Comparison of two direct-count techniques for enumerating aquatic bacteria

Direct Cell Mass Measurements Expand the Role of Small Microorganisms in Nature

Microbial biomass is a key parameter needed for the quantification of microbial turnover rates and their contribution to the biogeochemical element cycles. However, estimates of microbial biomass rely on empirically derived mass-to-volume relationships, and large discrepancies exist between the available empirical conversion factors. Here we report a significant nonlinear relationship between carbon mass and cell volume ([Formula: see text]; [Formula: see text]) based on direct cell mass, volume, and elemental composition measurements of 12 prokaryotic species with average volumes between 0.011 and 0.705 μm3 The carbon mass density of our measured cells ranged from 250 to 1,800 fg of C μm-3 for the measured cell volumes. Compared to other currently used models, our relationship yielded up to 300% higher carbon mass values. A compilation of our and previously published data showed that cells with larger volumes (>0.5 μm3) display a constant (carbon) mass-to-volume ratio, whereas cells with volumes below 0.5 μm3 exhibit a nonlinear increase in (carbon) mass density with decreasing volume. Small microorganisms dominate marine and freshwater bacterioplankton as well as soils and marine and terrestrial subsurface. The application of our experimentally determined conversion factors will help to quantify the true contribution of these microorganisms to ecosystem functions and global microbial biomass.IMPORTANCE Microorganisms are a major component of Earth's biosphere, and their activity significantly affects the biogeochemical cycling of bioavailable elements. To correctly determine the flux of carbon and energy in the environment, reliable estimates of microbial abundances and cellular carbon content are necessary. However, accurate assessments of cellular carbon content and dry weight are not trivial to obtain. Here we report direct measurements of cell dry and carbon mass of environmentally relevant prokaryotic microorganisms using a microfluidic mass sensor. We show a significant nonlinear relationship between carbon mass and cell volume and discuss this relationship in the light of currently used cellular mass models.

Size and Carbon Content of Sub-seafloor Microbial Cells at Landsort Deep, Baltic Sea

The discovery of a microbial ecosystem in ocean sediments has evoked interest in life under extreme energy limitation and its role in global element cycling. However, fundamental parameters such as the size and the amount of biomass of sub-seafloor microbial cells are poorly constrained. Here we determined the volume and the carbon content of microbial cells from a marine sediment drill core retrieved by the Integrated Ocean Drilling Program (IODP), Expedition 347, at Landsort Deep, Baltic Sea. To determine their shape and volume, cells were separated from the sediment matrix by multi-layer density centrifugation and visualized via epifluorescence microscopy (FM) and scanning electron microscopy (SEM). Total cell-carbon was calculated from amino acid-carbon, which was analyzed by high-performance liquid chromatography (HPLC) after cells had been purified by fluorescence-activated cell sorting (FACS). The majority of microbial cells in the sediment have coccoid or slightly elongated morphology. From the sediment surface to the deepest investigated sample (~60 m below the seafloor), the cell volume of both coccoid and elongated cells decreased by an order of magnitude from ~0.05 to 0.005 μm³. The cell-specific carbon content was 19–31 fg C cell⁻¹, which is at the lower end of previous estimates that were used for global estimates of microbial biomass. The cell-specific carbon density increased with sediment depth from about 200 to 1000 fg C μm⁻³, suggesting that cells decrease their water content and grow small cell sizes as adaptation to the long-term subsistence at very low energy availability in the deep biosphere. We present for the first time depth-related data on the cell volume and carbon content of sedimentary microbial cells buried down to 60 m below the seafloor. Our data enable estimates of volume- and biomass-specific cellular rates of energy metabolism in the deep biosphere and will improve global estimates of microbial biomass.

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-mum-pore-size Nuclepore filters to remove larger particulate materials, and diluted for growth in 0.22-mum-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 mum, 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, 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.

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 x 10 g of C mum. 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.

Frequency of dividing cells as an estimator of bacterial productivity

It has recently been proposed that the frequency of dividing bacterial cells (FDC) can be used to predict growth rates of natural aquatic bacterial assemblages. We have examined the relationship between FDC and growth rate in bacteria from southern-temperate, coastal marine waters by using incubation under conditions of manipulated nutrient availability and exclusion of bacterivores. The regression of the natural logarithm of bacterial instantaneous growth rate (mu) on FDC resulted in a better fit than regression of untransformed mu on FDC. The regression equation was ln mu = 0.299FDC - 4.961. The coefficient of variation for predicted ln mu at mean FDC was 7%. The range of FDC-estimated bacterial instantaneous generation times for coastal Georgia waters was 12 to 68 h, and range of calculated bacterial production rates was 0.6 to 17.6 mg of C.m. h. Unresolved problems of and suggested improvements on the FDC method of predicting growth rate are discussed.

Assessing biomass and production of bacteria in eutrophic lake mendota, wisconsin

Estimates were made of the biomass and production of heterotrophic bacteria in the epilimnion of Lake Mendota, Wis. Cell counts were done with epifluorescence microscopy and varied from 3 x 10 bacteria per ml in winter to 3 x 10 bacteria per ml in summer. Cell volumes were measured in scanning electron micrographs. The average cell volume was 0.159 mum. Annual variations and depth distribution were studied. Production was estimated from the frequency of dividing cells and from dark radioactive sulfate uptake. Annual productivity and daily average productivity were very close with both methods: 107 to 205 g of C per m per year for sulfate and 89 to 117 g of C per m per year for frequency of dividing cells. Zooplankton feeding removed 2 to 10% of the bacterial net production annually. When compared with biomass changes and losses due to zooplankton feeding, production values were very high. Therefore, it was suggested that other loss factors have to be more important than zooplankton feeding in controlling the bacterial population. Bacterial heterotrophic production was about 50% of gross primary production.

Comparison of light and electron microscopic determinations of the number of bacteria and algae in lake water

Determinations of the number of microorganisms in lake water samples with the bright-field light microscope were performed using conventional counting chambers. Determinations with the fluorescence microscope were carried out after staining the organisms with acridine orange and filtering them onto Nuclepore filters. For transmission electron microscopy, a water sample was concentrated by centrifugation. The pellet was solidifed in agar, fixed, dehydrated, embedded in Epon, and cut into thin sections. The number and area of organism profiles per unit area of the sections were determined. The number of organisms per unit volume of the pellet was then calculated using stereological formulae. The corresponding number in the lake water was obtained from the ratio of volume of solidified pellet/volume of water sample. Control experiments with pure cultures of bacteria and algae showed good agreement between light and electron microscopic counts. This was also true for most lake water samples, but the electron microscopic preparations from some samples contained small vibrio-like bodies and ill-defined structures that made a precise comparison more difficult. Bacteria and small blue-green and green algae could not always be differentiated with the light microscope, but this was easily done by electron microscopy. Our results show that transmission electron microscopy can be used for checking light microscopic counts of microorganisms in lake water.

Seasonal variation in population density and heterotrophic activity of attached and free-living bacteria in coastal waters

The abundance and heterotrophic activity of attached and free-living bacteria were examined seasonally in coastal water. Heterotrophic activity was determined by the uptake of [C]glucose. The density of attached bacteria was always minor, not showing a seasonal variation, whereas the free-living bacteria were more numerous and showed a marked seasonal variation, their density being higher under warmer conditions. The contribution of the attached bacteria to the total assimilation of [C]glucose (from 10 to 38%) was lower than that of the free-living bacteria, neither of them showing a seasonal variation. On a cellular basis, attached bacteria were more active, since they assimilated more [C]glucose and showed, under warmer conditions, a higher cellular volume (0.102 versus 0.047 mum). We consider that the factors responsible for these observations were the amount and quality of the particulate material, the different availability of organic matter for the two types of bacteria, and in a fundamental way, the variation in water temperature.

Comparison of two direct-count methods for determining metabolizing bacteria in freshwater

Planktonic bacteria collected from several freshwater environments and cultured bacteria were used to compare two methods for determining the numbers of metabolizing bacteria. The methods used were (i) reduction of 2-(rho-iodophenyl)-3-(rho-nitrophenyl)-5-phenyl tetrazolium chloride 2-(rho-iodophenyl)-3-(rho-nitrophenyl)-5-phenyl to tetrazolium chloride-formazan and (ii) elongation of cells by using yeast extract and nalidixic acid. No statistically significant difference was found between methods in determining metabolizing bacteria, although significant differences (P < 0.05) were found when comparing numbers of total bacteria. A combination of the two methods yielded significant changes, both positive and negative, in the numbers of metabolizing bacteria.

Seasonal bacterial production in a dimictic lake as measured by increases in cell numbers and thymidine incorporation

Rates of primary and bacterial production in Little Crooked Lake were calculated from the rates of incorporation of HCO(3) and [methyl-H]thymidine, respectively. Growth rates of bacteria in diluted natural samples were determined for epilimnetic and metalimnetic bacterial populations during the summers of 1982 and 1983. Exponential growth was observed in these diluted samples, with increases in cell numbers of 30 to 250%. No lag was observed in bacterial growth in 14 of 16 experiments. Correlation of bacterial growth rates to corresponding rates of thymidine incorporation by natural samples produced a conversion factor of 2.2 x 10 cells produced per mole of thymidine incorporated. The mass of the average bacterial cell in the lake was 1.40 x 10 +/- 0.05 x 10 g of C cell. Doubling times of natural bacteria calculated from thymidine incorporation rates and in situ cell numbers ranged from 0.35 to 12.00 days (median, 1.50 days). Bacterial production amounted to 66.7 g of C m from April through September, accounting for 29.4% of total (primary plus bacterial) production during this period. The vertical and seasonal distribution of bacterial production in Little Crooked Lake was strongly influenced by the distribution of primary production. From April through September 1983, the depth of maximum bacterial production rates in the water column was related to the depth of high rates of primary production. On a seasonal basis, primary production increased steadily from May through September, and bacterial production increased from May through August and then decreased in September.

Measurement and significance of specific activity in the heterotrophic bacteria of natural waters

It is now possible to obtain accurate total counts of the bacteria of natural waters with the use of acridine orange staining and epifluorescence microscopy. This approach can be coupled to highly sensitive measurements of heterotrophic activity using radioisotopes. To accomplish this, three variations of a "specific activity index" are suggested, based on different approaches to measuring heterotrophic activity with radiolabeled organic solutes. The denominator of each index is the direct count of bacteria from a given natural sample. Three numerators are presented, each of which has been shown to vary directly with heterotrophic bacterial activity: V(max), turnover rate, and direct uptake (at high substrate concentrations). Each approach is illustrated with data from estuarine and coastal waters of northeastern Massachusetts. The data show major differences in specific activity that accompany such habitat differences as distances within or offshore from an estuary and vertical location in the water column. These and other data suggest that specific activity is a valid indicator of the physiological state and metabolic role of the bacteria. Some evidence is presented in support of the hypothesis that the natural bacteria are adapted to conditions of nutrient starvation by becoming "dormant," existing for an unknown period of time in a reversible physiological state that reflects the availability of organic nutrients.

Microcomputer-assisted biomass determination of plankton bacteria on scanning electron micrographs

Although biovolume is a better measure of biomass than is cell number, biovolumes have rarely been measured because their evaluation is extremely time-consuming. We developed a microcomputer system that assists cell size measurements on images of filtered plankton: scanning electron micrograph negatives were projected on a digitizer field, bacterial length and width were marked by a cursor, and coordinates were directly transferred to an MOS 6502 microcomputer (KIM 1). The dialogue program BABI organized and controlled the digitizer measurements in cooperation with the user, enabled corrections, and printed out results with 95% confidence limits and sample description. The time for scanning electron micrograph preparation was reduced to 15 min (quick transfer to Freon 113 during filtration and air drying). Altogether, this biovolume determination took about 2.5 h for confidence limits of +/-15%. Examples are given for applications of the method: (i) comparison of 10 lakes (with specific activities for glucose uptake and for heterotrophic CO(2) fixation); (ii) ranges of biomass parameters in one lake; (iii) diurnal cycles (with synchronizing effects, uptake of algal exudates, and calculation of daily growth). This method is discussed in relation to other biomass methods (epifluorescent microscopy, lipopolysaccharide technique, frequency of dividing cells) and the problem of biovolume-to-carbon conversions.

Mineralogy influences structure and diversity of bacterial communities associated with geological substrata in a pristine aquifer

Our understanding of mineralogical influences on subsurface microbial community structure and diversity has been difficult to assess due to difficulties in isolating this variable from others in the subsurface environment. In this study, biofilm coupons were used to isolate specific geological substrata from the surrounding geological matrix during colonization by microorganisms suspended in the surrounding groundwater for an 8-week period. Upon retrieval, the structure and diversity of the microbial community associated with each type of substratum was evaluated using 16S rDNA-based terminal-restriction fragment length polymorphism (T-RFLP). Phylogenetic affiliations of the populations associated with each type of substratum were established based on sequence analysis of near full-length 16S rDNA obtained through construction of a clone library. Hematite, quartz, and saprolite each harbored a community dominated by members of the division Proteobacteria (>67% of community). However, the different substrata selected for different subdivisions of bacteria within the Proteobacteria. After accounting for the influence exerted by substratum type on recovery of DNA from the attached populations, both phylogenetic data and Jaccard and Bray-Curtis similarity indices derived from terminal-restriction fragment (T-RF) profiles suggested a strong mineralogical influence on the structure and composition of the solid phase-associated community. The results suggest that mineralogical heterogeneity influences microbial community structure and diversity in pristine aquifers.

Survival and recovery of viable but nonculturable Listeria monocytogenes cells in a nutritionally depleted medium

Survival of a desiccated five-strain Listeria monocytogenes mixture during storage in sand at 4 degrees C for 2 months was determined using the acridine orange direct count method with novobiocin and plate counts. Samples of inoculated sand were taken every 2 weeks, incubated at 37 degrees C for 6 h, stained with acridine orange, and then examined with a fluorescence microscope. Elongated viable but nonculturable cells were most frequently observed during weeks 2 and 4. At weeks 6 and 8, most of the cells either remained viable or were dead. In each microscopic field, only one or two viable but nonculturable cells were observed among hundreds of other viable culturable cells, indicating that L. monocytogenes does not generally become viable but nonculturable. Therefore, viable but nonculturable cells are not a concern when plating environmental samples or desiccated L. monocytogenes cells on nonselective media. Tryptic soy agar with 0.6% (wt/vol) yeast extract (TSAYE) and Columbia agar were used as nonselective plate count media. Modified Oxford agar and TSAYE + 5% (wt/vol) sodium chloride were used as the selective plate count media. The effects of aerobic or anaerobic incubation and media supplementation with 0.1% or 1% (wt/vol) sodium pyruvate were tested to optimize recovery of desiccated cells. Nonselective media showed better recovery when TSAYE and Columbia agar contained 0.1% (wt/vol) pyruvate and were incubated aerobically. These two culture methods were equally effective (P > 0.05) for recovering desiccated L. monocytogenes cells.

Absolute Emission Spectra from Bacillus subtilis and Escherichia coli Vegetative Cells in Solution

Spectrally resolved emission (270-560 nm) from dilute suspensions of washed Bacillus subtilis and Escherichia coli were measured by use of tunable laser excitation between 270 and 300 nm. Integrated absolute emission cross sections increase with decreasing excitation wavelength and range from 1.8 x 10(-12) to 6.0 x 10(-11) cm(2)/(particle sr). An emission band near 340 nm dominates all observed spectra. At each excitation wavelength spectrally resolved emissions from the E. coli and B. subtilis suspensions are indistinguishable.

Photometric application of the Gram stain method to characterize natural bacterial populations in aquatic environments

The Gram stain method was applied to the photometric characterization of aquatic bacterial populations with a charge-coupled device camera and an image analyzer. Escherichia coli and Bacillus subtilis were used as standards of typical gram-negative and gram-positive bacteria, respectively. A mounting agent to obtain clear images of Gram-stained bacteria on Nuclepore membrane filters was developed. The bacterial stainability by the Gram stain was indicated by the Gram stain index (GSI), which was applicable not only to the dichotomous classification of bacteria but also to the characterization of cell wall structure. The GSI spectra of natural bacterial populations in water with various levels of eutrophication showed a distinct profile, suggesting possible staining specificity that indicates the presence of a particular bacterial population in the aquatic environment.

Comparison of methods for detection and enumeration of airborne microorganisms collected by liquid impingement

Bacterial agents and cell components can be spread as bioaerosols, producing infections and asthmatic problems. This study compares four methods for the detection and enumeration of aerosolized bacteria collected in an AGI-30 impinger. Changes in the total and viable concentrations of Pseudomonas fluorescens in the collection fluid with respect to time of impingement were determined. Two direct microscopic methods (acridine orange and BacLight) and aerodynamic aerosol-size spectrometry (Aerosizer) were employed to measure the total bacterial cell concentrations in the impinger collection fluid and the air, respectively. These data were compared with plate counts on selective (MacConkey agar) and nonselective (Trypticase soy agar) media, and the percentages of culturable cells in the collection fluid and the bacterial injury response to the impingement process were determined'. The bacterial collection rate was found to be relatively unchanged during 60 min of impingement. The aerosol measurements indicated an increased amount of cell fragments upstream of the impinger due to continuous bacterial nebulization. Some of the bacterial clusters, present in the air upstream of the impinger, deagglomerated during impingement, thus increasing the total bacterial count by both direct microscopic methods. The BacLight staining technique was also used to determine the changes in viable bacterial concentration during the impingement process. The percentage of viable bacteria, determined as a ratio of BacLight live to total counts was only 20% after 60 min of sampling. High counts on Trypticase soy agar indicated that most of the injured cells could recover. On the other hand, the counts from the MacConkey agar were very low, indicating that most of the cells were structurally damaged in the impinger. The comparison of data on the percentage of injured bacteria obtained by the traditional plate count with the data on percentage of nonviable bacteria obtained by the BacLight method showed good agreement.

Use of fluorochromes for direct enumeration of total bacteria in environmental samples: past and present

Understanding the role of bacteria in microbial food webs is intimately connected to the methods applied in the direct enumeration of bacteria. We have examined over 220 papers describing studies in which fluorochrome staining followed by epifluorescent microscopic direct counts was used to estimate total bacterial abundances. In this review, we summarize patterns in the use of 3,6-bis[dimethylamino]acridinium chloride (acridine orange) and 4',6-diamidino-2-phenylindole (DAPI), the two stains most frequently used in bacterial enumeration. The staining of samples with these fluorochromes, followed by filtration and direct counting of bacterial cells on filter surfaces, has become routine over the past 10 years. We examine trends in features of the standard direct count methods, such as sample preservation and preparation techniques, membrane filter types used, applied stain concentrations, duration of staining, and counting strategies, in relation to the types of samples being examined. The high variability in bacterial counts observed within similar sample types may be partially accounted for by differences in methods. Synthesizing review findings, we include a recommended method for the direct enumeration of bacteria in environmental samples.

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.

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 μm 3 ) in mid-winter, larger in spring (ca. 0.076 μm 3 ), larger again in summer (up to 0.102 μm 3 ), and largest (ca. 0.133 μm 3 ) 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.

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 (CO 2 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 μm 3 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.

On the relation between dry matter and volume of bacteria

Dry matter and volumes of 337 individual bacterial cells with volumes in the range 0.01-7μm(3) from different origins were measured. An allometric relation was established between dry matter and volume, such that smaller bacteria tended to have a higher dry matter to volume ratio than larger bacteria. The results are compared to results from similar work on algae. The implications for the use of conversion factors are discussed.

Bacterial dry matter content and biomass estimations

Approximately 20% dry-matter content appears to be an accepted standard value for bacterial cells. We have found that the dry-matter content of bacteria may be more than twice as high as generally assumed. The main reason for the low estimates seems to be that proper corrections for intercellular water have not been made when estimating the wet weight of the cells. Using three different bacterial strains, we determined a dry-matter content of cells ranging from 31 to 57%, suggesting not only that the accepted standard value is much too low but also that it is far from standard. To convert bacterial biovolume into biomass (carbon content), we suggest that 0.22 g of C cm-3 should be used as a conversion factor.

Size of Suspended Bacterial Cells and Association of Heterotrophic Activity with Size Fractions of Particles in Estuarine and Coastal Waters

The size of bacteria and the size distribution of heterotrophic activity were examined in estuarine, neritic, and coastal waters. The data indicated the small size of suspended marine bacteria and the predominance of free-living cells in numerical abundance and in the incorporation of dissolved amino acids. The average per-cell volume of suspended marine bacteria in all environments was less than 0.1 μm 3 . Cell volume ranged from 0.072 to 0.096 μm 3 at salinities of 0 to 34.3‰ in the Newport River estuary, N.C., and from 0.078 to 0.096 μm 3 in diverse areas of the Gulf of Mexico. Thus, the free-living bacteria were too small to be susceptible to predation by copepods. In the Newport River estuary, ca. 93 to 99% of the total number of cells and 75 to 97% of incorporated tritium (from 3 H-labeled mixed amino acids) retained by a 0.2-μm-pore-size filter passed through a 3.0-μm-pore-size filter. Although the amino acid turnover rate per cell was higher for the bacteria in the >3.0-μm size fraction than in the <3.0-μm size fraction, the small number of bacteria associated with the >3.0-μm size particles resulted in the low relative contribution of attached bacteria to total heterotrophic activity in the estuary. For coastal and neritic samples, collected off the coast of Georgia and northeast Florida and in the plume of the Mississippi River, 56 to 98% of incorporated label passed through a 3.0-μm-pore-size filter. The greatest activity in the >3.0-μm fraction in the Georgia Bight was at nearshore stations and in the bottom samples. Our data were consistent with the hypothesis that resuspension of bottom material is an important factor in influencing the proportion of heterotrophic activity attributable to particle-associated bacteria.

Malachite green-INT (MINT) method for determining active bacteria in sewage

A membrane filtration method was developed to determine the proportion of active (respiring) bacteria at various stages of sewage treatment. Samples were incubated in the presence of 2-(p-iodophenyl)-3-(p-nitrophenyl)-5-phenyl tetrazolium chloride (INT) and, after fixation, passed through membrane filters. Filters were counterstained with malachite green and then were examined by bright-field microscopy. The contrast between bacteria and the filter background was greatly improved by drying and then clearing the filter before counterstaining. By this method, it was found that active bacterial fractions in raw sewage, settled sewage, and secondary effluent were 40, 29, and 58%, respectively, whereas the proportion of respiring bacteria in chlorinated secondary effluent was 6.1%. The active bacterial fraction of activated sludge was found to be 16%. The proposed method represents a significant improvement in speed and simplicity over existing methods for determining active bacteria in sewage.

Comparison between direct methods for determination of microbial cell volume: electron microscopy and electronic particle sizing

Size frequency distributions of different phototrophic and heterotrophic microorganisms were determined by means of scanning and transmission electron microscopy and electronic particle sizing. Statistically significant differences existed among the three techniques used in this study. Cells processed for electron microscopy showed lower mean cellular volumes than those processed for electronic particle sizing, reflecting a shrinkage by factors ranging from 1.1 to 6.2 (mean, 2.3). Processing of cells for scanning electron microscopy caused higher shrinkage than processing for transmission electron microscopy. Shrinkage was dependent neither on the size nor on the cell wall type of the microorganism. When processed for scanning electron microscopy, phototrophic bacteria were strongly shrunken, whereas heterotrophic microorganisms were less affected. A direct relationship existed among phototrophic bacteria between percentage of shrinkage and specific pigment content. This was probably a consequence of the pigment extraction by organic solvents during the dehydration process, previous to the critical point drying, necessary to examine the specimens under the scanning electron microscope.

Sampling Design and Enumeration Statistics for Bacteria Extracted from Marine Sediments

The spatial and temporal distributions of marine bacteria were studied at both a muddy and a sandy subtidal site in North Inlet, S.C. The sampling design was hierarchical, since subsampling (by a dilution series) of the sediments was necessary to count bacterial cells using acridine orange epifluoresence microscopy. The cell count data fit a log-normal distribution. The abundance of bacteria was 10 11 g −1 (dry weight) of mud and 10 9 g −1 (dry weight) of sand. Variance component analyses demonstrated that variation due to the subsampling procedures was always statistically significant. Thus the common practice of counting 20 fields from one filter preparation is inadequate for estimating the true bacterial population variance in marine sediments. It is recommended that replication of the subsampling level be performed. Standardization of data (by dry weight of sediment) decreased sampling variance at the mud site but not at the sand site, implying that bacteria are more homogeneously distributed in sand than in mud.

Dissolved Hydrocarbons and Related Microflora in a Fjordal Seaport: Sources, Sinks, Concentrations, and Kinetics

The continuous addition of toluene as a solute of treated ballast water from oil tankers into a well-defined estuary facilitated the study of the dynamics of dissolved hydrocarbon metabolism in seawater. Most rates of toluene oxidation were in the range of 1 to 30 pg/liter per h at 0.5 μg of toluene per liter. Near the ballast water injection point, a layer of warm ballast water, rich in bacteria, that was trapped below the less-dense fresh surface water was located. Toluene residence times were approximately 2 weeks in this layer, 2 years elsewhere in Port Valdez, and 2 decades in the surface water of a more oceanic receiving estuary adjacent. Mixing was adequate for a steady-state treatment which showed that 98% of the toluene was flushed from Port Valdez before metabolism and gave a steady-state concentration of 0.18 μg/liter. Total bacterial biomass from direct counts and organism size data was usually near 0.1 mg/liter, but ranged up to 0.8 mg/liter in the bacteria-rich layer. The origin of bacteria in this layer was traced to growth in oil tanker ballast during shipments. The biomass of toluene oxidizers in water samples was estimated from the average affinity of pure-culture isolates for toluene (28 liters per g of cells per h) and observed toluene oxidation kinetics. Values ranged from nearly all of the total bacterial biomass within the bacteria-rich layer down to 0.2% at points far removed. Because the population of toluene oxidizers was large with respect to the amount of toluene consumed and because water from a nearby nonpolluted estuary was equally active in facilitating toluene metabolism, we searched for an additional hydrocarbon source. It was found that terpenes could be washed from spruce trees by simulated rainfall, which suggested that riparian conifers provide an additional and significant hydrocarbon source to seawater.

Sensitive and Accurate Methodology for Measuring the Kinetics of Concentration-Dependent Hydrocarbon Metabolism Rates in Seawater by Microbial Communities

A method having sufficient sensitivity to resolve the kinetic constants for dissolved nonpolar substrate metabolism, together with the related rate constants in natural waters, is presented. The method is based on the rate of 14 CO 2 recovery from radioactive dissolved substrate. Sensitivity is enhanced by using large seawater volumes, high-specific-activity isotopes, and by reducing background radioactivity. Before use, commercial isotopes are purified by mild alkaline hydrolysis followed by sublimation from base to remove 14 CO 2 as well as interfering polar 14 C-substrates. During sample analysis, chilled Tenax resin is used to remove volatile 14 C-substrate from the nitrogen stream containing 14 CO 2 recovered from substrate oxidation. Chromatographic evidence of purity, shown to be insufficient, is augmented by kinetic data from toluene utilization by mixed cultures and by rates in induced versus noninduced pure cultures. Accuracy is enhanced by using short (<10 h) incubation times and small hydrocarbon concentrations so that the metabolism rates in unamended natural water systems can be evaluated. Toluene metabolism rates in seawater as low as 1 pg/liter per h and at concentrations as low as 20 ng/liter have been determined.

Use of nuclepore filters for counting bacteria by fluorescence microscopy

Polycarbonate Nuclepore filters are better than cellulose filters for the direct counting of bacteria because they have uniform pore size and a flat surface that retains all of the bacteria on top of the filter. Although cellulose filters also retain all of the bacteria, many are trapped inside the filter where they cannot be counted. Before use, the Nuclepore filters must be dyed with irgalan black to eliminate autofluorescence. Direct counts of bacteria in lake and ocean waters are twice as high with Nuclepore filters as with cellulose filters.