Bacterial Biovolume and Biomass Estimations

The biomass of bacterial populations in aquatic ecosystems is often estimated by measuring bacterial biovolume and converting this into biomass in terms of carbon. A reliable conversion factor relating the measured bacterial biovolume to bacterial carbon content is essential for this approach. Based on direct measurements of bacterial cell carbon content, cell number, and biovolume, I have derived an average conversion factor of 5.6 × 10⁻¹³ g of C μm⁻³. This conversion factor is 3.4 to 6.6 times higher than most theoretically derived factors currently in use. Both bacterial biomass and bacterial production in aquatic ecosystems may thus have been seriously underestimated.     

Relationships between Biovolume and Biomass of Naturally Derived Marine Bacterioplankton

Microscopic estimation of bacterial biomass requires determination of both biovolume and biovolume-to-biomass conversion. Both steps have uncertainty when applied to the very small bacteria typically found in natural seawater. In the present study, natural bacterioplankton assemblages were freshly collected, passed through 0.6-μm-pore-size Nuclepore filters to remove larger particulate materials, and diluted for growth in 0.22-μm-pore-size Millipore filter-sterilized unenriched seawater. This provided cells comparable in size and morphology to those in natural seawater, but the cultures were free of the interfering particulate detritus naturally present. Cells were collected on glass-fiber GF/F filters, and biovolumes were corrected for cells passing these filters; C and N were measured with a CHN analyzer. Our criteria for size measurement by epifluorescence photomicrography were confirmed with fluorescent microspheres of known diameters. Surprisingly, in six cultures with average per-cell biovolumes ranging from 0.036 to 0.073 μm³, the average per-cell carbon biomass was relatively constant at 20 ± 0.08 fg of C (mean ± standard error of the mean). The biovolume-to-biomass conversion factor averaged 0.38 ± 0.05 g of C cm⁻³, which is about three times higher than the value previously estimated from Escherichia coli, and decreased with increasing cell volume. The C:N ratio was 3.7 ± 0.2. We conclude that natural marine bacterial biomass and production may be higher than was previously thought and that variations in bacterial size may not reflect variations in biomass per cell.     

Determination of Bacterial Cell Dry Mass by Transmission Electron Microscopy and Densitometric Image Analysis

We applied transmission electron microscopy and densitometric image analysis to measure the cell volume (V) and dry weight (DW) of single bacterial cells. The system was applied to measure the DW of Escherichia coli DSM 613 at different growth phases and of natural bacterial assemblages of two lakes, Piburger See and Gossenköllesee. We found a functional allometric relationship between DW (in femtograms) and V (in cubic micrometers) of bacteria (DW = 435 · V^0.86); i.e., smaller bacteria had a higher ratio of DW to V than larger cells. The measured DW of E. coli cells ranged from 83 to 1,172 fg, and V ranged from 0.1 to 3.5 μm³ (n = 678). Bacterial cells from Piburger See and Gossenköllesee (n = 465) had DWs from 3 fg (V = 0.003 μm³) to 1,177 fg (V = 3.5 μm³). Between 40 and 50% of the cells had a DW of less than 20 fg. By assuming that carbon comprises 50% of the DW, the ratio of carbon content to V of individual cells varied from 466 fg of C μm⁻³ for Vs of 0.001 to 0.01 μm³ to 397 fg of C μm⁻³ (0.01 to 0.1 μm³) and 288 fg of C μm⁻³ (0.1 to 1 μm³). Exponentially growing and stationary cells of E. coli DSM 613 showed conversion factors of 254 fg of C μm⁻³ (0.1 to 1 μm³) and 211 fg of C μm⁻³ (1 to 4 μm³), respectively. Our data suggest that bacterial biomass in aquatic environments is higher and more variable than previously assumed from volume-based measurements.     

Annual Bacterioplankton Biomasses and Productivities in a Temperate West Coast Canadian Fjord

Bacterioplankton numbers, biomasses, and productivities, as well as chlorophyll a concentrations and phytoplankton productivities, were assayed from 1 March 1984 to 12 August 1985 through a 250-m-deep seawater column in Howe Sound, a temperate fjord-sound on the southern coast of British Columbia, Canada. Primary production during this 18-month period was 845 g of C m⁻². Bacterial production was assayed over this same period as 193 g of C m⁻² (thymidine incorporation) and 77 g of C m⁻² (frequency of dividing cells). Bacterial productivities per cubic meter were usually greater in the euphotic zone than in deeper aphotic water, but when integrated through the water column, approximately half of the bacterial production occurred in the deeper aphotic portion. Bacterial production occurred throughout the year, although at reduced rates in late fall and early winter; primary production almost ceased during late fall and early winter. Because of this heterotrophic bacterioplankton production was a very large portion of the microbial (bacterial plus phyto-plankton) production at this time. In mid-summer bacterial production was a small proportion of the microbial production. Because of this asynchrony in peaks and troughs of bacterial and phytoplankton production through the year, data comparison is best done over an annual cycle. On this basis the bacterial production in the Howe Sound water column was between 23 and 9% of the phytoplankton production when a bacterial C to biovolume ratio of 0.107 pg of C μm⁻³ was assumed; the corresponding values were 64 and 29% when a ratio of 0.300 pg of bacterial C μm⁻³ was assumed.     

Carbon- and Nitrogen-to-Volume Ratios of Bacterioplankton Grown under Different Nutritional Conditions

Carbon- and nitrogen-to-volume (C/V and N/V) ratios were determined for freshwater bacterial assemblages grown in lake water filtrate or in water enriched with nutrients (aqueous extract of lake seston, glucose, arginine, phosphate, or ammonium). Biovolume was measured by epifluorescence microphotography, and carbon and nitrogen biomasses were measured with a CHN analyzer. Despite large variations of nutritional conditions (i.e., the composition and concentration of the dissolved organic carbon) and different mean cell sizes of the bacterial assemblage (0.17 to 1.8 μm³ per cell), the C/V, N/V, and carbon-to-nitrogen weight ratios varied little (C/V ratio, 0.14 pg of C per μm³ [standard deviation, 0.057; n = 15]; N/V ratio, 0.027 pg of N per μm³ [standard deviation; 0.011, n = 15]; carbon-to-nitrogen weight ratio, 5.6 [standard deviation, 2.2, n = 15]). An average C/V ratio of 0.12 pg of C per μm³ that was derived from natural and cultured bacterial assemblages is proposed as an appropriate conversion factor for estimation of the biomass of freshwater bacteria.     

Effects of Light on the Microcystin Content of Microcystis Strain PCC 7806

Many cyanobacteria produce microcystins, hepatotoxic cyclic heptapeptides that can affect animals and humans. The effects of photosynthetically active radiation (PAR) on microcystin production by Microcystis strain PCC 7806 were studied in continuous cultures. Microcystis strain PCC 7806 was grown under PAR intensities between 10 and 403 μmol of photons m⁻² s⁻¹ on a light-dark rhythm of 12 h -12 h. The microcystin concentration per cell, per unit biovolume and protein, was estimated under steady-state and transient-state conditions and on a diurnal timescale. The cellular microcystin content varied between 34.5 and 81.4 fg cell⁻¹ and was significantly positively correlated with growth rate under PAR-limited growth but not under PAR-saturated growth. Microcystin production and PAR showed a significant positive correlation under PAR-limited growth and a significant negative correlation under PAR-saturated growth. The microcystin concentration, as a ratio with respect to biovolume and protein, correlated neither with growth rate nor with PAR. Adaptation of microcystin production to a higher irradiance during transient states lasted for 5 days. During the period of illumination at a PAR of 10 and 40 μmol of photons m⁻² s⁻¹, the intracellular microcystin content increased to values 10 to 20% higher than those at the end of the dark period. Extracellular (dissolved) microcystin concentrations were 20 times higher at 40 μmol of photons m⁻² s⁻¹ than at 10 μmol of photons m⁻² s⁻¹ and did not change significantly during the light-dark cycles at both irradiances. In summary, our results showed a positive effect of PAR on microcystin production and content of Microcystis strain PCC 7806 up to the point where the maximum growth rate is reached, while at higher irradiances the microcystin production is inhibited.     

Experimental Evaluation of Conversion Factors for the [3H]Thymidine Incorporation Assay of Bacterial Secondary Productivity

The relationship between bacterial growth and incorporation of [methyl-³H]thymidine in oligotrophic lake water cultures was investigated. Prescreening, dilution, and addition of organic and inorganic nutrients were treatments used to prevent bacterivory and stimulate bacterial growth. Growth in unmanipulated samples was estimated through separate measurements of grazing losses. Both bacterial number and biovolume growth responses were measured, and incorporation of [³H]thymidine in both total macromolecules and nucleic acids was assayed. The treatments had significant effects on conversion factors used to relate thymidine incorporation to bacterial growth. Cell number-based factors ranged from 1.1 × 10¹⁸ to 38 × 10¹⁸ cells mol of total thymidine incorporation⁻¹ and varied with treatment up to 10-fold for the same initial bacterial assemblage. In contrast, cell biovolume-based conversion factors were similar for two treatment groups across a 16-fold range of [³H]thymidine incorporation rates: 5.54 × 10¹⁷ μm³ mol of total thymidine incorporation⁻¹ and 15.2 × 10¹⁷ μm³ mol of nucleic acid incorporation⁻¹. Much of the variation in cell number-based conversion factors was related to changes in apparent mean cell volume of produced bacteria. Phosphorus addition stimulated [³H]thymidine incorporation more than it increased bacterial growth, which resulted in low conversion factors.     

Annual Cycle of Bacterial Specific Biovolumes in Howe Sound, a Canadian West Coast Fjord Sound

The mean specific biovolumes (biovolume cell⁻¹) of the bacterioplankton within a 250-m-deep water column in Howe Sound, British Columbia, were determined for the period of 4 September 1984 to 23 October 1985. These bacteria had an annual cycle in mean specific biovolume; they were small (ca. 0.058 μm³) in mid-winter, larger in spring (ca. 0.076 μm³), larger again in summer (up to 0.102 μm³), and largest (ca. 0.133 μm³) in early fall (immediately after the decrease in phytoplankton production). The mean specific biovolumes changed coincidently through the water column with time, although the larger bacterioplankton tended to occur in the surface and deepest water. Although the mean specific biovolumes correlated better with in situ temperature (r = 0.65, a = 0.01) than with in situ chlorophyll a concentration (r = 0.34, a = 0.25), modeling experiments with batch cultures of the dinoflagellate Prorocentrum minimum (Pavillard) and the green alga Dunaliella tertiolecta (Butcher) indicated that the biomass and physiological condition of the phytoplankters may be more important than temperature in determining these bacterial specific biovolumes.     

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.     

Fast Fluorescence Quenching from Isolated Guard Cell Chloroplasts of Vicia faba Is Induced by Blue Light and Not by Red Light

Chlorophyll a fluorescence transients from isolated Vicia faba guard cell chloroplasts were used to probe the response of these organelles to light quality. Guard cell chloroplasts were isolated from protoplasts by passing them through a 10-μm nylon net. Intact chloroplasts were purified on a Percoll gradient. Chlorophyll a fluorescence transients induced by actinic red or blue light were measured with a fluorometer equipped with a measuring beam. Actinic red light induced a monophasic quenching, and transients induced by blue light showed biphasic kinetics having a slow and a fast component. The difference between the red and blue light-induced transients could be observed over a range of fluence rates tested (200-800 μmol m⁻² s⁻¹). The threshold fluence rate of blue light for the induction of the fast component of quenching was 200 μmol m⁻² s⁻¹, but in the presence of saturating red light, fluence rates as low as 25 μmol m⁻² s⁻¹ induced the fast quenching. These results indicate that guard cell chloroplasts have a specific response to blue light.     

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).     

Change in Size of Chromatium minus Cells in Relation to Growth Rate, Sulfur Content, and Photosynthetic Activity: A Comparison of Pure Cultures and Field Populations

The size frequency distribution of planktonic cells of purple sulfur phototrophic bacteria was measured at several depths in a bacterial layer of Lake Cisó (Spain). The bacterioplankton was dominated by Chromatium minus (87 to 94% of the total biomass). The largest cells of C. minus were found in the top part of the bacterial layer. In addition, the in situ and potential specific photosynthetic activity (CO₂ fixation and acetate uptake) and specific pigment content were measured in relation to several key environmental parameters that determine the activity of cells. Potential growth rates were estimated from production rates and biomass. A maximal specific growth rate of 0.074 h⁻¹ was found for the top part of the bacterial layer. Photosynthesis versus light and versus sulfide curves among field samples indicated that light was the main limiting factor controlling the activity of C. minus in Lake Cisó. The specific bacteriochlorophyll a content was very high in all samples (0.27 to 0.36 μg μg of C⁻¹). Results of laboratory experiments performed with pure cultures indicated that the average cell volume changes from 5.9 to 20.0 μm³ and that differences in growth rate, breakdown, or synthesis of sulfur and glycogen and degradation of the photosynthetic apparatus are the main factors accounting for the observed changes in cell volume across the bacterial layer.     

Carbon and Nitrogen Content of Natural Planktonic Bacteria

A method of estimating carbon and nitrogen content per unit of natural bacterial cell volume was developed. This method is based on the difference in the retentiveness of bacteria between two kinds of glass fiber filter, GF/C and GF/F (Whatman, Inc., Clifton, N.J.). Biovolume and biomass (carbon and nitrogen content) of bacteria which passed through the GF/C but not the GF/F filter were estimated with an epifluorescence microscopy and a CHN analyzer, respectively. From seasonal determinations of natural planktonic bacteria in epilimnetic waters of a mesotrophic lake, the conversion factors of 106 fg of C/μm³ and 25 fg of N/μm³ were derived as average values. By using these values, the contribution of bacteria to the biomass of lake plankton is discussed.     

Kinetic Parameters of Denitrification in a River Continuum

Kinetic parameters for nitrate reduction in intact sediment cores were investigated by using the acetylene blockage method at five sites along the Swale-Ouse river system in northeastern England, including a highly polluted tributary, R. Wiske. The denitrification rate in sediment containing added nitrate exhibited a Michaelis-Menten-type curve. The concentration of nitrate for half-maximal activity (K_map) by denitrifying bacteria increased on passing downstream from 13.1 to 90.4 μM in the main river, but it was highest (640 μM) in the Wiske. The apparent maximal rate (V_maxap) ranged between 35.8 and 324 μmol of N m⁻² h⁻¹ in the Swale-Ouse (increasing upstream to downstream), but it was highest in the Wiske (1,194 μmol N m⁻² h⁻¹). A study of nitrous oxide (N₂O) production at the same time showed that rates ranged from below the detection limit (0.05 μmol of N₂O-N m⁻² h⁻¹) at the headwater site to 27 μmol of N₂O-N m⁻² h⁻¹ at the downstream site. In the Wiske the rate was up to 570 μmol of N₂O-N m⁻² h⁻¹, accounting for up to 80% of total N gas production.     

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.     

Estimation of the Yield Coefficient of Pseudomonas sp. Strain DP-4 with a Low Substrate (2,4-Dichlorophenol [DCP]) Concentration in a Mineral Medium from Which Uncharacterized Organic Compounds Were Eliminated by a Non-DCP-Degrading Organism

The yield coefficient (YC) of Pseudomonas sp. strain DP-4, a 2,4-dichlorophenol (DCP)-degrading organism, was estimated from the number of CFU produced at the expense of 1 unit amount of DCP at low concentrations. At a low concentration of DCP, the YC can be overestimated in pure culture, because DP-4 assimilated not only DCP but also uncharacterized organic compounds contaminating a mineral salt medium. The concentration of these uncharacterized organic compounds was nutritionally equivalent to 0.7 μg of DCP-C ml⁻¹. A mixed culture with non-DCP-degrading organisms resulted in elimination of ca. 99.9% of the uncharacterized organic compounds, and then DP-4 assimilated only DCP as a substrate. In a mixed culture, DP-4 degraded an initial concentration of 0.1 to 10 μg of C ml of DCP⁻¹ and the number of CFU of DP-4 increased. In the mixed culture, DCP at an initial concentration of 0.07 μg of C ml⁻¹ was degraded. However, the number of CFU of DP-4 did not increase. DCP at an extremely low initial concentration of 0.01 μg of C ml⁻¹ was not degraded in mixed culture even by a high density, 10⁵ CFU ml⁻¹, of DP-4. When glucose was added to this mixed culture to a final concentration of 1 μg of C ml⁻¹, the initial concentration of 0.01 μg of C ml of DCP⁻¹ was degraded. These results suggested that DP-4 required cosubstrates to degrade DCP at an extremely low initial concentration of 0.01 μg of C ml⁻¹. The YCs of DP-4 at the expense of DCP alone decreased discontinuously with the decrease of the initial concentration of DCP, i.e., 1.5, 0.19, or 0 CFU per pg of DCP-C when 0.7 to 10, 0.1 to 0.5, or 0.07 μg of C ml of DCP⁻¹ was degraded, respectively. In this study, we developed a new method to eliminate uncharacterized organic compounds, and we estimated the YC of DP-4 at the expense of DCP as a sole source of carbon.     

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

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

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.     

Extracellular Polymeric Substance Architecture Influences Natural Genetic Transformation of Acinetobacter baylyi in Biofilms

Genetic exchange by natural transformation is an important mechanism of horizontal gene transfer in biofilms. Thirty-two biofilm metrics were quantified in a heavily encapsulated Acinetobacter baylyi strain and a miniencapsulated mutant strain, accounting for cellular architecture, extracellular polymeric substances (EPS) architecture, and their combined biofilm architecture. In general, transformation location, abundance, and frequency were more closely correlated to EPS architecture than to cellular or combined architecture. Transformation frequency and transformant location had the greatest correlation with the EPS metric surface area-to-biovolume ratio. Transformation frequency peaked when EPS surface area-to-biovolume ratio was greater than 3 μm²/μm³ and less than 5 μm²/μm³. Transformant location shifted toward the biofilm-bulk fluid interface as the EPS surface area-to-biovolume ratio increased. Transformant biovolume was most closely correlated with EPS biovolume and peaked when transformation occurred in close proximity to the substratum. This study demonstrates that biofilm architecture influences A. baylyi transformation frequency and transformant location and abundance. The major role of EPS may be to facilitate the binding and stabilization of plasmid DNA for cellular uptake.     

Vanadate inhibits blue light-stimulated swelling of vicia guard cell protoplasts

When supplied under low chloride concentrations, vanadate inhibits the blue light-stimulated swelling of Vicia faba L. guard cell protoplasts in a dose-dependent fashion. The volume of guard cell protoplasts incubated in 10 mm K-imino-diacetic acid, 0.4 m mannitol, and 1 mm CaCl₂ remained essentially constant under 1000 μmol m⁻² s⁻¹ red light, but increased an average of 27% after 8 min of the addition of 50 μmol m⁻² s⁻¹ blue light to the background red light. At 500 μm, vanadate completely inhibits the response to blue light. Vanadate also inhibits the swelling of guard cell protoplasts stimulated by the H⁺-ATPase agonist fusicoccin. The vanadate sensitivity of the blue light-stimulated swelling implicates a proton-pumping ATPase as a component of the sensory transduction of blue light in guard cells.