Osmotic Pressure in a Bacterial Swarm

Using Escherichia coli as a model organism, we studied how water is recruited by a bacterial swarm. A previous analysis of trajectories of small air bubbles revealed a stream of fluid flowing in a clockwise direction ahead of the swarm. A companion study suggested that water moves out of the agar into the swarm in a narrow region centered ∼30 μm from the leading edge of the swarm and then back into the agar (at a smaller rate) in a region centered ∼120 μm back from the leading edge. Presumably, these flows are driven by changes in osmolarity. Here, we utilized green/red fluorescent liposomes as reporters of osmolarity to verify this hypothesis. The stream of fluid that flows in front of the swarm contains osmolytes. Two distinct regions are observed inside the swarm near its leading edge: an outer high-osmolarity band (∼30 mOsm higher than the agar baseline) and an inner low-osmolarity band (isotonic or slightly hypotonic to the agar baseline). This profile supports the fluid-flow model derived from the drift of air bubbles and provides new (to our knowledge) insights into water maintenance in bacterial swarms. High osmotic pressure at the leading edge of the swarm extracts water from the underlying agar and promotes motility. The osmolyte is of high molecular weight and probably is lipopolysaccharide.     

Bacterial Community Composition in Central European Running Waters Examined by Temperature Gradient Gel Electrophoresis and Sequence Analysis of 16S rRNA Genes

The bacterial community composition in small streams and a river in central Germany was examined by temperature gradient gel electrophoresis (TGGE) with PCR products of 16S rRNA gene fragments and sequence analysis. Complex TGGE band patterns suggested high levels of diversity of bacterial species in all habitats of these environments. Cluster analyses demonstrated distinct differences among the communities in stream and spring water, sandy sediments, biofilms on stones, degrading leaves, and soil. The differences between stream water and sediment were more significant than those between sites within the same habitat along the stretch from the stream source to the mouth. TGGE data from an entire stream course suggest that, in the upper reach of the stream, a special suspended bacterial community is already established and changes only slightly downstream. The bacterial communities in water and sediment in an acidic headwater with a pH below 5 were highly similar to each other but deviated distinctly from the communities at the other sites. As ascertained by nucleotide sequence analysis, stream water communities were dominated by Betaproteobacteria (one-third of the total bacteria), whereas sediment communities were composed mainly of Betaproteobacteria and members of the Fibrobacteres/Acidobacteria group (each accounting for about 25% of bacteria). Sequences obtained from bacteria from water samples indicated the presence of typical cosmopolitan freshwater organisms. TGGE bands shared between stream and soil samples, as well as sequences found in bacteria from stream samples that were related to those of soil bacteria, demonstrated the occurrence of some species in both stream and soil habitats. Changes in bacterial community composition were correlated with geographic distance along a stream, but in comparisons of different streams and rivers, community composition was correlated only with environmental conditions.     

The Origin of the Low-Energy Form of Photosystem I Light-Harvesting Complex Lhca4: Mixing of the Lowest Exciton with a Charge-Transfer State

The peripheral light-harvesting complex of photosystem I contains red chlorophylls (Chls) that, unlike the typical antenna Chls, absorb at lower energy than the primary electron donor P700. It has been shown that the red-most absorption band arises from two excitonically coupled Chls, although this interaction alone cannot explain the extreme red-shifted emission (25 nm, ∼480 cm⁻¹ for Lhca4 at 4 K) that the red Chls present. Here, we report the electric field-induced absorption changes (Stark effect) on the Q_y region of the Lhca4 complex. Two spectral forms, centered around 690 nm and 710 nm, were necessary to describe the absorption and Stark spectra. The analysis of the lowest energy transition yields a high value for the change in dipole moment, Δμ_710nm ≈ 8 Df⁻¹, between the ground and excited states as compared with monomeric, Δμ = 1 D, or dimeric, Δμ = 5 D, Chl a in solution. The high value of the Δμ demonstrates that the origin of the red-shifted emission is the mixing of the lowest exciton state with a charge-transfer state of the dimer. This energetic configuration, an excited state with charge-transfer character, is very favorable for the trapping and dissipation of excitations and could be involved in the photoprotective mechanism(s) of the photosystem I complex.     

Dynamics of Bacterial Swarming

When vegetative bacteria that can swim are grown in a rich medium on an agar surface, they become multinucleate, elongate, synthesize large numbers of flagella, produce wetting agents, and move across the surface in coordinated packs: they swarm. We examined the motion of swarming Escherichia coli, comparing the motion of individual cells to their motion during swimming. Swarming cells' speeds are comparable to bulk swimming speeds, but very broadly distributed. Their speeds and orientations are correlated over a short distance (several cell lengths), but this correlation is not isotropic. We observe the swirling that is conspicuous in many swarming systems, probably due to increasingly long-lived correlations among cells that associate into groups. The normal run-tumble behavior seen in swimming chemotaxis is largely suppressed, instead, cells are continually reoriented by random jostling by their neighbors, randomizing their directions in a few tenths of a second. At the edge of the swarm, cells often pause, then swim back toward the center of the swarm or along its edge. Local alignment among cells, a necessary condition of many flocking theories, is accomplished by cell body collisions and/or short-range hydrodynamic interactions.     

Single-molecule imaging of β-actin mRNAs in the cytoplasm of a living cell

β-Actin mRNA labeled with an MS2-EGFP fusion protein was expressed in chicken embryo fibroblasts and its localization and movement were analyzed by single-molecule imaging. Most β-Actin mRNAs localized to the leading edge, while some others were observed in the perinuclear region. Singe-molecule tracking of individual mRNAs revealed that the majority of mRNAs were in unrestricted Brownian motion at the leading edge and in restricted Brownian motion in the perinuclear region. The macroscopic diffusion coefficient of mRNA (D_MACRO) at the leading edge was 0.3 μm²/s. On the other hand, D_MACRO in the perinuclear region was 0.02 μm²/s. The destruction of microfilaments with cytochalasin D, which is known to delocalize β-actin mRNAs, led to an increase in D_MACRO to 0.2 μm²/s in the perinuclear region. These results suggest that the microstructure, composed of microfilaments, serves as a barrier for the movement of β-actin mRNA.     

Preparation,Imaging, and Quantification of Bacterial Surface Motility Assays

Bacterial surface motility, such as swarming, is commonly examined in the laboratory using plate assays that necessitate specific concentrations of agar and sometimes inclusion of specific nutrients in the growth medium. The preparation of such explicit media and surface growth conditions serves to provide the favorable conditions that allow not just bacterial growth but coordinated motility of bacteria over these surfaces within thin liquid films. Reproducibility of swarm plate and other surface motility plate assays can be a major challenge. Especially for more “temperate swarmers” that exhibit motility only within agar ranges of 0.4%-0.8% (wt/vol), minor changes in protocol or laboratory environment can greatly influence swarm assay results. “Wettability”, or water content at the liquid-solid-air interface of these plate assays, is often a key variable to be controlled. An additional challenge in assessing swarming is how to quantify observed differences between any two (or more) experiments. Here we detail a versatile two-phase protocol to prepare and image swarm assays. We include guidelines to circumvent the challenges commonly associated with swarm assay media preparation and quantification of data from these assays. We specifically demonstrate our method using bacteria that express fluorescent or bioluminescent genetic reporters like green fluorescent protein (GFP), luciferase (lux operon), or cellular stains to enable time-lapse optical imaging. We further demonstrate the ability of our method to track competing swarming species in the same experiment.     

Heme-heme and heme-ligand interactions in the di-heme oxygen-reducing site of cytochrome bd from Escherichia coli revealed by nanosecond absorption spectroscopy

Cytochrome bd is a terminal quinol:O₂ oxidoreductase of respiratory chains of many bacteria. It contains three hemes, b₅₅₈, b₅₉₅, and d. The role of heme b₅₉₅ remains obscure. A CO photolysis/recombination study of the membranes of Escherichia coli containing either wild type cytochrome bd or inactive E445A mutant was performed using nanosecond absorption spectroscopy. We compared photoinduced changes of heme d-CO complex in one-electron-reduced, two-electron-reduced, and fully reduced states of cytochromes bd. The line shape of spectra of photodissociation of one-electron-reduced and two-electron-reduced enzymes is strikingly different from that of the fully reduced enzyme. The difference demonstrates that in the fully reduced enzyme photolysis of CO from heme d perturbs ferrous heme b₅₉₅ causing loss of an absorption band centered at 435 nm, thus supporting interactions between heme b₅₉₅ and heme d in the di-heme oxygen-reducing site, in agreement with previous works. Photolyzed CO recombines with the fully reduced enzyme monoexponentially with τ ∼ 12 μs, whereas recombination of CO with one-electron-reduced cytochrome bd shows three kinetic phases, with τ ∼ 14 ns, 14 μs, and 280 μs. The spectra of the absorption changes associated with these components are different in line shape. The 14 ns phase, absent in the fully reduced enzyme, reflects geminate recombination of CO with part of heme d. The 14-μs component reflects bimolecular recombination of CO with heme d and electron backflow from heme d to hemes b in ∼ 4% of the enzyme population. The final, 280-μs component, reflects return of the electron from hemes b to heme d and bimolecular recombination of CO in that population. The fact that even in the two-electron-reduced enzyme, a nanosecond geminate recombination is observed, suggests that namely the redox state of heme b₅₉₅, and not that of heme b₅₅₈, controls the pathway(s) by which CO migrates between heme d and the medium.     

The impact of long-term contact of Achromobacter sp. 4(2010) with diesel oil – Changes in biodegradation,surface properties and hexadecane monooxygenase activity

A bacterial strain was isolated from soil that was contaminated with diesel oil and was used in our experiments. The strain was then phenotypically, biochemically and genetically tested and named as Achromobacter 4(2011). In order to examine the impact of long-term contact with diesel oil of bacterial cells, the strain was stored under different conditions – on standard nutrient agar plates and on agar plates with 50 μl diesel oil as a sole carbon and energy source. The results clearly indicated that longer contact with diesel oil led to changes in both the bacterial surface and biochemical properties, as well as the hexadecane monooxygenase activity. Moreover, the fatty acid profiles also changed, leading to an increased content of saturated fatty acids. In addition, the rates of biodegradation of diesel oil were higher even when supplemented with the surfactants – rhamnolipids and saponins. This work demonstrates that prolonged contact of microorganisms with diesel oil can lead to many changes, not only in biodegradation potential, but also in their surface and genetic properties.     

Oxygen Concentration Inside a Functioning Photosynthetic Cell

The excess oxygen concentration in the photosynthetic membranes of functioning oxygenic photosynthetic cells was estimated using classical diffusion theory combined with experimental data on oxygen production rates of cyanobacterial cells. The excess oxygen concentration within the plesiomorphic cyanobacterium Gloeobactor violaceus is only 0.025 μM, or four orders of magnitude lower than the oxygen concentration in air-saturated water. Such a low concentration suggests that the first oxygenic photosynthetic bacteria in solitary form could have evolved ∼2.8 billion years ago without special mechanisms to protect them against reactive oxygen species. These mechanisms instead could have been developed during the following ∼500 million years while the oxygen level in the Earth’s atmosphere was slowly rising. Excess oxygen concentrations within individual cells of the apomorphic cyanobacteria Synechocystis and Synechococcus are 0.064 and 0.25 μM, respectively. These numbers suggest that intramembrane and intracellular proteins in isolated oxygenic photosynthetic cells are not subjected to excessively high oxygen levels. The situation is different for closely packed colonies of photosynthetic cells. Calculations show that the excess concentration within colonies that are ∼40 μm or larger in diameter can be comparable to the oxygen concentration in air-saturated water, suggesting that species forming colonies require protection against reactive oxygen species even in the absence of oxygen in the surrounding atmosphere.     

Spectral characteristic of fluorescence induction in a model cyanobacterium, Synechococcus sp. (PCC 7942)

We present here three-dimensional time-wavelength-intensity displays of changes in variable fluorescence, during the O(JI)PSMT transient, observed in cyanobacterium at room temperature. We were able to measure contributions of individual chromophores to fluorescence spectra at various times of fluorescence induction (FI). The method was applied to a freshwater cyanobacterium, Synechococcus sp. (PCC 7942). Analysis of our experimental results provides the following new conclusions: (i) the main chlorophyll (Chl) a emission band at ∼ 685 nm that originates in Photosystem (PS) II exhibits typical fast (OPS) and slow (SMT) FI kinetics with both orange (622 nm) and blue (464 nm) excitation. (ii) Similar kinetics are exhibited for its far-red emission satellite band centered at ∼ 745 nm, where the PS II contribution predominates. (iii) A significant OPS-SMT-type kinetics of C-phycocyanin emission at ∼ 650 nm are observed with the blue light excitation, but not with orange light excitation where the signal rose only slightly to a maximum. The induction of F650 was not caused by an admixture of the F685 fluorescence and thus our data show light-inducible and dark-reversible changes of phycobilin fluorescence in vivo. We discuss possible interpretations of this new observation.     

Disaggregation of Methanosarcina spp. and Growth as Single Cells at Elevated Osmolarity

The effect of medium osmolarity on the morphology and growth of Methanosarcina barkeri, Methanosarcina thermophila, Methanosarcina mazei, Methanosarcina vacuolata, and Methanosarcina acetivorans was examined. Each strain was adapted for growth in NaCl concentrations ranging from 0.05 to 1.0 M. Methanosarcina spp. isolated from both marine and nonmarine sources exhibited similar growth characteristics at all NaCl concentrations tested, demonstrating that these species are capable of adapting to a similar range of medium osmolarities. Concomitant with the adaptation in 0.4 to 1.0 M NaCl, all strains disaggregated and grew as single cells rather than in the characteristic multicellular aggregates. Aggregated cells had a methanochondroitin outer layer, while disaggregated single cells lacked the outer layer but retained the protein S-layer adjacent to the cell membrane. Synthesis of glucuronic acid, a major component of methanochondroitin, was reduced 20-fold in the single-cell form of M. barkeri when compared with synthesis in aggregated cells. Strains with the methanochondroitin outer cell layer exhibited enhanced stability at low (<0.2 M NaCl) osmolarity and grew at higher temperatures. Disaggregated cells could be converted back to aggregated cells by gradually readapting cultures to lower NaCl (<0.2 M) and Mg²⁺ (<0.005 M) concentrations. Disaggregated Methanosarcina spp. could also be colonized and replica plated with greater than 95% recovery rates on solidified agar basal medium that contained 0.4 to 0.6 M NaCl and either trimethylamine, methanol, or acetate as the substrate. The ability to disaggregate and grow Methanosarcina spp. as viable, detergent-sensitive, single cells on agar medium makes these species amenable to mutant selection and screening for genetic studies and enables cells to be gently lysed for the isolation of intact genetic material.     

A novel method for exploring elemental composition of microbial communities: Laser ablation-inductively coupled plasma-mass spectrometry of intact bacterial colonies

Bacterial colonies are spatially complex structures whose physiology is profoundly dependent on interactions between cells and with the underlying semi-solid substratum. Here, we use bacterial colonies as a model of a microbial community to evaluate the potential of laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) to delineate elemental distributions within colonies with minimal pre-treatment. To reduce water content of the colony and limit undesirable absorption of laser energy, we compared methods of preparing 24 h-old colonies of Escherichia coli TG1 on agar for laser ablation. Colonies on excised agar segments dried on chromatography paper were superior to colonies dried in a dessicator or by prolonged incubation, with respect to signal magnitude, signal:noise ratio and background signal. Having optimised laser scan speed (10 μm s^− 1) and laser beam diameter (100 μm), further improvements were achieved by growing colonies on nylon membranes over agar, which were then transferred to the ablation chamber without further treatment. Repeated line rasters across individual membrane-supported colonies yielded three-dimensional elemental maps of colonies, revealing a convex morphology consistent with visual inspection. By normalising isotope counts for P, Mn, Zn, Fe and Ca against Mg, the most abundant cellular divalent cation, we sought elemental heterogeneity within the colony. The normalised concentration of Mn in the perimeter was higher than in the colony interior, whereas the converse was true for Ca. LA-ICP-MS is a novel and powerful method for probing elemental composition and organisation within microbial communities and should find numerous applications in, for example, biofilm studies.     

Antimicrobial activity of highly stable silver nanoparticles embedded in agar-agar matrix as a thin film

Highly stable silver nanoparticles (Ag NPs) in agar-agar (Ag/agar) as inorganic-organic hybrid were obtained as free-standing film by in situ reduction of silver nitrate by ethanol. The antimicrobial activity of Ag/agar film on Escherichia coli (E. coli), Staphylococcus aureus (S. aureus), and Candida albicans (C. albicans) was evaluated in a nutrient broth and also in saline solution. In particular, films were repeatedly tested for antimicrobial activity after recycling. UV-vis absorption and TEM studies were carried out on films at different stages and morphological studies on microbes were carried out by SEM. Results showed spherical Ag NPs of size 15-25 nm, having sharp surface plasmon resonance (SPR) band. The antimicrobial activity of Ag/agar film was found to be in the order, C. albicans > E. coli > S. aureus, and antimicrobial activity against C. albicans was almost maintained even after the third cycle. Whereas, in case of E. coli and S. aureus there was a sharp decline in antimicrobial activity after the second cycle. Agglomeration of Ag NPs in Ag/agar film on exposure to microbes was observed by TEM studies. Cytotoxic experiments carried out on HeLa cells showed a threshold Ag NPs concentration of 60 μg/mL, much higher than the minimum inhibition concentration of Ag NPs (25.8 μg/mL) for E. coli. The mechanical strength of the film determined by nanoindentation technique showed almost retention of the strength even after repeated cycle.     

Water transport in epididymal and ejaculated rhesus monkey (Macaca mulatta) sperm during freezing

In the present study, we report the effects of cooling ejaculated and epididymal rhesus monkey (Macacamulatta) sperm with and without the presence of a cryoprotective agent, glycerol. Water transport data during freezing of ejaculated and epididymal sperm cell suspensions were obtained at a cooling rate of 20 °C/min in the absence of any cryoprotective agents and in the presence of 0.7 M of glycerol, as well. Using previously published values, the macaque sperm cell was modeled as a cylinder of length 73.83 μm with a radius of 0.40 μm and an osmotically inactive cell volume, V_b, of 0.772V_o, where V_o is the isotonic cell volume. This translated to a surface area, SA to initial water volume, WV ratio of ∼22 μm⁻¹. By fitting a model of water transport to the experimentally determined volumetric shrinkage data, the best-fit membrane permeability parameters (reference membrane permeability to water at 0 °C, L_pg or L_pg[cpa] and the activation energy, E_Lp or E_Lp[cpa]) were found to range from: L_pg or L_pg[cpa] = 0.0020-0.0029 μm/min-atm; E_Lp or E_Lp[cpa]) = 10.6-18.3 kcal/mole. By incorporating these membrane permeability parameters in a recently developed equation (optimal cooling rate, ; where the units of B_opt are °C/min, E_Lp or E_Lp[cpa] are kcal/mole, L_pg or L_pg[cpa] are μm/min-atm and SA/WV are μm⁻¹), we determined the optimal rates of freezing macaque sperm to be ∼23 °C/min (ejaculated sperm in the absence of CPAs), ∼29 °C/min (ejaculated sperm in the presence of glycerol), ∼24 °C/min (epididymal sperm in the absence of CPAs) and ∼24 °C/min (epididymal sperm in the presence of glycerol). In conclusion, the subzero water transport response and consequently the subzero water transport parameters are not significantly different between the ejaculated and epididymal macaque spermatozoa under corresponding cooling conditions.     

Water-to-Air Transfer and Enrichment of Bacteria in Drops from Bursting Bubbles

An electrostatic induction technique was used to determine both drop size distribution and concentration of bacteria in the film drops produced by bubbles bursting at the surface of a suspension of Serratia marcescens. Film drops are produced from the collapse of the thin film of water that just before bursting separates the air in the bubble from the atmosphere. Bubbles of 1.7-mm diameter produced from 10 to 20 film drops which ranged from <2 μm to over 30 μm in diameter. Half the drops were <10 μm. For bubbles rising a distance of less than 2 cm through the bacterial suspension, bacterial enrichment factors in the drops were between 10 and 20. Electrostatic methods can be used to determine the enrichment of bacteria in film drops as a function of bubble size and distance of rise through the bacterial suspension.     

Evidence for the fragmentation of VtgAb LvH in common dentex (Dentex dentex), a marine pelagophil teleost

Since the timing of the lipovitellin heavy chain (LvH) of vitellogenin Ab (VtgAb) degradation during marine teleosts ontogeny is poorly understood, the current study was planned to address some aspects of this shortcoming. Fertilized eggs (before epiboly) of Dentex dentex were analysed. Vtg-derived proteins, that appeared as 6 protein bands after SDS-PAGE, were purified using specific ion exchange chromatography. Five bands (i.e., ∼78, ∼57, ∼22, ∼19, and ∼17 kDa) were merely stained with Coomassie Blue and one band (i.e., ∼16 kDa) only with Periodic Acid Schiff (PAS). The ∼16 kDa protein band was subjected to a mass spectrometry-based sequencing. These results showed that the ∼16 kDa protein band contains a mixture of “LvH-Ab” derivatives with the same molecular weight. The specific staining system (i.e., PAS) also revealed the glycosylation of some of the LvH-Ab fragments inside the ∼16 kDa protein band. The study provides new data about fragmentation of the LvH-Ab in marine pelagophil teleosts.     

Accommodation of NO in the active site of mammalian and bacterial cytochrome c oxidase aa3

Following different reports on the stoichiometry and configuration of NO binding to mammalian and bacterial reduced cytochrome c oxidase aa₃ (CcO), we investigated NO binding and dynamics in the active site of beef heart CcO as a function of NO concentration, using ultrafast transient absorption and EPR spectroscopy. We find that in the physiological range only one NO molecule binds to heme a₃, and time-resolved experiments indicate that even transient binding to Cu_B does not occur. Only at very high (∼ 2 mM) concentrations a second NO is accommodated in the active site, although in a different configuration than previously observed for CcO from Paracoccus denitrificans [E. Pilet, W. Nitschke, F. Rappaport, T. Soulimane, J.-C. Lambry, U. Liebl and M.H. Vos. Biochemistry 43 (2004) 14118-14127], where we proposed that a second NO does bind to Cu_B. In addition, in the bacterial enzyme two NO molecules can bind already at NO concentrations of ∼ 1 μM. The unexpected differences highlighted in this study may relate to differences in the physiological relevance of the CcO-NO interactions in both species.     

Laser stimulation of auditory neurons: effect of shorter pulse duration and penetration depth

We have pioneered what we believe is a novel method of stimulating cochlear neurons, using pulsed infrared radiation, based on the hypothesis that optical radiation can provide more spatially selective stimulation of the cochlea than electric current. Very little of the available optical parameter space has been used for optical stimulation of neurons. Here, we use a pulsed diode laser (1.94 μm) to stimulate auditory neurons of the gerbil. Radiant exposures measured at CAP threshold are similar for pulse durations of 5, 10, 30, and 100 μs, but greater for 300-μs-long pulses. There is evidence that water absorption of optical radiation is a significant factor in optical stimulation. Heat-transfer-based analysis of the data indicates that potential structures involved in optical stimulation of cochlear neurons have a dimension on the order of ∼10 μm. The implications of these data could direct further research and design of an optical cochlear implant.     

Effects of grinding processes on enzymatic degradation of wheat straw

The effectiveness of wheat straw fine to ultra-fine grindings at pilot scale was studied. The produced powders were characterised by their particle-size distribution (laser diffraction), crystallinity (WAXS) and enzymatic degradability (Trichoderma reesei enzymatic cocktail). A large range of wheat-straw powders was produced: from coarse (median particle size ∼800 μm) to fine particles (∼50 μm) using sieve-based grindings, then ultra-fine particles ∼20 μm by jet milling and ∼10 μm by ball milling. The wheat straw degradability was enhanced by the decrease of particle size until a limit: ∼100 μm, up to 36% total carbohydrate and 40% glucose hydrolysis yields. Ball milling samples overcame this limit up to 46% total carbohydrate and 72% glucose yields as a consequence of cellulose crystallinity reduction (from 22% to 13%). Ball milling appeared to be an effective pretreatment with similar glucose yield and superior carbohydrate yield compared to steam explosion pretreatment.     

Exciting Cell Membranes with a Blustering Heat Shock

Brief heat shocks delivered to cells by pulsed laser light can evoke action potentials in neurons and contraction in cardiomyocytes, but the primary biophysical mechanism has been elusive. In this report we show in the neuromuscular junction of Caenorhabditis elegans that application of a 500°C/s heat shock for 500 μs evoked ∼35 pA of excitatory current and injected ∼23 fC(femtocoulomb) of charge into the cell while raising the temperature only 0.25°C. The key variable driving the current was the rate of change of temperature (dT/dt heat shock), not temperature itself. The photothermal heat shock current was voltage-dependent and was from thermally driven displacement of ions near the plasma membrane. The charge movement was rapid during the heat shock and slow during thermal relaxation, thus leading to an asymmetrical capacitive current that briefly depolarized the cell. A simple quantitative model is introduced to describe modulation of the membrane potential and facilitate practical application of optical heat shock stimuli.