When it is too hot for photosynthesis: heat-induced instability of photosynthesis in relation to respiratory burst, cell permeability changes and H₂O₂ formation

Photosynthesis rate (A_n) becomes unstable above a threshold temperature, and the recovery upon return to low temperature varies because of reasons not fully understood. We investigated responses of A_n, dark respiration and chlorophyll fluorescence to supraoptimal temperatures of varying duration and kinetics in Phaseolus vulgaris asking whether the instability of photosynthesis under severe heat stress is associated with cellular damage. Cellular damage was assessed by Evans blue penetration (enhanced membrane permeability) and by H₂O₂ generation [3,3′‐diaminobenzidine 4HCl (DAB)‐staining]. Critical temperature for dark fluorescence (F₀) rise (T_F) was at 46–48 °C, and a burst of respiration was observed near T_F. However, A_n was strongly inhibited already before T_F was reached. Membrane permeability increased with temperature according to a switch‐type response, with enhanced permeability observed above 48 °C. Experiments with varying heat pulse lengths and intensities underscored the threshold‐type loss of photosynthetic function, and indicated that the degree of photosynthetic deterioration and cellular damage depended on accumulated heat‐dose. Beyond the ‘point of no return’, propagation of cellular damage and reduction of photosynthesis continued upon transfer to lower temperatures and photosynthetic recovery was slow or absent. We conclude that instability of photosynthesis under severe heat stress is associated with time‐dependent propagation of cellular lesions.     

Effects of temperature and irradiance on quantum yield of PSII photochemistry and xanthophyll cycle in a tropical and a temperate species

The effect of a wide range of temperatures (?15 and 60°C) in darkness or under strong irradiation [1,600 μmol(photon) m^?2 s^?1] on quantum yield of photosystem II photochemistry and xanthophyll cycle pigments was investigated in a tropical fruit crop (Musa sp.) and a temperate spring flowering plant (Allium ursinum L.). In darkness within the nonlethal thermal window of A. ursinum (from ?6.7 to 47.7°C; 54.5 K) and of Musa sp. (from ?2.2°C to 49.5°C; 51.7 K) maximal quantum yield of PSII photochemistry (F_v/F_m) was fairly unaffected by temperature over more than 40 K. At low temperature F_v/F_m started to drop with ice nucleation but significantly only with initial frost injuries (temperature at 10% frost damage; LT₁₀). The critical high temperature threshold for PSII (T_c) was 43.8°C in A. ursinum and 44.7°C in Musa sp. Under strong irradiation, exposure to temperatures exceeding the growth ones but being still nonlethal caused photoinhibition in both species. Severity of photoinhibition increased with increasing distance to the growth temperature range. ΔF/F_m′ revealed distinctly different optimum temperature ranges: 27–36°C for Musa sp. and 18–27°C for A. ursinum exceeding maximum growth temperature by 2–7 K. In both species only at temperatures > 30°C zeaxanthin increased and violaxanthin decreased significantly. At nonlethal low temperature relative amounts of xanthophylls remained unchanged. At temperatures > 40°C β-carotene increased significantly in both species. In Musa sp. lutein and neoxanthin were significantly increased at 45°C, in A. ursinum lutein remained unchanged, neoxanthin levels decreased in the supraoptimal temperature range. In darkness, F_v/F_m was highly temperature-insensitive in both species. Under strong irradiation, whenever growth temperature was exceeded, photoinhibition occurred with xanthophylls being changed only under supraoptimal temperature conditions as an antiradical defence mechanism.     

Black leaf-clips of a commercial fluorometer increased leaf temperature during dark adaptation under high solar radiation

The use of black leaf-clips for dark adaptation under high solar radiation conditions is reported to underestimate the maximum quantum yield of PSII photochemistry (F_v/F_m) measured by the continuous-excitation fluorometer Pocket PEA. The decrease in F_v/F_m was due to a rise in minimum fluorescence emission (F_o), probably resulting from increased leaf temperature (T_l). In field-grown tomato and pepper, fluorescence parameters and T_l in the region covered by the black leaf clip were measured in clipped leaves exposed to solar radiation during dark adaptation (clipped-only leaves) and in clipped leaves protected from solar radiation by aluminium foil (shrouded clipped leaves). Results confirmed significant F_v/F_m underestimates in clipped-only leaves primarily due to increased F_o. In one tomato experiment, T_l increased from 30 to 44.5°C in clipped-only leaves, with a negligible rise in shrouded clipped leaves. In two respective pepper experiments, T_l in clipped-only leaves increased from 27 to 36.2°C and 33 to 40.9°C. Based on the results of this study, a clip-effect parameter (P_CE) on fluorescence emission is proposed as the difference for F_v/F_m (or ?F_o/F_m) between shrouded clipped leaves and clipped-only leaves, which resulted to be 0.706 for tomato, and 0.241 and 0.358 for the two pepper experiments.     

Estimating heat tolerance among plant species by two chlorophyll fluorescence parameters

The heat tolerance of 8 temperate- and 1 subtropical-origin C₃ species as well as 17 tropical-origin ones, including C₃, C₄, and CAM species, was estimated using both F₀-T curve and the ratio of chlorophyll fluorescence parameters, prior to and after high temperature treatment. When leaves were heated at the rate of ca. 1 °C min⁻¹ in darkness, the critical temperature (T_c) varied extensively among species. The T_c's of all 8 temperate-origin species ranged between 40–46 °C in winter (mean temperature 16–19 °C), and between 32–48 °C in summer (mean temperature ca. 30 °C). Those for 1 subtropical- and 12 tropical-origin C₃ species ranged between 25–44 °C and 35–48 °C, and for 1 CAM and 4 C₄ species were 41–47 and 45–46 °C, respectively. Acclimating three C₃ herbaceous plants at high temperature (33/28 °C, day/night) for 10 d in winter caused their T_c's rising to nearly the values measured in summer. When leaves were exposed to 45 °C for 20 min and then kept at room temperature in darkness for 1 h, a significant correlation between RF_v/m (the ratio of F_v/F_m before and after 45 °C treatment) and T_c was observed for all tested temperate-origin C₃ species as well as tropical-origin CAM and C₄ species. However, F₀ and F_v/F_m of the tropical-origin C₃ species were less sensitive to 45 °C treatment, regardless of a large variation of T_c; thus no significant correlation was found between their RF_v/m and T_c. Thus T_c might not be a suitable index of heat tolerance for plants with wide range of environmental adaptation. Nevertheless, T_c's of tropical origin C₃ species, varying and showing high plasticity to seasonal changes and temperature treatment, appeared suitable for the estimation of the degree of temperature acclimation in the same species.     

Temperature response of CO<Subscript>2</Subscript> exchange and dissolved organic carbon release in a maritime Antarctic tundra ecosystem

We examined the temperature response of CO₂ exchange and soil biogeochemical processes in an Antarctic tundra ecosystem using laboratory incubations of intact tundra cores. The cores were collected from tundra near Anvers Island along the west coast of the Antarctic Peninsula that was dominated by the vascular plants Colobanthus quitensis and Deschampsia antarctica. After the initial 8-week incubation at moderate growth temperatures (12/7°C, day/night), the tundra cores were incubated for another 8 weeks at either a higher (17/12°C) or lower (7/4°C) temperature regime. Temperature responses of CO₂ exchange were measured at five temperatures (4, 7, 12, 17, and 27°C) following each incubation and soil leachates were collected biweekly over the second incubation. Daytime net ecosystem CO₂ exchange (NEE) per unit core surface area was higher across the five measurement temperatures after the warmer incubation (17/12°C > 7/4°C). Responses of ecosystem respiration (ER) were similar at each measurement temperature irrespective of incubation temperature regimes. ER, expressed on a leaf-area basis, however, was significantly lower following the warmer incubation, suggesting a downregulation of ER. Warmer incubation resulted in a greater specific leaf area and N concentration, and a lower δ¹³C in live aboveground C. quitensis, but a higher δ¹³C in D. antarctica, implying species-specific responses to warming. Concentrations of dissolved organic C and N and inorganic N in soil leachates showed that short-term temperature changes had no noticeable effect on soil biogeochemical processes. The results suggest that downregulation of ER, together with plant species differences in leaf-area production and N use, can play a crucial role in constraining the C-cycle response of Antarctic tundra ecosystems to warming.     

Is survival after ice encasement related with sugar distribution in organs of the Antarctic plants <Emphasis Type="Italic">Deschampsia antarctica</Emphasis> Desv. (Poaceae) and <Emphasis Type="Italic">Colobanthus quitensis</Emphasis> (Kunth) Bartl. (Caryophyllaceae)?

Deschampsia antarctica and Colobanthus quitensis are usually covered by snow from April to November. It is unknown whether the leaves survive ice encasement. This study proposes that day length influences sugar distribution in C. quitensis and that sugar accumulation favors re-growths after an ice encasement period. The objectives of this work were: (1) to study the effect of day length and low temperature on sugar distribution in organs of C. quitensis and (2) to study the survival and recovery of D. antarctica and C. quitensis after a period of ice encasement. Extremely short day length (SD) (8/16 h) and long day length (LD) (21/3 h) was used, medium (MD) (16/8 h) corresponding at control day length. Also two temperatures: 4°C (cold acclimated) and 15°C (control) were evaluated. Both factors: day length and cold acclimation significantly affected sugar distribution in C. quitensis. Both species presented a high rate of survival after ice encasement. D. antarctica conserved most of their leaves green, while C. quitensis presented dead leaves and new shoots in plants from cold acclimated under SD. Only in D. antarctica the number of green leaves after ice encasement was positively correlated with sugar content in underground organs. The high sugar content in green leaves of both species suggested fast activity recovery after snow melting.     

Low-temperature limitation of primary photosynthetic processes in Antarctic lichens <Emphasis Type="Italic">Umbilicaria antarctica</Emphasis> and <Emphasis Type="Italic">Xanthoria elegans</Emphasis>

Temperature response curves of chlorophyll a fluorescence parameters were used to assess minimum sub-zero temperature assuring functioning of photosynthetic photochemical processes in photosystem II (PS II) of Antarctic lichens. Umbilicaria Antarctica and Xanthoria elegans were measured within the temperature range from −20 to +10°C by a fluorometric imaging system. For potential (F _V/F _M) and actual (Φ _II) quantum yields of photochemical processes the minimum temperature was found to be between −10 and −20°C. Non-photochemical quenching (NPQ) of absorbed excitation energy increased with temperature drop reaching maximum NPQ at −15°C. Image analysis revealed intrathalline heterogeneity of chlorophyll a fluorescence parameters with temperature drop. Temperature response of Φ _II exhibited an S-curve with pronounced intrathalline differences in X. elegans. The same relation was linear with only limited intrathalline difference in U. antarctica. The results showed that Antarctic lichen species were well adapted to sub-zero temperatures and capable of performing primary photosynthesis at −15°C.     

Temperature response and photoinhibition investigated by chlorophyll fluorescence measurements for four distinct species of dipterocarp trees

The effects of strong light in combination with elevated temperatures on the photosynthetic system were examined in 4 dipterocarp tree species with ecologically different habitats. The 4 dipterocarp tree species were: Shorea platyclados originated from upper dipterocarp forests, Shorea parvifolia– lowland and hill dipterocarp forests, Shorea assamica– lowland dipterocarp forests, and Dipterocarpus oblongifolius– riparian fringes. S. platyclados and D. oblongifolius have higher growth and survival rates in open sites than S. parvifolia and S. assamica. Tolerance of high temperature among the species was assessed by determining the critical temperatures (T_c) at which the minimal fluorescence (F_o) began to rise sharply. This was measured by exposing plants to an increasing temperature of about 1°C min^?1. The intrinsic thermotolerance of the thylakoid membrane appears to be the highest for D. oblongifolius (T_c=46.4°C), intermediate for S. platyclados (45.7°C), and lowest for S. parvifolia and S. assamica (45.2 and 45.3°C, respectively). The temperature‐dependent efficiency of PSII electron transport (ΔF/F′_m), photochemical quenching (q_P), and the efficiency of light capture of open PSII (F′_v/F′_m) were measured at the photosynthetic steady state at least 10 min after the light exposure (180 μmol m^?2 s^?1 PFD). Stable temperature responses of ΔF/F′_m and q_P were observed in S. platyclados and D. oblongifolius, while those in S. parvifolia and S. assamica were more temperature‐dependent and severely affected at 45°C. Little difference was observed in temperature‐dependent F′_v/F′_m among species. Photoinhibitory light exposure (1600 μmol m^?2 s^?1 PFD) for 2 h at 40°C had little effect on the recovery kinetics from photoinhibition of S. platyclados and D. oblongifolius compared with those at 35°C. In contrast, the recovery from photoinhibition was retarded in S. parvifolia and S. assamica. These findings suggest that even at 40°C, a temperature below T_c, an exposure to strong light exacerbated photoinhibition in S. parvifolia and S. assamica corresponding to the closure of PSII reaction centers, as indicated by the decrease in q_P at this temperature. Thus, S. platyclados and D. oblongifolius, which occur at uplands and riparian fringes with frequent disturbances, are suggested to have higher photosynthetic tolerance to elevated temperatures contributing to a circumvention of photoinhibition.     

Growth and reproduction of Antarctic vascular plants in response to warming and UV radiation reductions in the field

Along the west coast of the Antarctic Peninsula springtime ozone depletion events can lead to a two-fold increase in biologically effective UV-B radiation (UV-B_BE) and summer air temperatures have risen ≈1.5°C during the past 50 years. We manipulated levels of UV radiation and temperature around Colobanthus quitensis (a cushion-forming plant, Caryophyllaceae) and Deschampsia antarctica (a tussock grass) along the Peninsula near Palmer Station for two field seasons. Ambient levels of UV were manipulated by placing filters that either transmitted UV (filter control), absorbed UV-B (reducing diurnal levels of UV-B_BE by about 82%), or absorbed both UV-B and UV-A (reducing UV-B_BE and UV-A_BE by about 88 and 78%, respectively) on frames over naturally growing plants from November to March. Half the filters of each material completely surrounded the frames and raised diurnal and diel air temperatures around plants by an average of 2.3°C and 1.3°C, respectively. Reducing UV or warming had no effect on leaf concentrations of soluble UV-B absorbing compounds, UV-B absorbing surface waxes or chlorophylls. Warming had few effects on growth of either species over the first season. However, over the second field season warming improved growth of C. quitensis, leading to a 50% increase in leaf production (P < 0.10), a 26% increase in shoot production, and a 6% increase in foliar cover. In contrast, warming reduced growth of D. antarctica, leading to a 20% decline in leaf length, a 17% decline in leaf production (P < 0.10), and a 5% decline in foliar cover. Warming improved sexual reproduction in both species, primarily through faster development of reproductive structures and greater production of heavier seeds. Over the second field season, the percentage of reproductive structures that had reached the most developed (seed) stage in C. quitensis and D. antarctica was 20% and 15% higher, respectively, under warming. Capsules of C. quitensis produced 45% more seeds under warming and these seeds were 11% heavier. Growth of D. antarctica was improved when UV was reduced and these effects appeared to be cumulative over field seasons. Over the second season, tillers produced 55% more leaves and these leaves were 32% longer when UV-B was reduced. Tillers produced 137% more leaves that were 67% longer when both UV-B and UV-A were reduced. The effects of UV reduction were not as pronounced on C. quitensis, although over the second season cushions tended to be 17% larger and produce 21% more branches when UV-B was reduced, and tended to be 27% larger and produce 38% more branches when both UV-B and UV-A were reduced (P < 0.10). Few interactions were found between UV reduction and warming, although in the absence of warming, reducing UV led to slower development of reproductive structures in both species. The effects of warming and UV reduction were species specific and were often cumulative over the two field seasons, emphasizing the importance of long-term field manipulations in predicting the impacts of climate change. Received: 4 August 1998 / Accepted: 1 December 1998     

Growth at moderately elevated temperature alters the physiological response of the photosynthetic apparatus to heat stress in pea (Pisum sativum L.) leaves

The impact of heat stress on the functioning of the photosynthetic apparatus was examined in pea (Pisum sativum L.) plants grown at control (25 °C; 25 °C-plants) or moderately elevated temperature (35 °C; 35 °C-plants). In both types of plants net photosynthesis (P_n) decreased with increasing leaf temperature (LT) and was more than 80% reduced at 45 °C as compared to 25 °C. In the 25 °C-plants, LTs higher than 40 °C could result in a complete suppression of P_n. Short-term acclimation to heat stress did not alter the temperature response of P_n. Chlorophyll a fluorescence measurements revealed that photosynthetic electron transport (PET) started to decrease when LT increased above 35 °C and that growth at 35 °C improved the thermal stability of the thylakoid membranes. In the 25 °C-plants, but not in the 35 °C-plants, the maximum quantum yield of the photosystem II primary photochemistry, as judged by measuring the F_v/F_m ratio, decreased significantly at LTs higher than 38 °C. A post-illumination heat-induced reduction of the plastoquinone pool was observed in the 25 °C-plants, but not in the 35 °C-plants. Inhibition of P_n by heat stress correlated with a reduction of the activation state of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco). Western-blot analysis of Rubisco activase showed that heat stress resulted in a redistribution of activase polypeptides from the soluble to the insoluble fraction of extracts. Heat-dependent inhibition of P_n and PET could be reduced by increasing the intercellular CO₂ concentration, but much more effectively so in the 35 °C-plants than in the 25 °C-plants. The 35 °C-plants recovered more efficiently from heat-dependent inhibition of P_n than the 25 °C-plants. The results show that growth at moderately high temperature hardly diminished inhibition of P_n by heat stress that originated from a reversible heat-dependent reduction of the Rubisco activation state. However, by improving the thermal stability of the thylakoid membranes it allowed the photosynthetic apparatus to preserve its functional potential at high LTs, thus minimizing the after-effects of heat stress.     

Contrasting thermal acclimation of leaf dark respiration and photosynthesis of Antarctic vascular plant species exposed to nocturnal warming

Leaf respiration and photosynthesis will respond differently to an increase in temperature during night, which can be more relevant in sensitive ecosystems such as Antarctica. We postulate that the plant species able to colonize the Antarctic Peninsula – Colobanthus quitensis (Kunth) Bartl. and Deschampsia antarctica Desv. – are able to acclimate their foliar respiration and to maintain photosynthesis under nocturnal warming to sustain a positive foliar carbon balance. We conducted a laboratory experiment to evaluate the effect of time of day (day and night) and nocturnal warming on dark respiration. Short (E₀ and Q₁₀) and long‐term acclimation of respiration, leaf carbohydrates, photosynthesis (A_sat) and foliar carbon balance (R/A) were evaluated. The results suggest that the two species have differential thermal acclimation respiration, where D. antarctica showed more thermosensitivity to short‐term changes in temperature than C. quitensis. Experimental nocturnal warming affected respiration at daytime differentially between the two species, with a significant increase of R₁₀ and A_sat in D. antarctica, while no changes on respiration were observed in C. quitensis. Long thermal treatments of the plants indicated that nocturnal but not diurnal respiration could acclimate in both species, and to a greater extent in C. quitensis. Non‐structural carbohydrates were related with respiration in C. quitensis but not in D. antarctica, suggesting that respiration in the former species is likely controlled by total soluble sugars and starch during day and night, respectively. Finally, foliar carbon balance was differentially improved under warming conditions in Antarctic plants by different mechanisms, with C. quitensis deploying respiratory acclimation, while D. antarctica increased its A_sat.     

Photosynthesis in cold acclimated leaves of plants with various degrees of freezing tolerance

The objective of this study was to compare the photosynthetic changes during cold acclimation in various plant types able to acquire different degrees of freezing tolerance. Four herbaceous and six woody plants were hardened under natural or artificial conditions and – after determination of their frost resistance (LT₅₀) – the net photosynthetic rate at an ambient CO₂ of 33 Pa (P_n33), the dependencies of P_n to light and to CO₂ and the room temperature chlorophyll a fluorescence were recorded under optimal conditions. Herbaceous plants acquired freezing tolerances to temperatures between ?10 and ?15°C when hardened at temperatures around 0°C. Most leaves fully developed prior to frost hardening exhibited typical symptoms of senescence after frost hardening. In non-senescing leaves P_n33 was reduced by 15 to 50% mainly due to a reduced stomatal conductance. After hardening at temperatures around ?10°C Brassica survived down to ?24°C, but P_n33 was almost abolished as a result of disturbances in the chloroplasts. After transferring the plants to 20/15°C P_n33 recovered completely within a few days. Woody plants hardened at temperatures around 0°C tolerated – 15 to ?36°C: P_n33 was reduced by 25 to 60% and hardly recovered at 20/15°C. Hardening at ?10°C induced a tolerance of ?32 to n33 was almost totally blocked, but at 20/15°C it returned to the values of the plants hardened at 0°C within a few days. In woody plants disturbances were invariably localized in the chloroplasts. Thus, conifers, and especially Pinus cembra, can survive much lower temperatures than herbaceous plants and, at the same level of freezing tolerance, exhibit appreciably less restriction in relative P_n33.     

Resilience of rice (Oryza spp.) pollen germination and tube growth to temperature stress

Resilience of rice cropping systems to potential global climate change will partly depend on the temperature tolerance of pollen germination (PG) and tube growth (PTG). Pollen germination of high temperature‐susceptible Oryza glaberrima Steud. (cv. CG14) and Oryza sativa L. ssp. indica (cv. IR64) and high temperature‐tolerant O. sativa ssp. aus (cv. N22), was assessed on a 5.6–45.4 °C temperature gradient system. Mean maximum PG was 85% at 27 °C with 1488 μm PTG at 25 °C. The hypothesis that in each pollen grain, the minimum temperature requirements (T_n) and maximum temperature limits (T_x) for germination operate independently was accepted by comparing multiplicative and subtractive probability models. The maximum temperature limit for PG in 50% of grains (T_x(50)) was the lowest (29.8 °C) in IR64 compared with CG14 (34.3 °C) and N22 (35.6 °C). Standard deviation (s_x) of T_x was also low in IR64 (2.3 °C) suggesting that the mechanism of IR64's susceptibility to high temperatures may relate to PG. Optimum germination temperatures and thermal times for 1 mm PTG were not linked to tolerating high temperatures at anthesis. However, the parameters T_x(50) and s_x in the germination model define new pragmatic criteria for successful and resilient PG, preferable to the more traditional cardinal (maximum and minimum) temperatures.     

A modern pollen–climate calibration set based on lake sediments from the Tibetan Plateau and its application to a Late Quaternary pollen record from the Qilian Mountains

Aim Fossil pollen spectra from lake sediments on the Tibetan Plateau have been used for qualitative climate reconstruction, but no modern pollen–climate calibration set based on lake sediments is available to infer past climate quantitatively. This study aims to develop such a dataset and apply it to fossil data. Location The Tibetan Plateau, between 30 and 40° N and 87 and 103° E. Methods We collected surface sediments from 112 lakes and analysed them palynologically. The lakes span a wide range of mean annual precipitation (P_ann; 31–1022 mm), mean annual temperature (T_ann; −6.5 to 1 °C), and mean July temperature (T_July; 2.6–19.7 °C). Redundancy analysis showed that the modern pollen spectra are characteristic of their respective vegetation types and local climate. Transfer functions for P_ann, T_ann and T_July were developed with weighted averaging partial least squares. Model performance was assessed by leave-one-out cross-validation. Results The root mean square errors of prediction (RMSEP) were 104 mm (P_ann), 1.18 °C (T_ann) and 1.17 °C (T_July). The RMSEPs, when expressed as percentages of the gradient sampled, were 10.6% (P_ann), 15.7% (T_ann) and 11.9% (T_July). These low values indicate the good performance of our models. An application of the models to fossil pollen spectra covering the last c. 50 kyr yielded realistic results for Luanhaizi Lake in the Qilian Mountains on the north-eastern Tibetan Plateau (modern P_ann 480 mm; T_ann−1 °C). T_ann and P_ann values similar to present ones were reconstructed for late Marine Isotope Stage 3, with minimum values for the Last Glacial Maximum (c. 300 mm and 2 °C below present), and maximum values for the early Holocene (c. 70 mm and 0.5 °C greater than present). Main conclusions The modern pollen–climate calibration set will potentially be useful for quantitative climate reconstructions from lake-sediment pollen spectra from the Tibetan Plateau, an area of considerable climatic and biogeographical importance.     

Effect of incubation temperature on growth parameters of Pseudoalteromonas antarctica NF and its production of extracellular polymeric substancesdagger

Aim: To evaluate the effect of temperature on growth parameters and on extracellular polymeric substance (EPS) production for Pseudoalteromonas antarctica NF₃. Methods and Results: For this purpose, three growth parameters, lag time (λ), maximum growth rate (μ) and maximum population density (A), were calculated with the predictive Gompertz model. To evaluate the variations in μ with respect to temperature, the secondary Arrhenius and the square root models were used. Below the optimal growth temperature (17·5°C), the growth of P. antarctica was separated into two domains at the critical temperature of 12°C. Within the suboptimal domain (12–17·5°C), the temperature characteristic was the lowest (5·29 kcal mol^?1). Growth population densities were maintained over the entire physiological portion assayed (5–17·5°C). Higher crude EPS production was found at temperatures included in the cold domain (5–12°C). Conclusions: All calculated parameters revealed an optimal adaptation of this strain to cold temperatures. Significance and Impact of the Study: The knowledge of the influence of temperature on growth parameters of P. antarctica NF₃ and on EPS production could improve the production of this extracellular polymeric substance that is currently being used in the cosmetic and pharmaceutical industries.     

Protein expression in the obligate hydrocarbon-degrading psychrophile Oleispira antarctica RB-8 during alkane degradation and cold tolerance

In cold marine environments, the obligate hydrocarbon-degrading psychrophile Oleispira antarctica RB-8, which utilizes aliphatic alkanes almost exclusively as substrates, dominates microbial communities following oil spills. In this study, LC–MS/MS shotgun proteomics was used to identify changes in the proteome induced during growth on n-alkanes and in cold temperatures. Specifically, proteins with significantly higher relative abundance during growth on tetradecane (n-C₁₄) at 16°C and 4°C have been quantified. During growth on n-C₁₄, O. antarctica expressed a complete pathway for the terminal oxidation of n-alkanes including two alkane monooxygenases, two alcohol dehydrogenases, two aldehyde dehydrogenases, a fatty-acid-CoA ligase, a fatty acid desaturase and associated oxidoreductases. Increased biosynthesis of these proteins ranged from 3- to 21-fold compared with growth on a non-hydrocarbon control. This study also highlights mechanisms O. antarctica may utilize to provide it with ecological competitiveness at low temperatures. This was evidenced by an increase in spectral counts for proteins involved in flagella structure/output to overcome higher viscosity, flagella rotation to accumulate cells and proline metabolism to counteract oxidative stress, during growth at 4°C compared with 16°C. Such species-specific understanding of the physiology during hydrocarbon degradation can be important for parameterizing models that predict the fate of marine oil spills.     

Thermal sensitivity of white muscle lactate dehydrogenase isolated from a lake trout, (<Emphasis Type="Italic">Salmo trutta</Emphasis>), inhabiting lake Plav,Montenegro

To shed light on thermoadaptive properties of Salmo trutta from lake Plav (Montenegro), we undertook kinetic studies of pyruvate reduction rates and thermal stability analyses of white muscle LDH. We compared these with the data obtained for trout of the same, confirmed by us, Danubian lineage living in rivers and streams of Serbia and Montenegro. We also tested the effect of acclimation in captivity at 4 and 14 °C. The lake trout was of a typical smoltified phenotype (the size, the elongated silver colored body). At physiological substrate concentration, the breaks in the Arrhenius plots (critical temperature - T_c) correlated with acclimation temperatures or habitat water temperatures. Q₁₀ values for temperatures above T_c were close to one, in all cases except 4 °C acclimated trout. At temperatures below T_c Q₁₀ was close to two, except in the case of 14 °C acclimated trout. Lake trout had a highest Q₁₀ values at temperatures below T_c. It was conspicuous that within the entire range of tested temperatures the differences in Q₁₀ resulted from the effect of environmental temperature. Higher Q₁₀ values were obtained with LDH isolated from trout acclimated to 4 °C compared with LDH acclimated to 14 °C. E_a values were much lower at a temperature below T_c compared with temperatures above Tc. Thermal stability of muscle LDH was lower after acclimation to 14 compared to 4 °C, while extremely high thermostability was obtained with the lake trout enzyme. Our data support the concept that T_c values have distinct physiological significance.     

Temperature profiles of cellular growth and exopolysaccharide synthesis by Botryococus braunii Kütz. UC 58

Temperature profiles (range 20–33 °C) were obtained for growth and exopolysaccharide (EPS) biosynthesis of the microalga Botryococcus braunii strain UC 58 under photoautotrophic conditions. The maximum temperature for growth was 32 °C and the temperature dependence of the specific growth rate was described by the Hinshelwood equation based on the Arrhenius relationship. The optimal range of temperatures for growth and extracellular EPS synthesis (25–30 °C) concurred and production of 4.5–5 g l⁻¹ of EPS was obtained routinely, leading to high broth viscosities. Below 23 °C EPS biosynthesis was negligible, although the specific growth rate maintained high values. At supraoptimal temperatures EPS biosynthesis decreased, accompanying the increase in doubling time. The polymers formed at temperatures within the optimal range for production, when dissolved in water, produced solutions (2 gl⁻¹) with the highest viscosity, suggesting that their molecular weight showed the highest values. The degree of polymerization of the EPS synthesized at suboptimal and supraoptimal temperatures was significantly below the values within the optimal range.     

Integumental buffering in an alpine grasshopper

In most freeze tolerant insects, the tolerance of the formation of internal body ice is arrived at by a two‐step process: (S‐1) a period of supercooling of the body fluids that is followed by (S‐2) the freezing event. To date, the necessity of S‐1 remains to be questioned seriously. The present study reports evidence that S‐1 may be almost completely substituted or superseded in large‐bodied insects by integumental buffering. In the New Zealand alpine grasshopper Sigaus australis Hutton, there is a substantial difference between external and body core temperatures at the moment when internal ice nucleation is registered. Using the invagination of the pleural suture as a nondetrimental proxy for the core and the sclerotized postnotum as a measure of surface temperature, comparisons of the temperature of crystallization (T_c) show a highly significant difference (P < 0.001; Kolmogorov–Smirnov test). Proxy core T_c values are in the range from ?0.11 to ?4.78 °C compared with the range of ?4.1 to ?14.2 °C in external proxy T_c values. Although a thermal lag may sometimes be quietly assumed in measurements of T_c, a temperature differential of this size (approximately 6 °C), which is equivalent to the entire supercooling potential of many freeze tolerant insects, is of particular note. These findings have wider application to other large‐bodied insects with similarly well‐developed integumental protection.