Eretmoptera murphyi: pre‐adapted to survive a colder climate

During the late 1960s, larvae of the flightless midge Eretmoptera murphyi Schaeffer were accidentally transferred from the sub‐Antarctic island of South Georgia to Signy Island in the maritime Antarctic. Higher insects are rare in the Antarctic and the introduction and establishment of a new species is an unusual event. The fly has overcome the two major barriers to colonization of the Antarctic by new species: the geographical isolation of the region and its severe climate. Larvae of the flightless midge overwinter in the surface layers of soil on Signy Island where the temperature may fall to below ?10 °C, compared with as little as ?1.5 °C on South Georgia. This suggests the possession of a level of pre‐adaption to colder conditions. Summer‐collected larvae have a supercooling point (SCP or whole body freezing point) of approximately ?5.0 °C but survive experimental exposure to ?13 °C, giving them a level of freeze tolerance. After acclimation at ?4 °C for 4 days, the SCP changes little but the temperature at which 50% of the population would die decreases to lower than ?19 °C. Larvae are also resistant to dehydration. Under experimental conditions of 88% relative humidity at 5 °C, larvae lose water linearly (0.42% h^?1) over the first 30 h but resist further water loss once their water content decreases to approximately 1.4 g g^?1 dry weight. All larvae survive these conditions for the duration of the experiment (55 h). Eretmoptera murphyi is well adapted to survive on Signy Island, and these studies suggest that it has the ability to survive at more extreme locations at higher latitudes if it were to be inadvertently transferred to a suitable habitat.     

Antarctic marine molluscs do have an HSP70 heat shock response

The success of any organism depends not only on niche adaptation but also the ability to survive environmental perturbation from homeostasis, a situation generically described as stress. Although species-specific mechanisms to combat “stress” have been described, the production of heat shock proteins (HSPs), such as HSP70, is universally described across all taxa. Members of the HSP70 gene family comprising the constitutive (HSC70) and inducible (HSP70) members, plus GRP78 (glucose-regulated protein, 78 kDa), a related HSP70 family member, were cloned using degenerate polymerase chain reaction (PCR) from two evolutionary divergent Antarctic marine molluscs (Laternula elliptica and Nacella concinna), a bivalve and a gastropod, respectively. The expression of the HSP70 family members was surveyed via quantitative PCR after an acute 2-h heat shock experiment. Both species demonstrated significant up-regulation of HSP70 gene expression in response to increased temperatures. However, the temperature level at which these responses were induced varied with the species (+6–8°C for L. elliptica and +8–10°C for N. concinna) compared to their natural environmental temperature). L. elliptica also showed tissue-specific expression of the genes under study. Previous work on Antarctic fish has shown that they lack the classical heat shock response, with the inducible form of HSP70 being permanently expressed with an expression not further induced under higher temperature regimes. This study shows that this is not the case for other Antarctic animals, with the two molluscs showing an inducible heat shock response, at a level probably set during their temperate evolutionary past.     

The non-native chironomid Eretmoptera murphyi in Antarctica: erosion of the barriers to invasion

Antarctica is the continent least affected by invasive species, but climate change and increasing human activity are increasing this threat. Antarctic terrestrial ecosystems generally have low biodiversity with simple community structures and little competition for resources. Consequently, species with pre-adaptations or capabilities that allow them to tolerate polar conditions may have disproportionately large ecosystem impacts when introduced to Antarctica compared with other regions of the Earth. Here we investigate the invasion risk associated with the flightless chironomid midge, Eretmoptera murphyi, which was accidentally introduced from South Georgia (54°S) to Signy Island, South Orkney Islands (61°S), probably during plant transplantation experiments in the 1960s. Larval size class distribution analysis indicated that E. murphyi has a 2 year life cycle on Signy Island, supporting previous suggestions. Estimates of litter turnover show that recent large increases in E. murphyi population density and extent are likely to increase nutrient cycling rates on Signy Island substantially. Existing physiological adaptations may allow E. murphyi to colonise higher latitude locations. Growth rate and microhabitat climatic modelling show that temperature constraints on larval development on Anchorage Island (68°S) are theoretically similar to those on Signy Island even though it is ~750 km further south. Establishment of this non-native midge at climatically similar intervening locations along the western Antarctic Peninsula is therefore plausible. Currently, lack of effective natural dispersal mechanisms is probably limiting the spread of the midge. However, dispersal to other areas of the Antarctic Peninsula may occur via human-assisted transportation, highlighting the importance of appropriate biosecurity measures.     

COMPARISONS OF THE BIOLOGY OF THE INTERTIDAL SUBANTARCTIC LIMPETS NACELLA CONCINNA AND KERGUELENELLA LATERALIS

Two limpet species occur intertidally on subantarctic SouthGeorgia, the patellid Nacella concinna and the siphonarlid Kerguelenellalateralis. N. concinna is confined to the lower shore closeto LWS; K. lateralis occurs in middle shore pools, so theirdistributions do not overlap. N. concinna has a much narrowerthermal niche (–12.9°C to +15.6°C) than K. lateralis(–17.8°C to +31.8°C). Environmental data are presentedto show that the upper lethal temperature of N. concinna islow enough to prevent the limpet living higher on the shore.Both limpet species are slow-moving, but K. lateralis showsincreasing speed with rising temperature, peaking at 15–20°C.In contrast, N. concinna moves actively down to –1.9°C(when sea water freezes), but there is a steady decrease inspeed of locomotion above +2°C. Locomotion ceases at 14°Cin N. concinna (c.f. 30°C in K. lateralis). Both speciesexhibit very low tenacities, but in N. concinna tenacity decreaseswith increasing shell length. In K. lateralis there is no effectof temperature on tenacity. Both species show a positive allometricrelationship between foot area and shell length. N. concinnafeeds upon microbial films and microepiflora, but K. lateraliseats colonial diatoms and Enteromorpha bulbosa. Observationson shell middens of the kelp gull Larus dominicanus showed thatthe gulls did not eat K. lateralis, though they ate great quantitiesof the less accessible N. concinna. Gulls ate N. concinna assmall as 11 mm shell length (within the size range of K. lateralis).Experiments on gulls demonstrated an unwillingness to eat K.lateralis, probably because the siphonariid extrudes a viscidwhite mucus when the foot is touched. (Received 9 May 1996; accepted 8 July 1996)     

Growth in the limpet <Emphasis Type="Italic">Nacella concinna</Emphasis> from contrasting sites in Antarctica

Annual shell growth was determined by mark and recapture in the limpet Nacella concinna (Strebel 1908) at two contrasting sites in Antarctica. At Signy Island, 60°S, growth was moderately fast, comparable with some limpets in more temperate areas. The fluorescent calcium marker calcein was used to validate the results from the mark/recapture study, and fine-scale growth increments showed that shell growth was seasonal. Further south at Rothera Point, 67°S, mean annual growth over a 3-year period was significantly slower than at Signy, and in 1 year was the slowest yet reported for a limpet. Comparison with an earlier mark/recapture study at Arthur Harbour, Palmer Station (64°S) revealed a cline of decreasing growth performance with increasing latitude along the Antarctic Peninsula. It is not clear whether the slower annual growth rate at higher latitude was caused by physiological constraints, a reduced length of growing season, or a combination of both. Limpets show a global cline in growth performance, which decreases towards higher latitudes.     

Effects of temperature acclimation on cardiorespiratory performance of the Antarctic notothenioid Trematomus bernacchii

Notothenioid fishes of the Southern Ocean have evolved under cold and stable temperatures for millions of years. Due to rising temperatures in the Southern Ocean, investigating thermal limits and the capacities for inducing a temperature acclimation response in notothenioids has become of increasing interest. Here, we investigated effects of temperature acclimation on cardiorespiratory responses and cardiac and skeletal muscle energy metabolism in a benthic Antarctic notothenioid, Trematomus bernacchii. We acclimated specimens to ?1, 2 and 4.5 °C for 14 days and quantified heart rates and ventilation rates during an acute increase in temperature. Ventilation rates showed an effect of acclimation both at initial steady-state acclimation conditions and during an acute temperature increase, suggesting a partial thermal compensatory response. However, acclimation did not affect heart rates at steady-state acclimation conditions and the temperatures at which onset of cardiac arrhythmia occurred, suggesting lack of inducible thermal tolerance in cardiac performance. Citrate synthase (CS), lactate dehydrogenase (LDH) and 3-hydroxyacyl dehydrogenase activities in skeletal muscle tissues suggested acclimation-induced shifts in metabolic fuel preferences, and a marked increase in LDH activity with acclimation to 4.5 °C showed an increase in anaerobic metabolism. In heart tissue, CS and LDH activities decreased with acclimation to 4.5 °C, suggesting reduced cardiac ATP production. Overall, the data suggest a partial acclimatory response to temperature by T. bernacchii and support the hypothesis that reduced cardiac acclimatory capacity may play a role in limiting the thermal plasticity of T. bernacchii.     

Trace metals in the Antarctic soft-shelled clam Laternula elliptica: implications for metal pollution from Antarctic research stations

The concentrations of Cd, Cr, Cu, Fe, Mn, Ni, Pb and Zn were measured in several soft-tissue types of the Antarctic soft-shelled clam, Laternula elliptica, which had been collected from Rothera Research Station, Adelaide Island, Antarctic Peninsula. Concentrations of Mn, Ni, Cr and Pb were significantly higher in the kidney than in any other soft tissue and highest concentrations of Cu were observed in the digestive gland. In general, the total tissue concentrations of heavy metals in L. elliptica were considered to be at baseline levels, except for Cu in organisms near the end of the runway. Copper concentrations were an order of magnitude greater (357 µg/g dry weight) than baseline levels, suggesting anthropogenic contamination or an unidentified natural source. However, there was no indication of anthropogenic metal contamination occurring close to the sewage outfall at Rothera, which is a significant metal source. The results indicate that L. elliptica is a useful long-term biomonitor of heavy metal contamination in Antarctic coastal waters.     

Exploring Three PIPs and Three TIPs of Grapevine for Transport of Water and Atypical Substrates through Heterologous Expression in aqy-null Yeast

Aquaporins are membrane channels that facilitate the transport of water and other small molecules across the cellular membranes. We examined the role of six aquaporins of Vitis vinifera (cv. Touriga nacional) in the transport of water and atypical substrates (other than water) in an aqy-null strain of Saccharomyces cerevisiae. Their functional characterization for water transport was performed by stopped-flow fluorescence spectroscopy. The evaluation of permeability coefficients (P_f) and activation energies (Ea) revealed that three aquaporins (VvTnPIP2;1, VvTnTIP1;1 and VvTnTIP2;2) are functional for water transport, while the other three (VvTnPIP1;4, VvTnPIP2;3 and VvTnTIP4;1) are non-functional. TIPs (VvTnTIP1;1 and VvTnTIP2;2) exhibited higher water permeability than VvTnPIP2;1. All functional aquaporins were found to be sensitive to HgCl₂, since their water conductivity was reduced (24–38%) by the addition of 0.5 mM HgCl₂. Expression of Vitis aquaporins caused different sensitive phenotypes to yeast strains when grown under hyperosmotic stress generated by KCl or sorbitol. Our results also indicate that Vitis aquaporins are putative transporters of other small molecules of physiological importance. Their sequence analyses revealed the presence of signature sequences for transport of ammonia, boron, CO₂, H₂O₂ and urea. The phenotypic growth variations of yeast cells showed that heterologous expression of Vitis aquaporins increased susceptibility to externally applied boron and H₂O₂, suggesting the contribution of Vitis aquaporins in the transport of these species.     

Effects of moderate high-temperature stress on photosynthesis in three saplings of the constructive tree species of subtropical forest

During the exposition to moderate high-temperature stress, photosynthetic rates and fluorescence of chlorophyll a were measured with a photosynthetic measurement system (Li-Cor 6400) and leaf chamber fluorometer (Li-Cor6400 LCF), respectively, in leaves of saplings, sun-adapted species (Schima superba), shade-adapted species (Cryptocarya concinna), and in mesophytic plant (Castanopsis hystrix) (42°C). The results showed that moderate high-temperature stress led to a decrease in F_v/F_m, namely the primary photochemical quantum efficiency, indicating that moderate high-temperature stress causes a partial inhibition of PSII. It also showed that such an effect was more severe in the shade-adapted plant C. concinna than in the sun-adapted species S. superba. However, except for the sun-grown leaves of C. concinna, the moderate high-temperature stress increased the photosynthetic rate of leaves at high light intensity. Simultaneously, less photoinhibition was found to occur under high-light intensity, suggesting that the capacity of resistant-photoinhibition was stimulated by moderate high-temperature stress. The quantum yield of PSII (ϕ_PSII) decreased in the sun-grown leaves of S. superba and C. hystrix but did not show any significant change in leaves of the shade plant C. concinna and shade-grown leaves of sun plant S. superba or the mesophytic plant C. hystrix because they already had a very low ϕ_PSII under this condition. Moderate high-temperature stress led to a decrease in ϕ_PSII/ϕ_CO2 ratios, an estimate of the quantum requirement for CO₂ assimilation, in the sun plant S. superba and the mesophytic plant C. hystrix because they were associated with the dissipation of a lower fraction of excitation energy. However, no significant changes were found in shade plant C. concinna and in shade-grown leaves of the other examined plants. The effect of moderate high-temperature (42°C) on photosynthesis depends on species and leaf type (sun and shade leaves) in the saplings of subtropical broad-leaved trees.     

Can acclimation of thermal tolerance,in adults and across generations,act as a buffer against climate change in tropical marine ectotherms?

Thermal acclimation capacity was investigated in adults of three tropical marine invertebrates, the subtidal barnacle Striatobalanus amaryllis, the intertidal gastropod Volegalea cochlidium and the intertidal barnacle Amphibalanus amphitrite. To test the relative importance of transgenerational acclimation, the developmental acclimation capacity of A. amphitrite was investigated in F₁ and F₂ generations reared at a subset of the same incubation temperatures. The increase in CT_max (measured through loss of key behavioural metrics) of F₀ adults across the incubation temperature range 25.4–33.4 °C was low: 0.00 °C (V. cochlidium), 0.05 °C (S. amaryllis) and 0.06 °C (A. amphitrite) per 1 °C increase in incubation temperature (the acclimation response ratio; ARR). Although the effect of generation was not significant, across the incubation temperature range of 29.4–33.4 °C, the increase in CT_max in the F₁ (0.30 °C) and F₂ (0.15 °C) generations of A. amphitrite was greater than in the F₀ (0.10 °C). These correspond to ARR's of 0.03 °C (F₀), 0.08 °C (F₁) and 0.04 °C (F₂), respectively. The variability in CT_max between individuals in each treatment was maintained across generations, despite the high mortality of progeny. Further research is required to investigate the potential for transgenerational acclimation to provide an extra buffer for tropical marine species facing climate warming.     

Effects of ambient temperature and of temperature acclimation on nitrogen excretion and differential catabolism of protein and nonprotein resources in the intertidal snails,Littorina saxatilis (Olivi) and L. obtusata (L.)

Nitrogenous excretion in two snails, Littorina saxatilis (high intertidal) and L. obtusata (low intertidal) was studied in relation to temperature acclimation (at 4° and 21°C), including total N excretion rates, the fraction of urea in N excretion, corresponding O:N ratios and the partitioning of deaminated protein between catabolic and anabolic processes at 4°, 11° and 21°C. Aggregate N excretion rates in both species showed no significant compensatory adjustments following acclimation. Total weight specific N excretion rates at 21°C were higher in standard 3 mg L. saxatilis (739 ng N mg⁻¹ h⁻¹) than standard 5 mg L. obtusata (257 ng N mg⁻¹ h⁻¹) for snails acclimated to 21°C. Comparisons of Q₁₀ values of total weight specific N excretion to Q₁₀ values for weight specific oxygen consumption ({xxV}O₂) between 4° to 11 °C and 11° to 21°C indicated that, while total rates of catabolic metabolism ({xxV}O₂) and protein deamination in L. obtusata were essentially parallel, the relationship between N excretion and {xxV}O₂ in L. saxatilis revealed the partitioning of a larger share of deaminated protein carbon into anabolism at 4° and 21°C than at 11°C. Urea N accounted for a larger share of aggregate N excreted in L. saxatilis than in L. obtusata, but in both species urea N is a greater proportion of total N excreted when acclimated at 4°C (urea N: ammonia N ratio range: 1 to 2.15) than in snails acclimated to 21°C (urea N: ammonia N ratio range: 0.46 to 1.39). Molar O:N ratios indicate that the proportion of metabolism supported by protein catabolism is greater in L. saxatilis (O:N range: 2.5–8.4) than in L. obtusata (O:N range: 7.3–13.0). In both species, regardless of acclimation temperature, the O:N ratios are generally lowest (high protein catabolism) at 4°C and highest at 21°C.     

Development and characterization of a catalytically inactive cysteine protease domain of RtxA1/MARTXVv as a potential vaccine for Vibrio vulnificus

Recent studies have defined several virulence factors as vaccine candidates against Vibrio vulnificus. However, most of these factors have the potential to cause pathogenic effects in the vaccinees or induce incomplete protection. To overcome these drawbacks, a catalytically inactive form, CPD_Vv(C3725S), of the well‐conserved cysteine protease domain (CPD) of V. vulnificus multifunctional autoprocessing repeats‐in‐toxin (MARTX_Vv/RtxA1) was recombinantly generated and characterized. Notably, active and passive immunization with CPD_Vv(C3725S) conferred protective immunity against V. vulnificus strains. These results may provide a novel framework for developing safe and efficient subunit vaccines and/or therapeutics against V. vulnificus that target the CPD of MARTX toxins.     

Behavioural thermoregulation and critical thermal limits of Macrobrachium acanthurus (Wiegman)

• (1)The preferred temperatures of Macrobrachium acanthurus were determined for prawns acclimated to 20°C, 23°C, 26°C, 29°C and 32°C, and the final preferendum estimate was (29.5°C).
• (2)The critical thermal minima (CTMin) and maxima (CTMax) were 11.0°C, 12.1°C, 13.0°C and 14.8°C, and 34.2°C, 35.0°C, 36.1°C and 39.8°C, respectively.
• (3)The zone of thermal tolerance assessed using the CTMin and CTMax boundaries was 644°C².
• (4)The acclimation response ratio was between 0.33 and 0.62.
• (5)To cultivate this species in the southeastern region of México it should be done in not <15°C (CTMin) during the winter and below 38°C in summer (CTMax).

     

Duration tenacity: A method for assessing acclimatory capacity of the Antarctic limpet, Nacella concinna

The limpet, Nacella concinna, collected from the Antarctic Peninsula (67°S), was incubated at − 0.3 °C and 2.9 °C for 9 months to test if the previously reported absence of acclimation capacity in Antarctic marine ectotherms could be due to the extended time it takes for them to adjust their physiology to a new stable state. Acclimation was tested through acute measurements of upper lethal limit and a modified measure of tenacity, that tested muscle capacity by measuring the length of time that N. concinna were able to remain attached to the substratum at different temperatures. Both measures acclimated in response to incubation to the higher temperature. Lethal limits were elevated in N. concinna incubated at 2.9 °C (8.1 ± 0.3 °C) compared to those incubated at − 0.3 °C (6.9 ± 0.4 °C). 2.9 °C incubated N. concinna also had a maximum tenacity at 2.1 °C, a higher temperature than the maximum tenacity of those incubated at − 0.3 °C, which occurred at − 1.0 °C. This study is the first to show that the Antarctic limpet can acclimate its physiology, but that it requires a greater period of time for acclimation to occur than previous studies have allowed for.     

Involvement of nitric oxide in acquired thermotolerance of rice seedlings

The role of nitric oxide (NO) in thermotolerance acquired by heat acclimation (38°C) was investigated. Results showed that 38°C acclimation, on the one hand, obviously reduced hydrogen peroxide (H₂O₂) and MDA contents and ion leakage degree in rice leaves; however, on the other hand, it increased the survival of rice (Oryza sativa L.) seedlings under 50°C heat stress. Application of nitric oxide donor, sodium nitroprusside (SNP), prior to 38°C acclimation dramatically increased the acquired thermotolerance. To elucidate the role of endogenous NO in acquired thermotolerance, 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (PTIO, a specific NO scavenger) was used (scavengers are used to control the level of both exogenous and endogenous NO). Results showed that PTIO pretreatment resulted in the elimination of acquired thermotolerance induced by 38°C acclimation in rice seedlings. Nitric oxide (NO) release measurement indicated that there was indeed an abrupt elevation in the NO content in 40 min after 38°C acclimation, proving the involvement of NO in acquired thermotolerance inducement in rice seedling.     

Critical thermal maximum of seven estuarine fishes

Critical thermal maximum (CTM) and loss of righting response (LRR) were determined in seven estuarine fishes. The critical thermal maxima (CTM) ranged from 39.5°C to 44.5°C for fishes acclimated to 28°C. Lates calcarifer and Liza dussumeri had the highest CTM (44.5°C) and Siganus javus had the lowest CTM (39.5°C). The rate of change of CTM due to thermal acclimation was determined for Etroplus suratensis and Therapon jarbua. The CTM for E.suratensis increased from 39.75°C to 43.5°C on a 15°C increase in acclimation temperature and the CTM of T. jarbua shifted from 40.75°C to 43.15°C on a 15°C increase in acclimation. The acclimation response ratio (ARR) as a result of increasing the acclimation temperature was about 0.25 in E. suratensis and 0.2 in T. jarbua.     

The distribution,growth, and reproduction of the Antarctic limpet Nacella (Patinigera) concinna (Strebel, 1908)

Twenty-one monthly collections of the Antarctic limpet Nacella (Patinigera) concinna (Strebel, 1908) were obtained by divers at Signy Island, South Orkney Islands. A mean monthly population density of 123.7 ± 21.2 · m^?2, mean biomass of 13.7 ± 2.7 g dry tissue wt · m^?2, and annual production of 2.9 g · m^?2 were recorded in the depth range 2–12 m below mean low water.Shell growth was slow with a maximum growth rate, in the first 3–5 yr of life, of 3 4 mm per year. Maturity was attained at 7–8 yr (21 mm length), and maximum size (41 mm length) at about 21 yr. Unique spawning behaviour was observed in two Austral springs, and data relating spawning to the spring increase in sea temperature were obtained.     

Synthesis,characterization and bioactivity of four novel trinuclear copper(II) and nickel(II) complexes with pentadentate ligands derived from N-acylsalicylhydrazide

Four novel trinuclear copper(II)/nickel(II) complexes with four trianionic pentadentate ligands, N-(3-t-butylbenzoyl)-5-nitrosalicylhydrazide (H₃3-t-bbznshz), N-(3,5-dimethylbenzoyl)salicylhydrazide (H₃3,5-dmbzshz), N-(phenylacetyl)-5-bromosalicylhydrazide (H₃pabshz) and N-(3-t-butylbenzoyl)salicylhydrazide (H₃3-t-bbzshz) have been synthesized and characterized by X-ray crystallography. These trinuclear compounds all have an M–N–N–M–N–N–M core formed by three metal ions and two ligands. The geometries of three Cu(II) ions in compound Cu₃(3-t-bbznshz)₂(H₂O)(DMF)(py)₂ · DMF (1) alternate between distorted square pyramidal and square planar, while in compound Cu₃(3,5-dmbzshz)₂(py)₂ (2), they are all square planar. Three Ni(II) ions in compound Ni₃(pabshz)₂(DMF)₂(py)₂ (3) and Ni₃(3-t-bbzshz)₂(py)₄ · 2H₂O (4) follow square-planar/octahedral/square-planar coordination geometry. Compounds 1, 2 and 4 are bent trinuclear, with the bend angles of 156.4°, 141.49° and 127.1°, respectively, while the three nickel ions in compound 3 are strictly linear, with an angle of 180°. Studies on the trinuclear Ni(II) complexes show that the β-branched N-acylsalicylhydrazide ligands with sterically flexible Cα methylene groups are easier to yield linear trinuclear Ni(II) complexes, while α-branched N-acylsalicylhydrazides ligands tend to form bent trinuclear Ni(II) complexes. Antibacterial screening data indicate that the trinuclear Cu(II) compound 2 is more active than 1 and mononuclear Cu(II) compound, bent trinuclear Ni(II) compound 4 is more active than linear compound 3 and less active than tetranuclear nickel compound in the previous study.     

Acclimation effects of heat waves and elevated [CO2] on gas exchange and chlorophyll fluorescence of northern red oak (Quercus rubra L.) seedlings

Heat wave frequency and intensity are predicted to increase. We investigated whether repeated exposure to heat waves would induce acclimation in Quercus rubra seedlings and considered [CO₂] as an interacting factor. We measured gas exchange and chlorophyll fluorescence of seedlings grown in 380 (C _A) or 700 (C _E) μmol CO₂ mol^?1, and three temperature treatments (ambient, ambient +3 °C, and an ambient +12 °C heat wave every fourth week). Measurements were performed during the third and fourth +12 °C heat waves (July and August 2010) at Whitehall Forest, GA, USA. Additionally, previously unexposed seedlings were subjected to the August heat wave to serve as a control to determine acclimation of seedlings which were previously exposed. Seedlings with a history of heat wave exposure showed lower net photosynthesis (A _net) and stomatal conductance (on average ?47 and ?38 %, respectively) than seedlings with no such history, when both were subjected to the same +12 °C heat wave. During both heat waves, A _net significantly declined in the +12 °C treatment compared with the other treatments. Additionally, the A _net decline during the August compared with the July heat wave was stronger in C _E than in C _A, suggesting that elevated [CO₂] might have had a negative effect on acclimation capacity. We conclude that seedlings subjected to consecutive heat waves will moderate stomatal conductance outside the heat wave, to reduce water usage at lower temperatures, increasing survival at the expense of carbon assimilation.     

ECOPHYSIOLOGY OF TROPICAL RHODOPHYTES. II. MICROSCALE ACCLIMATION IN PHOTOSYNTHESIS1

Parameters of photosynthesis vs. irradiance curves varied markedly between tissues from microsites along the < 10-cm axes of the tropical intertidal red algae Ahnfeltiopsis concinna (J. Ag.) Silva et DeCew and Laurencia mcdermidiae (J. Ag.) Abbott. Differences in photosynthetic performance between tissues from canopy and understory microsites indicates that L. mcdermidiae exhibited an expected sun-to-shade acclimation but over the space of < 10 cm. Respiration, I_c, I_k, and P_max values were significantly lower in tissues from the understory relative to tissues from the canopy of L. mcdermidiae, while photosystem I (PS I) sizes (PSU I) were significantly higher in tissues from understory microsites. Quantum efficiency was unchanged. Ahnfeltiopsis concinna, in contrast, exhibited higher α in tissues from understory rather than canopy microsites. The values of P_max for tissues from the canopy of A. concinna were not higher than the understory, while PSU O₂(PS II size) of tissues from canopy microsites were unusually higher than those of understory microsites. These characteristics suggest a high degree of irradiance stress in tissues from the canopy of A. concinna, the highest tidal alga in Hawaiian coastal zones. Acclimation to high photosynthetically active radiation and ultraviolet irradiance levels especially in tropical regions appears to be an essential mechanism(s) for stress resistance and persistence of intertidal algae. Algal turfs acclimate at microscales in part fostered by their dense stands that create sharp irradiance gradients as well as adjust physiologically to canopy irradiance levels as mechanisms for optimal photosynthetic performance and stress tolerance.