Motility of Marichromatium gracile in Response to Light, Oxygen, and Sulfide

The motility of the purple sulfur bacterium Marichromatium gracile was investigated under different light regimes in a gradient capillary setup with opposing oxygen and sulfide gradients. The gradients were quantified with microsensors, while the behavior of swimming cells was studied by video microscopy in combination with a computerized cell tracking system. M. gracile exhibited photokinesis, photophobic responses, and phobic responses toward oxygen and sulfide. The observed migration patterns could be explained solely by the various phobic responses. In the dark, M. gracile formed an ~500-μm-thick band at the oxic-anoxic interface, with a sharp border toward the oxic zone always positioned at ~10 μM O₂. Flux calculations yielded a molar conversion ratio S_tot/O₂ of 2.03:1 (S_tot = [H₂S] + [HS⁻] + [S²⁻]) for the sulfide oxidation within the band, indicating that in darkness the bacteria oxidized sulfide incompletely to sulfur stored in intracellular sulfur globules. In the light, M. gracile spread into the anoxic zone while still avoiding regions with >10 μM O₂. The cells also preferred low sulfide concentrations if the oxygen was replaced by nitrogen. A light-dark transition experiment demonstrated a dynamic interaction between the chemical gradients and the cell's metabolism. In darkness and anoxia, M. gracile lost its motility after ca. 1 h. In contrast, at oxygen concentrations of >100 μM with no sulfide present the cells remained viable and motile for ca. 3 days both in light and darkness. Oxygen was respired also in the light, but respiration rates were lower than in the dark. Observed aggregation patterns are interpreted as effective protection strategies against high oxygen concentrations and might represent first stages of biofilm formation.     

Propagation of Rhizopus javanicus Biosorbent

After propagation of Rhizopus javanicus in defined media containing glucose, urea, and mineral salts in deionized distilled water, the ability of the nonliving biomass to sequester cupric ion was assayed. Growth, uptake capacity (saturation uptake at >1 mM Cu²⁺ concentration in solution), and biosorptive yield (biomass concentration × uptake capacity) were increased by augmentation of the growth medium with mineral salts once growth was under way. In the stationary phase, the uptake capacity of mycelia, which were normally a poor biosorbent, was improved within 4 h of trace metal addition to the growth medium. Growth of the culture was inhibited by excessive concentrations (0.04 to 40 μM) of metals in the medium in the following order: Cu > Co ≥ Ni > Mn > Mo; zinc was not inhibitory at 40 μM, and chromium was stimulatory at 0.53 μM but slightly inhibitory at higher levels. Iron and potassium phosphate stimulated growth at levels of 0.53 and 40 mM, respectively. When R. javanicus was propagated in a medium with a high salt concentration, exponential growth (0.23 h⁻¹) to a biomass concentration of >3 g/liter and a biosorptive yield of >500 μmol/liter was achieved. It is evident that the powerful biosorbent characteristics of Rhizopus biomass led to depletion of available trace minerals in suspension culture, which in turn limited growth.     

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.     

Electrophysiological Characterization of the Rat Epithelial Na+ Channel (rENaC) Expressed in MDCK Cells : Effects of Na+ and Ca2+

The epithelial Na⁺ channel (ENaC), composed of three subunits (α, β, and γ), is expressed in several epithelia and plays a critical role in salt and water balance and in the regulation of blood pressure. Little is known, however, about the electrophysiological properties of this cloned channel when expressed in epithelial cells. Using whole-cell and single channel current recording techniques, we have now characterized the rat αβγENaC (rENaC) stably transfected and expressed in Madin-Darby canine kidney (MDCK) cells. Under whole-cell patch-clamp configuration, the αβγrENaC-expressing MDCK cells exhibited greater whole cell Na⁺ current at −143 mV (−1,466.2 ± 297.5 pA) than did untransfected cells (−47.6 ± 10.7 pA). This conductance was completely and reversibly inhibited by 10 μM amiloride, with a Ki of 20 nM at a membrane potential of −103 mV; the amiloride inhibition was slightly voltage dependent. Amiloride-sensitive whole-cell current of MDCK cells expressing αβ or αγ subunits alone was −115.2 ± 41.4 pA and −52.1 ± 24.5 pA at −143 mV, respectively, similar to the whole-cell Na⁺ current of untransfected cells. Relaxation analysis of the amiloride-sensitive current after voltage steps suggested that the channels were activated by membrane hyperpolarization. Ion selectivity sequence of the Na⁺ conductance was Li⁺ > Na⁺ >> K⁺ = N-methyl-d-glucamine⁺ (NMDG⁺). Using excised outside-out patches, amiloride-sensitive single channel conductance, likely responsible for the macroscopic Na⁺ channel current, was found to be ∼5 and 8 pS when Na⁺ and Li⁺ were used as a charge carrier, respectively. K⁺ conductance through the channel was undetectable. The channel activity, defined as a product of the number of active channel (n) and open probability (P _o), was increased by membrane hyperpolarization. Both whole-cell Na⁺ current and conductance were saturated with increased extracellular Na⁺ concentrations, which likely resulted from saturation of the single channel conductance. The channel activity (nP _o) was significantly decreased when cytosolic Na⁺ concentration was increased from 0 to 50 mM in inside-out patches. Whole-cell Na⁺ conductance (with Li⁺ as a charge carrier) was inhibited by the addition of ionomycin (1 μM) and Ca²⁺ (1 mM) to the bath. Dialysis of the cells with a pipette solution containing 1 μM Ca²⁺ caused a biphasic inhibition, with time constants of 1.7 ± 0.3 min (n = 3) and 128.4 ± 33.4 min (n = 3). An increase in cytosolic Ca²⁺ concentration from <1 nM to 1 μM was accompanied by a decrease in channel activity. Increasing cytosolic Ca²⁺ to 10 μM exhibited a pronounced inhibitory effect. Single channel conductance, however, was unchanged by increasing free Ca²⁺ concentrations from <1 nM to 10 μM. Collectively, these results provide the first characterization of rENaC heterologously expressed in a mammalian epithelial cell line, and provide evidence for channel regulation by cytosolic Na⁺ and Ca²⁺.     

Effect of Tungstate on Nitrate Reduction by the Hyperthermophilic Archaeon Pyrobaculum aerophilum

Pyrobaculum aerophilum, a hyperthermophilic archaeon, can respire either with low amounts of oxygen or anaerobically with nitrate as the electron acceptor. Under anaerobic growth conditions, nitrate is reduced via the denitrification pathway to molecular nitrogen. This study demonstrates that P. aerophilum requires the metal oxyanion WO₄²⁻ for its anaerobic growth on yeast extract, peptone, and nitrate as carbon and energy sources. The addition of 1 μM MoO₄²⁻ did not replace WO₄²⁻ for the growth of P. aerophilum. However, cell growth was completely inhibited by the addition of 100 μM MoO₄²⁻ to the culture medium. At lower tungstate concentrations (0.3 μM and less), nitrite was accumulated in the culture medium. The accumulation of nitrite was abolished at higher WO₄²⁻ concentrations (<0.7 μM). High-temperature enzyme assays for the nitrate, nitrite, and nitric oxide reductases were performed. The majority of all three denitrification pathway enzyme activities was localized to the cytoplasmic membrane, suggesting their involvement in the energy metabolism of the cell. While nitrite and nitric oxide specific activities were relatively constant at different tungstate concentrations, the activity of nitrate reductase was decreased fourfold at WO₄²⁻ levels of 0.7 μM or higher. The high specific activity of the nitrate reductase enzyme observed at low WO₄²⁻ levels (0.3 μM or less) coincided with the accumulation of nitrite in the culture medium. This study documents the first example of the effect of tungstate on the denitrification process of an extremely thermophilic archaeon. We demonstrate here that nitrate reductase synthesis in P. aerophilum occurs in the presence of high concentrations of tungstate.     

Effect of Sarcoplasmic Reticulum (SR) Calcium Content on SR Calcium Release Elicited by Small Voltage-Clamp Depolarizations in Frog Cut Skeletal Muscle Fibers Equilibrated with 20 mM EGTA

Cut muscle fibers from Rana temporaria (sarcomere length, 3.5–3.9 μm; 14–16°C) were mounted in a double Vaseline-gap chamber and equilibrated with an external solution that contained tetraethyl ammonium– gluconate and an internal solution that contained Cs as the principal cation, 20 mM EGTA, and 0 Ca. Fibers were stimulated with a voltage-clamp pulse protocol that consisted of pulses to −70, −65, −60, −45, and −20 mV, each separated by 400-ms periods at −90 mV. The change in total Ca that entered into the myoplasm (Δ[Ca_T]) and the Ca content of the SR ([Ca_SR]) were estimated with the EGTA/phenol red method (Pape, P.C., D.-S. Jong, and W.K. Chandler. 1995. J. Gen. Physiol. 106:259–336). Fibers were stimulated with the pulse protocol, usually every 5 min, so that the resting value of [Ca_SR] decreased from its initial value of 1,700–2,300 μM to values near or below 100 μM after 18–30 stimulations. Three main findings for the voltage pulses to −70, −65, and −60 mV are: (a) the depletion-corrected rate of Ca release (release permeability) showed little change when [Ca_SR] decreased from its highest level (>1,700 μM) to ∼1,000 μM; (b) as [Ca_SR] decreased below 1,000 μM, the release permeability increased to a maximum level when [Ca_SR] was near 300 μM that was on average about sevenfold larger than the values observed for [Ca_SR] > 1,000 μM; and (c) as [Ca_SR] decreased from ∼300 μM to <100 μM, the release permeability decreased, reaching half its maximum value when [Ca_SR] was ∼110 μM on average. It was concluded that finding b was likely due to a decrease in Ca inactivation, while finding c was likely due to a decrease in Ca-induced Ca release.     

Differences in SOM Decomposition and Temperature Sensitivity among Soil Aggregate Size Classes in a Temperate Grasslands

The principle of enzyme kinetics suggests that the temperature sensitivity (Q₁₀) of soil organic matter (SOM) decomposition is inversely related to organic carbon (C) quality, i.e., the C quality-temperature (CQT) hypothesis. We tested this hypothesis by performing laboratory incubation experiments with bulk soil, macroaggregates (MA, 250–2000 μm), microaggregates (MI, 53–250 μm), and mineral fractions (MF, <53 μm) collected from an Inner Mongolian temperate grassland. The results showed that temperature and aggregate size significantly affected on SOM decomposition, with notable interactive effects (P<0.0001). For 2 weeks, the decomposition rates of bulk soil and soil aggregates increased with increasing incubation temperature in the following order: MA>MF>bulk soil >MI(P <0.05). The Q₁₀ values were highest for MA, followed (in decreasing order) by bulk soil, MF, and MI. Similarly, the activation energies (E_a) for MA, bulk soil, MF, and MI were 48.47, 33.26, 27.01, and 23.18 KJ mol⁻¹, respectively. The observed significant negative correlations between Q₁₀ and C quality index in bulk soil and soil aggregates (P<0.05) suggested that the CQT hypothesis is applicable to soil aggregates. Cumulative C emission differed significantly among aggregate size classes (P <0.0001), with the largest values occurring in MA (1101 μg g⁻¹), followed by MF (976 μg g⁻¹) and MI (879 μg g⁻¹). These findings suggest that feedback from SOM decomposition in response to changing temperature is closely associated withsoil aggregation and highlights the complex responses of ecosystem C budgets to future warming scenarios.     

Clotrimazole as a Potent Agent for Treating the Oomycete Fish Pathogen Saprolegnia parasitica through Inhibition of Sterol 14α-Demethylase (CYP51)

A candidate CYP51 gene encoding sterol 14α-demethylase from the fish oomycete pathogen Saprolegnia parasitica (SpCYP51) was identified based on conserved CYP51 residues among CYPs in the genome. It was heterologously expressed in Escherichia coli, purified, and characterized. Lanosterol, eburicol, and obtusifoliol bound to purified SpCYP51 with similar binding affinities (K_s, 3 to 5 μM). Eight pharmaceutical and six agricultural azole antifungal agents bound tightly to SpCYP51, with posaconazole displaying the highest apparent affinity (K_d, ≤3 nM) and prothioconazole-desthio the lowest (K_d, ∼51 nM). The efficaciousness of azole antifungals as SpCYP51 inhibitors was confirmed by 50% inhibitory concentrations (IC₅₀s) of 0.17 to 2.27 μM using CYP51 reconstitution assays. However, most azole antifungal agents were less effective at inhibiting S. parasitica, Saprolegnia diclina, and Saprolegnia ferax growth. Epoxiconazole, fluconazole, itraconazole, and posaconazole failed to inhibit Saprolegnia growth (MIC₁₀₀, >256 μg ml⁻¹). The remaining azoles inhibited Saprolegnia growth only at elevated concentrations (MIC₁₀₀ [the lowest antifungal concentration at which growth remained completely inhibited after 72 h at 20°C], 16 to 64 μg ml⁻¹) with the exception of clotrimazole, which was as potent as malachite green (MIC₁₀₀, ∼1 μg ml⁻¹). Sterol profiles of azole-treated Saprolegnia species confirmed that endogenous CYP51 enzymes were being inhibited with the accumulation of lanosterol in the sterol fraction. The effectiveness of clotrimazole against SpCYP51 activity (IC₅₀, ∼1 μM) and the concentration inhibiting the growth of Saprolegnia species in vitro (MIC₁₀₀, ∼1 to 2 μg ml⁻¹) suggest that clotrimazole could be used against Saprolegnia infections, including as a preventative measure by pretreatment of fish eggs, and for freshwater-farmed fish as well as in leisure activities.     

Biphasic Behavior of Anammox Regulated by Nitrite and Nitrate in an Estuarine Sediment

The production of N₂ gas via anammox was investigated in sediment slurries at in situ NO₂⁻ concentrations in the presence and absence of NO₃⁻. With single enrichment above 10 μM ¹⁴NO₂⁻ or ¹⁴NO₃⁻ and ¹⁵NH₄⁺, anammox activity was always linear (P < 0.05), in agreement with previous findings. In contrast, anammox exhibited a range of activity below 10 μM NO₂⁻ or NO₃⁻, including an elevated response at lower concentrations. With 100 μM NO₃⁻, no significant transient accumulation of NO₂⁻ could be measured, and the starting concentration of NO₂⁻ could therefore be regulated. With dual enrichment (1 to 20 μM NO₂⁻ plus 100 μM NO₃⁻), there was a pronounced nonlinear response in anammox activity. Maximal activity occurred between 2 and 5 μM NO₂⁻, but the amplitude of this peak varied across the study (November 2003 to June 2004). Anammox accounted for as much as 82% of the NO₂⁻ added at 1 μM in November 2003 but only for 15% in May 2004 and for 26 and 5% of the NO₂⁻ added at 5 μM for these two months, respectively. Decreasing the concentration of NO₃⁻ but holding NO₂⁻ at 5 μM decreased the significance of anammox as a sink for NO₂⁻. The behavior of anammox was explored by use of a simple anammox-denitrification model, and the concept of a biphasic system for anammox in estuarine sediments is proposed. Overall, anammox is likely to be regulated by the availability of NO₃⁻ and NO₂⁻ and the relative size or activity of the anammox population.     

Denitrification in San Francisco Bay Intertidal Sediments

The acetylene block technique was employed to study denitrification in intertidal estuarine sediments. Addition of nitrate to sediment slurries stimulated denitrification. During the dry season, sediment-slurry denitrification rates displayed Michaelis-Menten kinetics, and ambient NO₃⁻ + NO₂⁻ concentrations (≤26 μM) were below the apparent K_m (50 μM) for nitrate. During the rainy season, when ambient NO₃⁻ + NO₂⁻ concentrations were higher (37 to 89 μM), an accurate estimate of the K_m could not be obtained. Endogenous denitrification activity was confined to the upper 3 cm of the sediment column. However, the addition of nitrate to deeper sediments demonstrated immediate N₂O production, and potential activity existed at all depths sampled (the deepest was 15 cm). Loss of N₂O in the presence of C₂H₂ was sometimes observed during these short-term sediment incubations. Experiments with sediment slurries and washed cell suspensions of a marine pseudomonad confirmed that this N₂O loss was caused by incomplete blockage of N₂O reductase by C₂H₂ at low nitrate concentrations. Areal estimates of denitrification (in the absence of added nitrate) ranged from 0.8 to 1.2 μmol of N₂ m⁻² h⁻¹ (for undisturbed sediments) to 17 to 280 μmol of N₂ m⁻² h⁻¹ (for shaken sediment slurries).     

The ultraviolet action spectrum for stomatal opening in broad bean

The ultraviolet action spectrum for stomatal opening was measured using epidermal peels from leaves of broad bean (Vicia faba). The spectrum was calculated from hyperbolic fluence response curves using 11 wavelengths ranging from 275 to 459 nm. The action spectrum exhibits a major peak at approximately 280 nm and a minor peak at approximately 360 nm. The response at 280 nm is about three times greater than the response at 459 nm. Under the conditions utilized (i.e. the absence of saturating red light), stomatal opening saturated at extremely low fluence rates: <0.2 μmol m⁻² s⁻¹ at 280 nm, and approximately 1.0 μmol m⁻² s⁻¹ at 459 nm. The threshold for blue-light-induced stomatal opening was approximately 0.02 μmol m⁻² s⁻¹. In light-mixing experiments, the addition of 280 nm light to saturating 650 nm (red) light caused additional stomatal opening, which is indicative of separate photoreceptors. In contrast, adding 280 nm of light to saturating 459 nm (blue) light did not increase stomatal opening, suggesting that they both excite the same receptor. The results with white light were similar to those with blue light. We infer that ultraviolet light acts via the blue light photoreceptor rather than through photosynthesis. The additional absorbance peak at 360 nm suggests that the chromophore is either a flavin or a cis-carotenoid, both of which exhibit peaks in this region. It is proposed that the chromophore can be excited either directly by blue light or by energy transferred from the protein portion of the protein-pigment complex after it absorbs 280 nm light.     

Genetic Variation of the Bloom-Forming Cyanobacterium Microcystis aeruginosa within and among Lakes: Implications for Harmful Algal Blooms

To measure genetic variation within and among populations of the bloom-forming cyanobacterium Microcystis aeruginosa, we surveyed a suite of lakes in the southern peninsula of Michigan that vary in productivity (total phosphorus concentrations of ~10 to 100 μg liter⁻¹). Survival of M. aeruginosa isolates from lakes was relatively low (i.e., mean of 7% and maximum of 30%) and positively related to lake total phosphorus concentration (P = 0.014, r² = 0.407, n = 14). In another study (D. F. Raikow, O. Sarnelle, A. E. Wilson, and S. K. Hamilton, Limnol. Oceanogr. 49:482-487, 2004), survival rates of M. aeruginosa isolates collected from an oligotrophic lake (total phosphorus of ~10 μg liter⁻¹ and dissolved inorganic nitrogen:total phosphorus ratio of 12.75) differed among five different medium types (G test, P of <0.001), with higher survival (P = 0.003) in low-nutrient media (28 to 37% survival) than in high-nutrient media. Even with the relatively low isolate survivorship that could select against detecting the full range of genetic variation, populations of M. aeruginosa were genetically diverse within and among lakes (by analysis of molecular variance, Φ_sc = 0.412 [Φ_sc is an F-statistic derivative which evaluates the correlation of haplotypic diversity within populations relative to the haplotypic diversity among all sampled populations], P = 0.001), with most clones being distantly related to clones collected from lakes directly attached to Lake Michigan (a Laurentian Great Lake) and culture collection strains collected from Canada, Scotland, and South Africa. Ninety-one percent of the 53 genetically unique M. aeruginosa clones contained the microcystin toxin gene (mcyA). Genotypes with the toxin gene were found in all lakes, while four lakes harbored both genotypes possessing and genotypes lacking the toxin gene.     

Active Transport and Accumulation of Iodide by Newly Isolated Marine Bacteria

Iodide (I⁻)-accumulating bacteria were isolated from marine sediment by an autoradiographic method with radioactive ¹²⁵I⁻. When they were grown in a liquid medium containing 0.1 μM iodide, 79 to 89% of the iodide was removed from the medium, and a corresponding amount of iodide was detected in the cells. Phylogenetic analysis based on 16S rRNA gene sequences indicated that iodide-accumulating bacteria were closely related to Flexibacter aggregans NBRC15975 and Arenibacter troitsensis, members of the family Flavobacteriaceae. When one of the strains, strain C-21, was cultured with 0.1 μM iodide, the maximum iodide content and the maximum concentration factor for iodide were 220 ± 3.6 (mean ± standard deviation) pmol of iodide per mg of dry cells and 5.5 × 10³, respectively. In the presence of much higher concentrations of iodide (1 μM to 1 mM), increased iodide content but decreased concentration factor for iodide were observed. An iodide transport assay was carried out to monitor the uptake and accumulation of iodide in washed cell suspensions of iodide-accumulating bacteria. The uptake of iodide was observed only in the presence of glucose and showed substrate saturation kinetics, with an apparent affinity constant for transport and a maximum velocity of 0.073 μM and 0.55 pmol min⁻¹ mg of dry cells⁻¹, respectively. The other dominant species of iodine in terrestrial and marine environments, iodate (IO₃⁻), was not transported.     

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.     

Specific Growth Rate Determines the Sensitivity of Escherichia coli to Thermal, UVA, and Solar Disinfection

Knowledge about the sensitivity of the test organism is essential for the evaluation of any disinfection method. In this work we show that sensitivity of Escherichia coli MG1655 to three physical stresses (mild heat, UVA light, and sunlight) that are relevant in the disinfection of drinking water with solar radiation is determined by the specific growth rate of the culture. Batch- and chemostat-cultivated cells from cultures with similar specific growth rates showed similar stress sensitivities. Generally, fast-growing cells were more sensitive to the stresses than slow-growing cells. For example, slow-growing chemostat-cultivated cells (D = 0.08 h⁻¹) and stationary-phase bacteria from batch culture that were exposed to mild heat had very similar T₉₀ (time until 90% of the population is inactivated) values (T_{90, chemostat} = 2.66 h; T_{90, batch} = 2.62 h), whereas T₉₀ for cells growing at a μ of 0.9 h⁻¹ was 0.2 h. We present evidence that the stress sensitivity of E. coli is correlated with the intracellular level of the alternative sigma factor RpoS. This is also supported by the fact that E. coli rpoS mutant cells were more stress sensitive than the parent strain by factors of 4.9 (mild heat), 5.3 (UVA light), and 4.1 (sunlight). Furthermore, modeling of inactivation curves with GInaFiT revealed that the shape of inactivation curves changed depending on the specific growth rate. Inactivation curves of cells from fast-growing cultures (μ = 1.0 h⁻¹) that were irradiated with UVA light showed a tailing effect, while for slow-growing cultures (μ = 0.3 h⁻¹), inactivation curves with shoulders were obtained. Our findings emphasize the need for accurate reporting of specific growth rates and detailed culture conditions in disinfection studies to allow comparison of data from different studies and laboratories and sound interpretation of the data obtained.     

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.     

Role of Sulfate Reduction Versus Methanogenesis in Terminal Carbon Flow in Polluted Intertidal Sediment of Waimea Inlet, Nelson, New Zealand

An investigation of the terminal anaerobic processes occurring in polluted intertidal sediments indicated that terminal carbon flow was mainly mediated by sulfate-reducing organisms in sediments with high sulfate concentrations (>10 mM in the interstitial water) exposed to low loadings of nutrient (equivalent to <10² kg of N · day⁻¹) and biochemical oxygen demand (<0.7 × 10³ kg · day⁻¹) in effluents from different pollution sources. However, in sediments exposed to high loadings of nutrient (>10² kg of N · day⁻¹) and biochemical oxygen demand (>0.7 × 10³ kg · day⁻¹), methanogenesis was the major process in the mediation of terminal carbon flow, and sulfate concentrations were low (≤2 mM). The respiratory index [¹⁴CO₂/(¹⁴CO₂ + ¹⁴CH₄)] for [2-¹⁴C]acetate catabolism, a measure of terminal carbon flow, was ≥0.96 for sediment with high sulfate, but in sediments with sulfate as little as 10 μM in the interstitial water, respiratory index values of ≤0.22 were obtained. In the latter sediment, methane production rates as high as 3 μmol · g⁻¹ (dry weight) · h⁻¹ were obtained, and there was a potential for active sulfate reduction.     

Carbon gain and photosynthetic response of chrysanthemum to photosynthetic photon flux density cycles

Most models of carbon gain as a function of photosynthetic irradiance assume an instantaneous response to increases and decreases in irradiance. High- and low-light-grown plants differ, however, in the time required to adjust to increases and decreases in irradiance. In this study the response to a series of increases and decreases in irradiance was observed in Chrysanthemum × morifolium Ramat. “Fiesta” and compared with calculated values assuming an instantaneous response. There were significant differences between high- and low-light-grown plants in their photosynthetic response to four sequential photosynthetic photon flux density (PPFD) cycles consisting of 5-minute exposures to 200 and 400 micromoles per square meter per second (μmol m⁻²s⁻¹). The CO₂ assimilation rate of high-light-grown plants at the cycle peak increased throughout the PPFD sequence, but the rate of increase was similar to the increase in CO₂ assimilation rate observed under continuous high-light conditions. Low-light leaves showed more variability in their response to light cycles with no significant increase in CO₂ assimilation rate at the cycle peak during sequential cycles. Carbon gain and deviations from actual values (percentage carbon gain over- or underestimation) based on assumptions of instantaneous response were compared under continuous and cyclic light conditions. The percentage carbon gain overestimation depended on the PPFD step size and growth light level of the leaf. When leaves were exposed to a large PPFD increase, the carbon gain was overestimated by 16 to 26%. The photosynthetic response to 100 μmol m⁻² s⁻¹ PPFD increases and decreases was rapid, and the small overestimation of the predicted carbon gain, observed during photosynthetic induction, was almost entirely negated by the carbon gain underestimation observed after a decrease. If the PPFD cycle was 200 or 400 μmol m⁻² s⁻¹, high- and low-light leaves showed a carbon gain overestimation of 25% that was not negated by the underestimation observed after a light decrease. When leaves were exposed to sequential PPFD cycles (200-400 μmol m⁻² s⁻¹), carbon gain did not differ from leaves exposed to a single PPFD cycle of identical irradiance integral that had the same step size (200-400-200 μmol m⁻² s⁻¹) or mean irradiance (200-300-200 μmol m⁻² s⁻¹).     

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.