distribution, Abundance, and Diversity of Ice Bacteria in McMurdo Sound, Antarctica, in 1980

An abundant and diverse bacterial community was found within brine channels of annual sea ice and at the ice-seawater interface in McMurdo Sound, Antarctica, in 1980. The mean bacterial standing crop was 1.4 × 10¹¹ cells m⁻² (9.8 mg of C m⁻²); bacterial concentrations as high as 1.02 × 10¹² cells m⁻³ were observed in ice core melt water. Vertical profiles of ice cores 1.3 to 2.5 m long showed that 47% of the bacterial numbers and 93% of the bacterial biomass were located in the bottom 20 cm of sea ice. Ice bacterial biomass concentration was more than 10 times higher than bacterioplankton from the water column. Scanning electron micrographs showed a variety of morphologically distinct cell types, including coccoid, rod, fusiform, filamentous, and prosthecate forms; dividing cells were commonly observed. Approximately 70% of the ice bacteria were free-living, whereas 30% were attached to either living algal cells or detritus. Interactions between ice bacteria and microalgae were suggested by a positive correlation between bacterial numbers and chlorophyll a content of the ice. Scanning and transmission electron microscopy revealed a close physical association between epibacteria and a dominant ice alga of the genus Amphiprora. We propose that sea ice microbial communities are not only sources of primary production but also sources of secondary microbial production in polar ecosystems. Furthermore, we propose that a detrital food web may be associated with polar sea ice.     

Contribution of Fungi and Bacteria to Leaf Litter Decomposition in a Polluted River

The contribution of fungi and bacteria to the decomposition of alder leaves was examined at two reference and two polluted sites in the Ave River (northwestern Portugal). Leaf mass loss, microbial production from incorporation rates of radiolabeled compounds into biomolecules, fungal biomass from ergosterol concentration, sporulation rates, and diversity of aquatic hyphomycetes associated with decomposing leaves were determined. The concentrations of organic nutrients and of inorganic nitrogen and phosphorus in the stream water was elevated and increased at downstream sites. Leaf decomposition rates were high (0.013 day⁻¹ < k < 0.042 day⁻¹), and the highest value was estimated at the most downstream polluted site, where maximum values of microbial production and fungal biomass and sporulation were found. The slowest decomposition occurred at the other polluted site, where, along with the nutrient enrichment, the lowest current velocity and dissolved-oxygen concentration in water were observed. At this site, fungal production, biomass, and sporulation were depressed, suggesting that stimulation of fungal activity by increased nutrient concentrations might be offset by other factors. Although bacterial production was higher at polluted sites, fungi accounted for more than 94% of the total microbial net production. Fungal yield coefficients varied from 10.2 to 13.6%, while those of bacteria were less than 1%. The contribution of fungi to overall leaf carbon loss (29.0 to 38.8%) greatly exceeded that of bacteria (4.2 to 13.9%).     

Hydrogen Metabolism by Decomposing Cyanobacterial Aggregates in Big Soda Lake, Nevada

Hydrogen production by incubated cyanobacterial epiphytes occurred only in the dark, was stimulated by C₂H₂, and was inhibited by O₂. Addition of NO₃⁻ inhibited dark, anaerobic H₂ production, whereas the addition of NH₄⁺ inhibited N₂ fixation (C₂H₂ reduction) but not dark H₂ production. Aerobically incubated cyanobacterial aggregates consumed H₂, but light-incubated rates (3.6 μmol of H₂ g⁻¹ h⁻¹) were statistically equivalent to dark uptake rates (4.8 μmol of H₂ g⁻¹ h⁻¹), which were statistically equivalent to dark, anaerobic production rates (2.5 to 10 μmol of H₂ g⁻¹ h⁻¹). Production rates of H₂ were fourfold higher for aggregates in a more advanced stage of decomposition. Enrichment cultures of H₂-producing fermentative bacteria were recovered from freshly harvested, H₂-producing cyanobacterial aggregates. Hydrogen production in these cyanobacterial communities appears to be caused by the resident bacterial flora and not by the cyanobacteria. In situ areal estimates of dark H₂ production by submerged epiphytes (6.8 μmol of H₂ m⁻² h⁻¹) were much lower than rates of light-driven N₂ fixation by the epiphytic cyanobacteria (310 μmol of C₂H₄ m⁻² h⁻¹).     

Chemostat Enrichments of Human Feces with Resistant Starch Are Selective for Adherent Butyrate-Producing Clostridia at High Dilution Rates

Resistant starch (RS) enrichments were made using chemostats inoculated with human feces from two individuals at two dilution rates (D = 0.03 h⁻¹ and D = 0.30 h⁻¹) to select for slow- and fast-growing amylolytic communities. The fermentations were studied by analysis of short-chain fatty acids, amylase and α-glucosidase activities, and viable counts of the predominant culturable populations and the use of 16S rRNA-targeted oligonucleotide probes. Considerable butyrate was produced at D = 0.30 h⁻¹, which corresponded with reduced branched-chain fatty acid formation. At both dilution rates, high levels of extracellular amylase activity were produced, while α-glucosidase was predominantly cell associated. Bacteroides and bifidobacteria predominated at the low dilution rate, whereas saccharolytic clostridia became more important at D = 0.30 h⁻¹. Microscopic examination showed that within 48 h of inoculation, one particular bacterial morphotype predominated in RS enrichments at D = 0.30 h⁻¹. This organism attached apically to RS granules and formed rosette-like structures which, with glycocalyx formation, agglomerated to form biofilm networks in the planktonic phase. Attempts to isolate this bacterium in pure culture were repeatedly unsuccessful, although a single colony was eventually obtained. On the basis of its 16S rDNA sequence, this RS-degrading, butyrate-producing organism was identified as being a previously unidentified group I Clostridium sp. A 16S rRNA-targeted probe was designed using this sequence and used to assess the abundance of the population in the enrichments. At 240 h, its contributions to total rRNA in the chemostats were 5 and 23% at D = 0.03 and 0.30 h⁻¹, respectively. This study indicates that bacterial populations with significant metabolic potential can be overlooked using culture-based methodologies. This may provide a paradigm for explaining the discrepancy between the low numbers of butyrate-producing bacteria that are isolated from fecal samples and the actual production of butyrate.     

Grazing of a Tetrahymena sp. on Adhered Bacteria in Percolated Columns Monitored by In Situ Hybridization with Fluorescent Oligonucleotide Probes

Predation of attached Pseudomonas putida mt2 by the small ciliate Tetrahymena sp. was investigated with a percolated column system. Grazing rates were examined under static and dynamic conditions and were compared to grazing rates in batch systems containing suspended prey. The prey densities were 2 × 10⁸ bacteria per ml of pore space and 2 × 10⁸ bacteria per ml of suspension, respectively. Postingestion in situ hybridization of bacteria with fluorescent oligonucleotide probes was used to quantify ingestion. During 30 min, a grazing rate of 1,382 ± 1,029 bacteria individual⁻¹ h⁻¹ was obtained with suspended prey; this was twice the grazing rate observed with attached bacteria under static conditions. Continuous percolation at a flow rate of 73 cm h⁻¹ further decreased the grazing rate to about 25% of the grazing rate observed with suspended prey. A considerable proportion of the protozoans fed on neither suspended bacteria nor attached bacteria. The transport of ciliates through the columns was monitored at the same time that predation was monitored. Less than 20% of the protozoans passed through the columns without being retained. Most of these organisms ingested no bacteria, whereas the retained protozoans grazed more efficiently. Retardation of ciliate transport was greater in columns containing attached bacteria than in bacterium-free columns. We propose that the correlation between grazing activity and retardation of transport is a consequence of the interaction between active predators and attached bacteria.     

Primary and Bacterial Secondary Production in a Southwestern Reservoir

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

High Motility Reduces Grazing Mortality of Planktonic Bacteria

We tested the impact of bacterial swimming speed on the survival of planktonic bacteria in the presence of protozoan grazers. Grazing experiments with three common bacterivorous nanoflagellates revealed low clearance rates for highly motile bacteria. High-resolution video microscopy demonstrated that the number of predator-prey contacts increased with bacterial swimming speed, but ingestion rates dropped at speeds of >25 μm s⁻¹ as a result of handling problems with highly motile cells. Comparative studies of a moderately motile strain (<25 μm s⁻¹) and a highly motile strain (>45 μm s⁻¹) further revealed changes in the bacterial swimming speed distribution due to speed-selective flagellate grazing. Better long-term survival of the highly motile strain was indicated by fourfold-higher bacterial numbers in the presence of grazing compared to the moderately motile strain. Putative constraints of maintaining high swimming speeds were tested at high growth rates and under starvation with the following results: (i) for two out of three strains increased growth rate resulted in larger and slower bacterial cells, and (ii) starved cells became smaller but maintained their swimming speeds. Combined data sets for bacterial swimming speed and cell size revealed highest grazing losses for moderately motile bacteria with a cell size between 0.2 and 0.4 μm³. Grazing mortality was lowest for cells of >0.5 μm³ and small, highly motile bacteria. Survival efficiencies of >95% for the ultramicrobacterial isolate CP-1 (≤0.1 μm³, >50 μm s⁻¹) illustrated the combined protective action of small cell size and high motility. Our findings suggest that motility has an important adaptive function in the survival of planktonic bacteria during protozoan grazing.     

Microbial food web in a large shallow lake (Lake Balaton, Hungary)

Seasonal variations of phyto-, bacterio- and colourless flagellate plankton were followed across a year in the large shallow Lake Balaton (Hungary). Yearly average chlorophyll-a concentration was 11 µg 1^–1, while the corresponding values of bacterioplankton and heterotrophic nanoflagellate (HNF) plankton biomass (fresh weight) were 0.24 mg 1^–1 and 0.35 mg 1^–1, respectively. About half of planktonic primary production was channelled through bacterioplankton on the yearly basis. However, there was no significant correlation between phytoplankton biomass and bacterial abundance. Bacterial specific growth rates were in the range of 0.009 and 0.09 h^–1, and ended to follow the seasonal changes in water temperature. In some periods of the year, predator-prey relationships between the HNF and bacterial abundance were obvious. The estimated HNF grazing on bacteria varied between 3% and 227% of the daily bacterial production. On an annual basis, 87% of bacterial cell production was grazed by HNF plankton.     

Growth Kinetics of Attached Iron-Oxidizing Bacteria

A model of growth and substrate utilization for ferrous-iron-oxidizing bacteria attached to the disks of a rotating biological contactor was developed and tested. The model describes attached bacterial growth as a saturation function in which the rate of substrate utilization is determined by a maximum substrate oxidation rate constant (P), a half-saturation constant (K_s), and the concentration of substrate within the rotating biological contactor (S₁). The maximum oxidation rate constant was proportional to flow rate, and the substrate concentration in the reactor varied with influent substrate concentration (S₀). The model allowed the prediction of metabolic constants and included terms for both constant and growth-rate-dependent maintenance energies. Estimates for metabolic constants of the attached population of acidophilic, chemolithotrophic, iron-oxidizing bacteria limited by ferrous iron were: maximum specific growth rate (μ_max), 1.14 h⁻¹; half-saturation constant (K_s) for ferrous iron, 54.9 mg/liter; constant maintenance energy coefficient (m₁), 0.154 h⁻¹; growth-rate-dependent maintenance energy coefficient (m′), 0.07 h⁻¹; maximum yield (Y_g), 0.063 mg of organic nitrogen per mg of Fe(II) oxidized.     

Bacterial Communities in Acidic and Circumneutral Streams

The relationship between pH and the abundance and activity of bacteria in streams was examined as part of a study of the effect of acidification on stream communities. Of the bacterial communities examined, the epilithic community appeared to be the most significantly affected by acidification. Microbial biomass, as quantified by measuring the ATP level, on rock surfaces was significantly correlated with pH. Also, bacterial production by the epilithic bacteria, indicated by incorporation of tritiated thymidine into DNA, was always higher at high-pH sites than at low-pH sites of the same stream order and elevation. Bacterioplankton concentrations varied between 0.53 × 10⁵ and 9.42 × 10⁵ cells · ml⁻¹ in the first- to fourth-order streams examined. The bacterioplankton concentration in one sample from a spring was 0.17 × 10⁵ cells · ml⁻¹. Bacterioplankton concentrations were not correlated with pH but were significantly correlated with seston concentrations. The correlation with seston is a result of increases in particle-associated bacteria at high seston concentrations. The proportion of bacterioplankton attached to particles varied from 0 to 70%. Bacterial numbers and production in the sediments were significantly correlated with the organic content of the sediment rather than with the pH of the overlying water. Thus, reduced abundance and activity of bacteria as a result of acidification could be detected only for the relatively active community on rock surfaces; this community was exposed to the low pH because of the unbuffered nature of its environment.     

Influence of Cyanobacterial Hyperscum on Heterotrophic Activity of Planktonic Bacteria in a Hypertrophic Lake

The response of the planktonic heterotrophic bacterial community to the buildup and breakdown of a semipermanent, crusted, floating cyanobacterial mat, or hyperscum, that covered 1 to 2 ha was studied in a hypertrophic lake (Hartbeespoort Dam, South Africa). The initial response of bacteria in the main basin to the release of dissolved organic carbon (DOC) from the hyperscum 1 km away was an increase in activity per cell from 35 × 10⁻¹² to 153 × 10⁻¹² μg of C cell⁻¹ h⁻¹ for total cell counts, while activity per cell for metabolically active cells increased from 19 × 10⁻¹¹ to 85 × 10⁻¹¹ μg of C cell⁻¹ h⁻¹. No major population growth occurred at this stage. Later, with the continuous supply of DOC from the hyperscum, total bacterial numbers increased from 6.6 × 10⁶ to 20 × 10⁶ cells ml⁻¹, while the activity per cell declined. Metabolically active bacteria followed the same trend. Shorter-term DOC increases caused only increases in bacterial activity per cell. The data from Hartbeespoort Dam demonstrate an interesting and little-documented mechanism by which aquatic bacteria respond to increased DOC concentration and which may be universal for aquatic systems.     

Rate of Bacterial Mortality in Aquatic Environments

A method is proposed which provides a minimum estimate of the rate of bacterial mortality in growing natural populations of planktonic bacteria. This estimate is given by the rate of decrease of radioactivity from the DNA of a [³H]thymidine-labeled natural assemblage of bacteria after all added thymidine has been exhausted from the medium. Results obtained from river water, estuarine water, and seawater show overall bacterial mortality rates in the range 0.010 to 0.030 h⁻¹, in good agreement with the range of growth rates measured in the same environments. Use of selective filtration through Nuclepore filters (pore size, 2 μm) allowed us to determine the contribution of microzooplankton grazing to overall bacterial mortality. Grazing rates estimated by this method ranged from 0 to 0.02 h⁻¹.     

Massive Regime Shifts and High Activity of Heterotrophic Bacteria in an Ice-Covered Lake

In winter 2009/10, a sudden under-ice bloom of heterotrophic bacteria occurred in the seasonally ice-covered, temperate, deep, oligotrophic Lake Stechlin (Germany). Extraordinarily high bacterial abundance and biomass were fueled by the breakdown of a massive bloom of Aphanizomenon flos-aquae after ice formation. A reduction in light resulting from snow coverage exerted a pronounced physiological stress on the cyanobacteria. Consequently, these were rapidly colonized, leading to a sudden proliferation of attached and subsequently of free-living heterotrophic bacteria. Total bacterial protein production reached 201 µg C L⁻¹ d⁻¹, ca. five times higher than spring-peak values that year. Fluorescence in situ hybridization and denaturing gradient gel electrophoresis at high temporal resolution showed pronounced changes in bacterial community structure coinciding with changes in the physiology of the cyanobacteria. Pyrosequencing of 16S rRNA genes revealed that during breakdown of the cyanobacterial population, the diversity of attached and free-living bacterial communities were reduced to a few dominant families. Some of these were not detectable during the early stages of the cyanobacterial bloom indicating that only specific, well adapted bacterial communities can colonize senescent cyanobacteria. Our study suggests that in winter, unlike commonly postulated, carbon rather than temperature is the limiting factor for bacterial growth. Frequent phytoplankton blooms in ice-covered systems highlight the need for year-round studies of aquatic ecosystems including the winter season to correctly understand element and energy cycling through aquatic food webs, particularly the microbial loop. On a global scale, such knowledge is required to determine climate change induced alterations in carbon budgets in polar and temperate aquatic systems.     

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

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