Regulation of Vacuolar pH of Plant Cells: I. Isolation and Properties of Vacuoles Suitable for P NMR Studies

For the first time, the ³¹P nuclear magnetic resonance technique has been used to study the properties of isolated vacuoles of plant cells, namely the vacuolar pH and the inorganic phosphate content. Catharanthus roseus cells incubated for 15 hours on a culture medium enriched with 10 millimolar inorganic phosphate accumulated large amounts of inorganic phosphate in their vacuoles. Vacuolar phosphate ions were largely retained in the vacuoles when protoplasts were prepared from the cells and vacuoles isolated from the protoplasts. Vacuolar inorganic phosphate concentrations up to 150 millimolar were routinely obtained. Suspensions prepared with 2 to 3 × 10⁶ vacuoles per milliliter from the enriched C. roseus cells have an internal pH value of 5.50 ± 0.06 and a mean trans-tonoplast ΔpH of 1.56 ± 0.07. Reliable determinations of vacuolar and external pH could be made by using accumulation times as low as 2 minutes. These conditions are suitable to follow the kinetics of H⁺ exchanges at the tonoplast. The ³¹P nuclear magnetic resonance technique also offered the possibility of monitoring simultaneously the stability of the trans-tonoplast pH and phosphate gradients. Both appeared to be reasonably stable over several hours. The buffering capacity of the vacuolar sap around pH 5.5 has been estimated by several procedures to be 36 ± 2 microequivalents per milliliter per pH unit. The increase of the buffering capacity due to the accumulation of phosphate in the vacuoles is, in large part, compensated by a decrease of the intravacuolar malate content.     

Procedure to Isolate Viable Sperm Cells from Corn (Zea mays L.) Pollen Grains

Sperm cells were isolated from corn (Zea mays L.) tricellular pollen grains. They were released using a light osmotic chock, and separated from pollen contaminants (especially starch grains) by a Percoll gradient centrifugation. Isolated sperm cells (3 × 10⁶ per milliliter) show a high viability score (90%) as demonstrated with the fluorochromatic reaction. They appeared as spherical cells which lack cell wall and plastids, and can be considered as haploid protoplasts.     

Useful Host-Vector Systems in Bacillus stearothermophilus

We isolated a highly transformable thermophile, Bacillus stearothermophilus SIC1, which exhibited the following features. The growth temperature ranged from 45 to 65°C in L broth. The maximum cell concentration in 2L broth (2% tryptone, 1% yeast extract, 0.5% NaCl, pH 7.2) was determined as an optical density at 660 nm of 7.8, and the generation time was 11 min at 60°C. Strain SIC1 was a prototroph and was transformed by the protoplast procedure not only with repB plasmids (high-copy-number plasmids such as pTB913 and pUB110) but also with repA plasmids (low-copy-number plasmids such as pTB53). Transformation efficiencies with repB and repA plasmids were about 2 × 10⁶ to 5 × 10⁶ and 5 × 10⁴ transformants per μg of DNA, respectively. The transformant carrying plasmid pTB913Y/K could grow at 63°C in the presence of kanamycin. The regeneration frequency of protoplasts was 60%, and only 1 day was needed for regeneration at 55°C.     

Regeneration of Transgenic Soybean (Glycine max) Plants from Electroporated Protoplasts

Transgenic soybean (Glycine max [L.] Merr.) plants were regenerated from calli derived from protoplasts electroporated with plasmid DNA-carrying genes for a selectable marker, neomycin phosphotransferase (NPTII), under the control of the cauliflower mosaic virus 35-Svedberg unit promoter, linked with a nonselectable mannityl opine synthesis marker. Following electroporation and culture, the protoplast-derived colonies were subjected to kanamycin selection (50 micrograms per milliliter) beginning on day 15 for 6 weeks. Approximately, 370 to 460 resistant colonies were recovered from 1 × 10⁶ electroporated protoplasts, giving an absolute transformation frequency of 3.7 to 4.6 × 10⁻⁴. More than 80% of the kanamycin-resistant colonies showed NPTII activity, and about 90% of these also synthesized opines. This indicates that the linked marker genes were co-introduced and co-expressed at a very high frequency. Plants were regenerated from the transformed cell lines. Southern blot analysis of the transformed callus and leaf DNA demonstrated the integration of both genes. Single-plant assays performed with different plant parts showed that both shoot and root tissues express NPTII activity and accumulate opines. Experiments with NPTII and mannityl opine synthesis marker genes on separate plasmids resulted in a co-expression rate of 66%. These results indicate that electroporation can be used to introduce both linked and unlinked genes into the soybean to produce transformed plants.     

Improved Cytoplasmic Delivery to Plant Protoplasts via pH-Sensitive Liposomes

We demonstrated that the liposomes composed of dioleolylphosphatidylethanolamine/cholesterol/oleic acid (4:4:2) dramatically release their contents at a pH of less than or equal to 6.0 and are capable of delivering their contents into the cytoplasm of higher plant protoplasts. This is shown by using a soluble fluorescent dye, calcein, as a liposome-entrapped marker. We found that calcein fluorescence was evenly distributed in the cytoplasm of wild carrot protoplasts after the incubation of protoplasts with liposomes in the presence of polyethylene glycol 6000. At 0.45 micro mole phospholipid per 6 × 10⁵ protoplast, for example, the percentage of protoplasts which took up liposomes was 89% which was much higher than that achieved by conventional pH-insensitive liposomes. In this study, liposomes were prepared by a detergent dialysis method which avoided sonication and organic solvents. Thus macromolecules such as proteins and nucleic acids could be entrapped in the liposomes and delivered to the cytoplasm of the protoplasts.     

Chlorine Demand and Inactivation of Fungal Propagules

Conidia of filamentous fungi, vegetative yeast cells, and coliform bacteria were tested to determine their chlorine demand and their sensitivity to chlorine inactivation. Levels of chlorine demand for the various conidia, yeast, and coliforms were, respectively, 3.6 × 10⁻⁹ to 3.2 × 10⁻⁸, 1.2 × 10⁻⁹ to 8.0 × 10⁻⁹, and 2.5 × 10⁻¹¹ to 6.3 × 10⁻¹⁰ mg of chlorine per propagule. Preliminary evidence suggests that the chlorine demand per propagule increases as the number of propagules per milliliter decreases. In general, conidia showed greatest resistance to chlorine inactiviation, followed by the yeast and coliforms. Inactivation by chlorine was influenced by pH, with inactivation (chlorine activity) falling in the order pH 5 > 7 > 8.     

Effect of coliform and Proteus bacteria on growth of Staphylococcus aureus

Cultures of coliform and Proteus bacteria, mostly from foods, were tested for their effect on growth of Staphylococcus aureus in Trypticase Soy Broth. Inhibition of the staphylococcus by these competitors increased with increasing proportions of inhibiting (effector) bacteria in the inoculum and decreasing incubation temperatures (37 to 10 C). Time required for 2 × 10⁴ staphylococci to increase to 5 × 10⁶ cells per milliliter, the minimal number assumed to be necessary for food poisoning, varied with the species of effector, the original ratio of effector bacteria to staphylococci in the medium, and the incubation temperature. When the original ratio was 100:1, the staphylococci did not reach 5 × 10⁶ cells per milliliter at 10, 15, 22, or 30 C (with one exception), when growing with cultures representing six species of coliform bacteria and two of Proteus. When the ratio was 1:1, all effectors either greatly delayed the staphylococcus or prevented it from reaching hazardous numbers at 15 C, six of the eight caused a delay of 2 to 3 hr at 22 C, and only Escherichia coli delayed the coccus at 30 C. All effectors were ineffective at 22 and 30 C when original numbers of effectors and staphylococci were in the ratio 1:100. Greatest overall inhibition was by E. coli, E. freundii, and Proteus vulgaris, and these species were more effective than the others at 22 and 30 C. Aerobacter cloacae and Paracolobactrum aerogenoides were more effective at 15 C. In general, results were similar with different strains of a species. Except for Aerobacter aerogenes, Klebsiella sp., and P. aerogenoides, which apparently did not compete for nutrients, inhibition of the staphylococcus was by a combination of antibiotic substances and competition for nutrients.     

Evaluation of selectable markers for obtaining stable transformants in the gramineae

Cell suspension cultures of Triticum monococcum, Panicum maximum, Saccharum officinarum, Pennisetum americanum, and a double cross trispecific hybrid between Pennisetum americanum, P. purpureum, and P. squamulatum were tested for resistance to kanamycin, hygromycin, and methotrexate for use in transformation studies. All cultures showed high natural levels of resistance to kanamycin, in excess of 800 milligrams per liter, and variable levels of resistance to hygromycin. Methotrexate was a potent growth inhibitor at low concentrations with all species. Kanamycin and hygromycin were growth inhibitory only if added early (within 5 days after protoplast isolation and culture). Protoplasts of T. monococcum, P. maximum, S. officinarum, and the tri-specific hybrid were electroporated with plasmid DNA containing hygromycin (pMON410), kanamycin (pMON273), or methotrexate (pMON806) resistance genes. Resistant colonies were obtained at low frequencies (1 × 10⁻⁵ to 2 × 10⁻⁶) when selected under conditions which were growth inhibitory to protoplasts electroporated without DNA. Southern blot hybridization confirmed stable integration of plasmid DNA into T. monococcum using hygromycin vectors and P. maximum using the methotrexate vector with 1 to 10 copies integrated per haploid genome.     

Inhibition of glucose phosphate isomerase by metabolic intermediates of fructose

1. Purified rabbit-muscle and -liver glucose phosphate isomerase, free of contaminating enzyme activities that could interfere with the assay procedures, were tested for inhibition by fructose, fructose 1-phosphate and fructose 1,6-diphosphate. 2. Fructose 1-phosphate and fructose 1,6-diphosphate are both competitive with fructose 6-phosphate in the enzymic reaction, the apparent K_i values being 1·37×10⁻³−1·67×10⁻³m for fructose 1-phosphate and 7·2×10⁻³−7·9×10⁻³m for fructose 1,6-diphosphate; fructose and inorganic phosphate were without effect. 3. The apparent K_m values for both liver and muscle enzymes at pH7·4 and 30° were 1·11×10⁻⁴−1·29×10⁻⁴m for fructose 6-phosphate, determined under the conditions in this paper. 4. In the reverse reaction, fructose, fructose 1-phosphate and fructose 1,6-diphosphate did not significantly inhibit the conversion of glucose 6-phosphate into fructose 6-phosphate. 5. The apparent K_m values for glucose 6-phosphate were in the range 5·6×10⁻⁴−8·5×10⁻⁴m. 6. The competitive inhibition of hepatic glucose phosphate isomerase by fructose 1-phosphate is discussed in relation to the mechanism of fructose-induced hypoglycaemia in hereditary fructose intolerance.     

Interactions between Bacillus thuringiensis subsp. israelensis and Fathead Minnows, Pimephales promelas Rafinesque, under Laboratory Conditions

Interactions between Bacillus thuringiensis subsp. israelensis and fathead minnows, Pimephales promelas, were studied in laboratory exposures to two commercial formulations, Vectobac-G and Mosquito Attack. Mortality among fatheads exposed to 2.0 × 10⁶ to 6.5 × 10⁶ CFU/ml with both formulations was attributed to severe dissolved oxygen depletion due to formulation ingredients rather than to direct toxicity from the parasporal crystal. No adverse effects were observed at 6.4 × 10⁵ CFU/ml and below. Fathead minnows rapidly accumulated high numbers of spores with 1 h of exposure to 2.2 × 10⁵ CFU of Mosquito Attack per ml, producing whole-body counts of 4.0 × 10⁶ CFU per fish. Comparison of counts on gastrointestinal tract samples and whole-body samples and high numbers of spores in feces indicated that ingestion was the major route of exposure. B. thuringiensis subsp. israelensis spore counts decreased rapidly after transfer of fish to clean water, with a drop of over 3 orders of magnitude in 1 day. Spores were rarely detected in fish after 8 days but were detectable in feces for over 2 weeks. These findings suggest that fish could influence the dissemination of B. thuringiensis subsp. israelensis, and possibly other microbial agents, in the aquatic environment.     

High-Frequency Phage-Mediated Gene Transfer among Escherichia coli Cells, Determined at the Single-Cell Level

Recent whole-genome analysis suggests that lateral gene transfer by bacteriophages has contributed significantly to the genetic diversity of bacteria. To accurately determine the frequency of phage-mediated gene transfer, we employed cycling primed in situ amplification-fluorescent in situ hybridization (CPRINS-FISH) and investigated the movement of the ampicillin resistance gene among Escherichia coli cells mediated by phage at the single-cell level. Phages P1 and T4 and the newly isolated E. coli phage EC10 were used as vectors. The transduction frequencies determined by conventional plating were 3 × 10⁻⁸ to 2 × 10⁻⁶, 1 × 10⁻⁸ to 4 × 10⁻⁸, and <4 × 10⁻⁹ to 4 × 10⁻⁸ per PFU for phages P1, T4, and EC10, respectively. The frequencies of DNA transfer determined by CPRINS-FISH were 7 × 10⁻⁴ to 1 × 10⁻³, 9 × 10⁻⁴ to 3 × 10⁻³, and 5 × 10⁻⁴ to 4 × 10⁻³ for phages P1, T4, and EC10, respectively. Direct viable counting combined with CPRINS-FISH revealed that more than 20% of the cells carrying the transferred gene retained their viabilities. These results revealed that the difference in the number of viable cells carrying the transferred gene and the number of cells capable of growth on the selective medium was 3 to 4 orders of magnitude, indicating that phage-mediated exchange of DNA sequences among bacteria occurs with unexpectedly high frequency.     

Ultrasound-mediated DNA transfer for bacteria

In environmental microbiology, the most commonly used methods of bacterial DNA transfer are conjugation and electroporation. However, conjugation requires physical contact and cell–pilus–cell interactions; electroporation requires low-ionic strength medium and high voltage. These limitations have hampered broad applications of bacterial DNA delivery. We have employed a standard low frequency 40 kHz ultrasound bath to successfully transfer plasmid pBBR1MCS2 into Pseudomonas putida UWC1, Escherichia coli DH5α and Pseudomonas fluorescens SBW25 with high efficiency. Under optimal conditions: ultrasound exposure time of 10 s, 50 mM CaCl₂, temperature of 22°C, plasmid concentration of 0.8 ng/µl, P. putida UWC1 cell concentration of 2.5 × 10⁹ CFU (colony forming unit)/ml and reaction volume of 500 µl, the efficiency of ultrasound DNA delivery (UDD) was 9.8 ± 2.3 × 10⁻⁶ transformants per cell, which was nine times more efficient than conjugation, and even four times greater than electroporation. We have also transferred pBBR1MCS2 into E. coli DH5α and P. fluorescens SBW25 with efficiencies of 1.16 ± 0.13 × 10⁻⁶ and 4.33 ± 0.78 × 10⁻⁶ transformants per cell, respectively. Low frequency UDD can be readily scaled up, allowing for the application of UDD not only in laboratory conditions but also on an industrial scale.     

Genetic variation at the FADS1-FADS2 gene locus influences delta-5 desaturase activity and LC-PUFA proportions after fish oil supplement

Delta-5 and delta-6 desaturases (D5D and D6D) are key enzymes in endogenous synthesis of long-chain PUFAs. In this sample of healthy subjects (n = 310), genotypes of single nucleotide polymorphisms (SNPs) rs174537, rs174561, and rs3834458 in the FADS1-FADS2 gene cluster were strongly associated with proportions of LC-PUFAs and desaturase activities estimated in plasma and erythrocytes. Minor allele carriage associated with decreased activities of D5D (FADS1) (5.84 × 10⁻¹⁹ ≤ P ≤ 4.5 × 10⁻¹⁸) and D6D (FADS2) (6.05 × 10⁻⁸ ≤ P ≤ 4.20 × 10⁻⁷) was accompanied by increased substrate and decreased product proportions (0.05 ≤ P ≤ 2.49 × 10⁻¹⁶). The significance of haplotype association with D5D activity (P = 2.19 × 10⁻¹⁷) was comparable to that of single SNPs, but haplotype association with D6D activity (P = 3.39 × 10⁻²⁸) was much stronger. In a randomized controlled dietary intervention, increasing eicosapentaenoic acid (EPA, 20:5n-3) and docosahexaenoic acid (DHA, 22:6n-3) intake significantly increased D5D (P = 4.0 × 10⁻⁹) and decreased D6D activity (P = 9.16 × 10⁻⁶) after doses of 0.45, 0.9, and 1.8 g/day for six months. Interaction of rs174537 genotype with treatment was a determinant of D5D activity estimated in plasma (P = 0.05). In conclusion, different sites at the FADS1-FADS2 locus appear to influence D5D and D6D activity, and rs174537 genotype interacts with dietary EPA+DHA to modulate D5D.     

Simultaneous Measurement of NH(4) Absorption and N(2) Fixation by Glycine max L. : RESPONSE TO TEMPERATURE, pH, AND EXTERNAL NITROGEN CONCENTRATION

Curvature, bending moment, and second moment of stem cross-sectional area were evaluated from photographic data and used to compute flexural rigidity and Young's modulus in the panicle rachis of rice, Oryza sativa L. `M-101.' Flexural rigidity C, and its components E, Young's modulus, and I, the moment of inertia of the area about the neutral axis, were evaluated 1.5 cm (tip), 9.5 cm (mid), and 16.5 cm (base) from the tip of the panicle rachis. In dynes per square centimeter, C increases from 1.1 × 10³ near the tip to 1.09 × 10⁴ in the middle to 5.35 × 10⁴ in the basal region of the rachis. Of the components of C, the I changes have the larger effect, increasing from 2.12 × 10⁻⁷ centimeters⁴ near the tip to 8.21 × 10⁻⁷ centimeters⁴ in mid regions to 6.0 × 10⁻⁶ centimeters⁴ in the basal regions. Young's modulus increases from 4.8 × 10⁹ dynes per square centimeter near the tip to 1.4 × 10¹⁰ dynes per square centimeter in mid regions then falls to 7.4 × 10⁹ dynes per square centimeter near the base of the main stem. Values of Young's modulus from Instron experiments were in satisfactory agreement with values calculated from the beam bending equation. Flexural rigidity in the curved region of the panicle proved independent of panicle load, indicating that the dissected panicle rachis behaves in some respects as a tapered loaded beam.     

Bacterial and Fungal Numbers in Ruminal and Cecal Contents of the Blue Duiker (Cephalophus monticola)

Total and cellulolytic bacterial and fungal numbers were determined in ruminal and cecal contents of 20 blue duikers (Cephalophus monticola). The animals were equally divided by sex and fed two diets, either high roughage or high concentrate. The mean concentration for total bacterial numbers in the rumen was 26.0 × 10⁸/g of contents, with values ranging from 2 × 10⁸/g to 93 × 10⁸/g. Cellulolytic numbers averaged 6.0 × 10⁸/g with a range of 1.5 × 10⁸/g to 24.0 × 10⁸/g. No differences related to sex or diet were found. In contrast, total bacterial numbers in the cecum differed between diets (P < 0.02), i.e., 1,046 × 10⁶ bacteria per g for animals fed the high-forage diet compared with 166 × 10⁶/g for those fed the high-concentrate diet. Cellulolytic bacterial counts in the cecal contents averaged 3.1 and 7.0% of the total counts for the high-forage and high-concentrate diets, respectively. Low concentrations of fungi were found in both ruminal and cecal contents of some, but not all, animals. Unexpectedly, concentrations of bacteria and fungi in the rumen and cecum were highly correlated with their total numbers (concentration multiplied by total weight of contents).     

Isolation and transport properties of protoplasts from cortical cells of corn roots

A procedure was developed for the enzymic isolation of large quantities of protoplasts from the cortex of Zea mays L. WF9 × MO 17 roots. Cortex was separated from the primary root, sectioned, and the cell walls digested for 3.5 hours in 2% (w/v) Cellulysin, 0.1% Pectolyase Y-23, 1 millimolar CaCl₂, 0.05% bovine serum albumin, 0.5 millimolar dithiothreitol in 0.6 molar mannitol (pH 5.6). Cortical cell protoplasts were collected by centrifugation and purified by flotation in a Ficoll step gradient. The yield of protoplasts was approximately 650 × 10³/gram fresh tissue. To obtain maximum yield it was essential to include an effective pectinase (Pectolyase Y-23) and protectants (bovine serum albumin and dithiothreitol) in the digestion medium.

Cortical cell protoplasts exhibited energy-dependent uptake of K⁺ (⁸⁶Rb), H₂³²PO₄⁻, and ³⁶Cl⁻ as well as net H⁺ extrusion. Ion fluxes were sustained for at least 3 hours. Influx of K⁺ was highest between pH 7.5 and 8.0, whereas the influx of H₂PO₄⁻ was greatest between pH 4.0 and 5.0. K⁺ and H₂PO₄⁻ influx and net H⁺ efflux were inhibited by respiratory poisons such as cyanide (0.1 millimolar) and oligomycin (5 micrograms per milliliter), and by inhibitors of plasma membrane ATPase such as diethylstilbestrol (50 micromolar). Calculated flux for Cl⁻ was low, but not greatly different from that observed for other plant cells. K⁺ flux was somewhat high, probably because the K⁺ concentration in the cortical cells was below steady-state. The results indicate that isolated cortical cell protoplasts retain transport properties which are similar to those of root tissue.

     

Plasmid Transfer into the Homoacetogen Acetobacterium woodii by Electroporation and Conjugation

Shuttle vectors (pMS3 and pMS4) which replicated in Escherichia coli and in gram-positive Acetobacterium woodii were constructed by ligating the replication origin of plasmid pAMβ1 with the E. coli cloning vector pUC19 and the tetM gene of streptococcal transposon Tn916. Electrotransformation of A. woodii was achieved at frequencies of 4.5 × 10³ transformants per μg of plasmid DNA. For conjugal plasmid transfer, the mobilizable shuttle vector pKV12 was constructed by cloning the tetM gene into pAT187. Mating of E. coli containing pKV12 with A. woodii resulted in transfer frequencies of 3 × 10^-6 to 7 × 10^-6 per donor or recipient.     

A new bioassay for auxins and cytokinins

The authors have developed a sensitive bioassay that can be used to detect auxins as well as cytokinins. The bioassay is based on the expression in transformed tobacco (Nicotiana tabacum) mesophyll protoplasts of a chimeric gene, consisting of the upstream sequences of the Agrobacterium tumefaciens gene 5, coupled to the coding sequence of the β-glucuronidase. The expression of this gene is induced by the presence of both auxin and cytokinin in the culture medium. Using this assay, indole-3-acetic acid was detected at 5 × 10⁻⁸ molar, whereas trans-zeatin could be detected at 5 × 10⁻¹¹ molar. The assay can be performed in microtiter plates, allowing numerous samples to be analyzed simultaneously. Only 2.5 × 10⁵ protoplasts are required for one individual assay in 250 microliters of culture medium and for qualitative results, the reaction is readily visualized by ultraviolet light.     

Estimation of the Spontaneous Mutation Rate per Nucleotide Site in a Drosophila melanogaster Full-Sib Family

We employed deep genome sequencing of two parents and 12 of their offspring to estimate the mutation rate per site per generation in a full-sib family of Drosophila melanogaster recently sampled from a natural population. Sites that were homozygous for the same allele in the parents and heterozygous in one or more offspring were categorized as candidate mutations and subjected to detailed analysis. In 1.23 × 10⁹ callable sites from 12 individuals, we confirmed six single nucleotide mutations. We estimated the false negative rate in the experiment by generating synthetic mutations using the empirical distributions of numbers of nonreference bases at heterozygous sites in the offspring. The proportion of synthetic mutations at callable sites that we failed to detect was <1%, implying that the false negative rate was extremely low. Our estimate of the point mutation rate is 2.8 × 10⁻⁹ (95% confidence interval = 1.0 × 10⁻⁹ − 6.1 × 10⁻⁹) per site per generation, which is at the low end of the range of previous estimates, and suggests an effective population size for the species of ∼1.4 × 10⁶. At one site, point mutations were present in two individuals, indicating that there had been a premeiotic mutation cluster, although surprisingly one individual had a G→A transition and the other a G→T transversion, possibly associated with error-prone mismatch repair. We also detected three short deletion mutations and no insertions, giving a deletion mutation rate of 1.2 × 10⁻⁹ (95% confidence interval = 0.7 × 10⁻⁹ − 11 × 10⁻⁹).     

Microbial Decomposition of Cellulose in Acidifying Lakes of South-Central Ontario

The rate of cellulose breakdown and density of bacterial populations were measured in the epilimnetic sediments and water columns of lakes in central Ontario that differ in pH, alkalinity, and nutrient status and are particularly sensitive to acidic inputs from atmospheric decomposition. There was no significant difference in decomposition rate in either oxic or anoxic sediment when mean epilimnetic pHs were in the range 5.5 to 6.9. The importance of these findings for the breakdown of autochthonous detritus in Canadian Shield lakes is discussed. Furthermore, the results of these experiments, in which dyed strips of cellophane (regenerated cellulose) were used as substrate, were compared with results of earlier decomposition studies carried out with coarse litter (leaves, twigs). Acridine orange direct counts of bacteria in the top 1 cm of sediment ranged from 5.5 × 10⁸ to 1.0 × 10⁹ per g and in planktonic water samples from 1.1 × 10⁶ to 1.8 × 10⁶ per ml. Bacterial densities were significantly higher in both the shallow sediment (P < 0.01) and the water column (P < 0.05) of dystrophic lakes than at these sites in oligotrophic lakes.