AUTOTROPHIC GROWTH AND PHOTOACCLIMATION IN KARLODINIUM MICRUM (DINOPHYCEAE) AND STOREATULA MAJOR (CRYPTOPHYCEAE)1

We compared autotrophic growth of the dinoflagellate Karlodinium micrum (Leadbeater et Dodge) and the cryptophyte Storeatula major (Butcher ex Hill) at a range of growth irradiances (E_g). Our goal was to determine the physiological bases for differences in growth–irradiance relationships between these species. Maximum autotrophic growth rates of K. micrum and S. major were 0.5 and 1.5 div.·d^?1, respectively. Growth rates were positively correlated with C‐specific photosynthetic performance (PP^C, g C·g C^?1·h^?1) (r²=0.72). Cultures were grouped as light‐limited (LL) and high‐light (HL) treatments to allow interspecific comparisons of physiological properties that underlie the growth–irradiance relationships. Interspecific differences in the C‐specific light absorption rate (E_a^C, mol photons·g C^?1·h^?1) were observed only among HL acclimated cultures, and the realized quantum yield of C fixation (φ_C(real.), mol C·mol photons^?1) did not differ significantly between species in either LL or HL treatments. The proportion of fixed C that was incorporated into new biomass was lower in K. micrum than S. major at each E_g, reflecting lower growth efficiency in K. micrum. Photoacclimation to HL in K. micrum involved a significant loss of cellular photosynthetic capacity (P_max^cell), whereas in S. major, P_max^cell was significantly higher in HL acclimated cells. We conclude that growth rate differences between K. micrum and S. major under LL conditions relate primarily to cell metabolism processes (i.e. growth efficiency) and that reduced chloroplast function, reflected in PP^C and photosynthesis–irradiance curve acclimation in K. micrum, is also important under HL conditions.     

Light History Influences the Response of Fluvial Biofilms to Zn Exposure

Fluvial biofilms are subject to multistress situations in natural ecosystems, such as the co‐occurrence of light intensity changes and metal toxicity. However, studies simultaneously addressing both factors are rare. This study evaluated in microcosm conditions the relationship between short‐term light intensity changes and Zn toxicity on fluvial biofilms with long‐term photoacclimation to different light conditions. Biofilms that had long‐term photoacclimation to 25 μmol photons · m^?2 · s^?1 (low light [LL] biofilms), 100 μmol photons · m^?2 · s^?1 (medium light [ML] biofilms), and 500 μmol photons · m^?2 · s^?1 (high light [HL] biofilms) were characterized by different structural (Chlorophyll‐a [Chl‐a], total biomass‐AFDW, EPS, algal groups, and diatom taxonomy) and physiological attributes (ETR‐I curves and photosynthetic pigments). HL biofilms showed higher light saturation intensity and a higher production of xanthophylls than LL biofilms. In contrast, LL biofilms had many structural differences; a higher proportion of diatoms and lower AFDW and EPS contents than ML and HL biofilms. A clear effect of light intensity changes on Zn toxicity was also demonstrated. Zn toxicity was enhanced when a sudden increase in light intensity also occurred, mainly with LL biofilms, causing higher inhibition of both the Φ′_PSII and the Φ_PSII. A decoupling of NPQ from de‐epoxidation reaction (DR) processes was also observed, indicating substantial damage to photoprotective mechanisms functioning in biofilms (i.e., xanthophyll cycle of diatoms) due to Zn toxicity. This study highlights the need to take into account environmental stress (e.g., light intensity changes) to better assess the environmental risks of chemicals (e.g., metals).     

THE RELATIONSHIP OF LIPIDS WITH LIGHT AND CHLOROPHYLL MEASUREMENTS IN FRESHWATER ALGAE AND PERIPHYTON1

Lipid content and lipid class composition were determined in stream periphyton and the filamentous green algae Cladophora sp. and Spirogyra sp, Sterols and phospholipids were compared to chlorophyll a (chl a) as predictors of biomass for stream periphyton and algae. Chlorophyll a, phospholipids, and sterols were each highly correlated with ash-free dry mass (AFDM) (r² > 0.98). Stream periphyton exposed naturally to high light (HL) and low light (LL) had chl a concentrations (μg chl a-mg^?1AFDM) of 7.9± 0.7 and 12.4 ± 2.9, respectively, while the sterol concentrations of these HL and LL stream periphyton (1.6 ± 0.4) were not significantly different (P > 0.05). Periphyton exposed to an irradiance of 300 μmol photons·m^?2s^?1 in the laboratory for 60 h had 5.6 ± 0.55 μg chl a·mg^?1 AFDM, but the same periphyton exposed to 2% incident light for the same amount of time had 11.0 ± 0.56 μg chl a·mg^?1 AFDM. Sterol concentrations in these periphyton communities remained unchanged (1.5 ± 0.3 μg·mg^?1AFDM), Similar results (i.e. changes in chl a but stability of sterol concentrations in response to irradiance changes) were also found for Cladophora and Spirogyra in laboratory experiments. Sterols can be quantified rapidly from a few milligrams of algae and appear to be a useful predictor of eukaryote biomass, whereas cellular levels of chl a vary substantially with light conditions. Phospholipids (or phospholipid fatty acids) are considered to be a reliable measure of viable microbial biomass. Nevertheless, phospholipid content varied substantially and unpredictably among algae and periphyton under different light regimes. Irradiance also had a significant effect on storage lipids: HL Cladophora and HL periphyton had 2 × and 5 × greater concentrations of triacylglycerols, respectively, compared to their LL forms. HL and LL algae also differed in the concentration of several major fatty acids. These light-induced changes in algal lipids and fatty acids have important implications for grazers.     

Winter photoinhibition in needles of <Emphasis Type="Italic">Taxus baccata</Emphasis> seedlings acclimated to different light levels

Seasonal variability of maximum quantum yield of PSII photochemistry (F_v/F_m) was studied in needles of Taxus baccata seedlings acclimated to full light (HL, 100% solar irradiance), medium light (ML, 18% irradiance) or low light (LL, 5% irradiance). In HL plants, F_v/F_m was below 0.8 (i.e. state of photoinhibition) throughout the whole experimental period from November to May, with the greatest decline in January and February (when F_v/F_m value reached 0.37). In ML seedlings, significant declines of F_v/F_m occurred in January (with the lowest level at 0.666), whereas the decline in LL seedlings (down to 0.750) was not significant. Full recovery of F_v/F_m in HL seedlings was delayed until the end of May, in contrast to ML and LL seedlings. F_v/F_m was significantly correlated with daily mean (T _mean), maximal (T _max) and minimal (T _min) temperature and T _min was consistently the best predictor of F_v/F_m in HL and ML needles. Temperature averages obtained over 3 or 5 days prior to measurement were better predictors of F_v/F_m than 1- or 30-day averages. Thus our results indicate a strong light-dependent seasonal photoinhibition in needles of T. baccata as well as suggest a coupling of F_v/F_m to cumulative temperature from several preceding days. The dependence of sustained winter photoinhibition on light level to which the plants are acclimated was further demonstrated when plants from the three light environments were exposed to full daylight over single days in December, February and April and F_v/F_m was followed throughout the day to determine residual sensitivity of electron transport to ambient irradiance. In February, the treatment revealed a considerable midday increase in photoinhibition in ML plants, much less in HL (already downregulated) and none in LL plants. This suggested a greater capacity for photosynthetic utilization of electrons in LL plants and a readiness for rapid induction of photoinhibition in ML plants. Further differences between plants acclimated to contrasting light regimes were revealed during springtime de-acclimation, when short term regeneration dynamics of F_v/F_m and the relaxation of nonphotochemical quenching (NPQ) indicated a stronger persistent thermal mechanism for energy dissipation in HL plants. The ability of Taxus baccata to sustain winter photoinhibition from autumn until late spring can be beneficial for protection against an excessive light occurring together with frosts but may also restrict photosynthetic carbon gain by this shade-tolerant species when growing in well illuminated sites.     

Photosynthetic acclimation in relation to nitrogen allocation in cucumber leaves in response to changes in irradiance

Leaves deep in canopies can suddenly be exposed to increased irradiances following e.g. gap formation in forests or pruning in crops. Studies on the acclimation of photosynthesis to increased irradiance have mainly focused on the changes in photosynthetic capacity (A_max), although actual irradiance often remains below saturating level. We investigated the effect of changes in irradiance on the photosynthesis irradiance response and on nitrogen allocation in fully grown leaves of Cucumis sativus. Leaves that fully developed under low (50 µmol m^?2 s^?1) or moderate (200 µmol m^?2 s^?1) irradiance were subsequently exposed to, respectively, moderate (LM‐leaves) or low (ML‐leaves) irradiance or kept at constant irradiance level (LL‐ and MM‐leaves). Acclimation of photosynthesis occurred within 7 days with final A_max highest in MM‐leaves, lowest in LL‐leaves and intermediate in ML‐ and LM‐leaves, whereas full acclimation of thylakoid processes underlying photosystem II (PSII) efficiency and non‐photochemical quenching occurred in ML‐ and LM‐leaves. Dark respiration correlated with irradiance level, but not with A_max. Light‐limited quantum efficiency was similar in all leaves. The increase in photosynthesis at moderate irradiance in LM‐leaves was primarily driven by nitrogen import, and nitrogen remained allocated in a similar ratio to Rubisco and bioenergetics, while allocation to light harvesting relatively decreased. A contrary response of nitrogen was associated with the decrease in photosynthesis in ML‐leaves. Net assimilation of LM‐leaves under moderate irradiance remained lower than in MM‐leaves, revealing the importance of photosynthetic acclimation during the leaf developmental phase for crop productivity in scenarios with realistic, moderate fluctuations in irradiance that leaves can be exposed to.     

Leaf age as a factor in anatomical and physiological acclimative responses of Taxus baccata L. needles to contrasting irradiance environments

To evaluate the acclimative ability of current-year and previous-year needles of a shade tolerant conifer Taxus baccata L. to contrasting irradiance conditions, seedlings were raised under 27% solar irradiance and at 3 years of age they were transferred to an experimental garden and grown for one season under full irradiance (HL), 18% irradiance (ML) or 5% irradiance (LL). Whereas previous year needles did not change anatomically, current year needles in HL were thicker and had a thicker palisade and spongy mesophyll, and greater leaf mass per area than ML or LL needles. LL needles had greater nitrogen concentration than HL needles irrespective of age but only previous year LL needles also had an increased N per area content, thanks to their lack of reduction in LMA. Adjustment of chlorophyll and carotenoid content occurred in both needle age classes with LL and ML needles having much higher concentrations but, in current year needles, only slightly higher per area content than HL needles. Chlorophyll a/b ratio was not affected by age or irradiance. These modifications had no significant effect on photosynthetic capacities, which did not significantly differ between the age classes in HL or LL treatment and between treatments. On the other hand, high growth irradiance resulted in a greater photochemical yield, photochemical quenching, apparent electron transport rate and inducible non-photochemical quenching in needles formed in the current season. In previous year needles, however, only inducible NPQ was enhanced by high irradiance with other parameters remaining identical among treatments. To test sensitivity to photoinhibition, at the end of the summer plants from the three irradiance levels were transferred to a HL situation and F _v/F _M was determined over the following 18 days. Sensitivity to photoinhibition was negatively related to growth irradiance and previous year needles were less photoinhibited than current year needles. Thus, differences in acclimation ability between needle age classes were most pronounced at the level of anatomy and light reactions of photosynthesis, both of which showed almost no plasticity in previous year needles but were considerably modified by irradiance in current year needles.     

Impact of irradiance on the C allocation in the coastal marine diatom Skeletonema marinoi Sarno and Zingone

Elemental stoichiometry and organic composition were investigated in an Adriatic strain of Skeletonema marinoi, cultured at 25 [low light (LL)] and 250 [high light (HL)]µmol photon m^?2 s^?1. Inorganic carbon acquisition, fixation and allocation, and silicic acid and orthophosphate uptake were also studied. The C : P ratio was below the Redfield ratio, especially at LL. In HL cells, N quota was halved, C quota was similar, silica quota was lower, growth rate and long‐term net primary productivity were almost doubled, relative to LL cells. The HL : LL cell quota ratios were 6 for lipid, 0.5 for protein and 0.4 for carbohydrate. Phosphoenolpyruvate carboxylase (PEPc) and glutamine synthetase (GS) activities were unaffected by the growth irradiance; phosphoenolpyruvate carboxykinase (PEPck) was 2.5‐fold more active in LL cells. This suggests that in S. marinoi, C₄ photosynthesis is unlikely, PEPc is anaplerotic and PEPck may be involved in the conversion of lipid C to carbohydrates, especially in LL cells. Because about 50% of the cost for the production of an HL cell is caused by lipid biosynthesis, we propose that the preferential allocation of C to lipid at HL takes advantage of the relatively high volume‐based energy content of lipids, in an organism that reduces its size at each vegetative cell division.     

PRODUCTION AND NUTRIENT REMOVAL BY PERIPHYTON GROWN UNDER DIFFERENT LOADING RATES OF ANAEROBICALLY DIGESTED FLUSHED DAIRY MANURE1

Growing algae to scrub nutrients from manure presents an alternative to the current practice of land application and provides utilizable algal biomass as an end product. The objective of this study was to assess algal growth, nutrient removal, and nitrification using higher light intensities and manure loading rates than in the previous experiments. Algal turfs, with periphyton mainly composed of green algal species, were grown under two light regimes (270 and 390 μmol photons·m^?2· s^?1) and anaerobically digested flushed dairy manure wastewater (ADFDMW) loading rates ranging from 0.8 to 3.7 g total N and 0.12 to 0.58 g total P·m^?2·d^?1. Filamentous cyanobacteria (Oscillatoria spp.) and diatoms (Navicula, Nitzschia, and Cyclotella sp.) partially replaced the filamentous green algae at relatively high ADFDMW loading rates and more prominently under low incident light. Mean algal production increased with loading rate and irradiance from 7.6±2.71 to 19.1±2.73 g dry weight· m^?2·d^?1. The N and P content of algal biomass generally increased with loading rate and ranged from 2.9%–7.3% and 0.5%–1.3% (by weight), respectively. Carbon content remained relatively constant at all loading rates (42%–47%). The maximum removal rates of N and P per unit algal biomass were 70 and 13 mg·g^?1 dry weight·m^?2·d^?1, respectively. Recovery of nutrients in harvested algal biomass accounted for about 31%–52% for N and 30%–59% for P. Recovery of P appeared to be uncoupled with N at higher loading rates, suggesting that algal potential for accumulation of P may have already been saturated. It appears that higher irradiance level enhancing algal growth was the overriding factor in controlling nitrification in the algal turf scrubber units.     

LIGHT‐INDUCED RESPONSES OF OXYGEN PHOTOREDUCTION,REACTIVE OXYGEN SPECIES PRODUCTION AND SCAVENGING IN TWO DIATOM SPECIES1

Diatoms are frequently exposed to high light (HL) levels, which can result in photoinhibition and damage to PSII. Many microalgae can photoreduce oxygen using the Mehler reaction driven by PSI, which could protect PSII. The ability of Nitzschia epithemioides Grunow and Thalassiosira pseudonana Hasle et Heimdal grown at 50 and 300 μmol photons · m^?2 · s^?1 to photoreduce oxygen was examined by mass spectrometric measurements of ¹⁸O₂. Both species exhibited significant rates of oxygen photoreduction at saturating light levels, with cells grown in HL exhibiting higher rates. HL‐grown T. pseudonana had maximum rates of oxygen photoreduction five times greater than N. epithemoides, with 49% of electrons transported through PSII being used to reduce oxygen. Exposure to excess light (1,000 μmol photons · m^?2 · s^?1) produced similar decreases in the operating quantum efficiency of PSII (F_q′/F_m′) of low light (LL)‐ and HL‐grown N. epithemoides, whereas HL‐grown T. pseudonana exhibited much smaller decreases in F_q′/F_m′ than LL‐grown cells. HL‐grown T. pseudonana and N. epithemioides exhibited greater superoxide and hydrogen peroxide production, higher activities (in T. pseudonana) of superoxide dismutase (SOD) and ascorbate peroxidase (APX), and increased expression of three SOD‐ and one APX‐encoding genes after 60 min of excess light compared to LL‐grown cells. These responses provide a mechanism that contributes to the photoprotection of PSII against photodamage.     

Plant Growth Under High Radiant Energy Fluxes

Radiant energy flux density equivalent to peak midday mid-summersunlight was simulated in an artificially lit controlled environmentchamber. High pressure multivapour discharge lamps togetherwith tungsten iodide lamps, installed to give 5·1 kWm^–2 of plant growing space, produced a mean photosyntheticallyactiveradiant energy flux density of 385 W m^–2, 2 m below thethermal barrier, during the course of the experiments. Maize, sorghum, paspalum, ryegrass and soya bean were grownfor 28 days from early seedling development at two temperatures(27·5/22·5 °C, 17·5/12·5 °C,day/night) and three irradiance levels simulating one quarter(LL), one-half (ML) and full (HL) peak mid-summer sunlight over12 h days. Shoot d. wt increased markedly between the LL and ML treatmentsbut only slightly, if at all, between the ML and HL treatments.In comparison with ML treatment plants, those from the HL treatmenthad shorter main stems, more tillers and fewer, thicker mainshoot leaves. Net assimilation rate increased and leaf arearatio decreased with increasing irradiance. As a consequence,relative growth rate did not increase when irradiance exceededthe ML treatment levels. These results suggest that irradiance levels of approximatelyhalf-full daylight values are adequate for most routine controlledenvironment studies where there is little mutual shading betweenor within plants. Major laboratories must, however, providehigh irradiance lighting systems to meet the needs of specialiststudies. Controlled environment, high radiant energy, plant growth, net assimilation rate     

INORGANIC CARBON REPLETION CONSTRAINS STEADY‐STATE LIGHT ACCLIMATION IN THE CYANOBACTERIUM SYNECHOCOCCUS ELONGATUS1

Cyanobacteria show high metabolic plasticity by re‐allocating macromolecular resources in response to variations in both environmental inorganic carbon (Ci) and light. We grew cultures of the picoplanktonic cyanobacterium Synechococcus elongatus Nägeli across a 50‐fold range of growth irradiance at either a dissolved [Ci] <0.1 mM, sufficient to induce strongly the carbon‐concentrating mechanism (CCM) or a dissolved [Ci] of ～4 mM, sufficient to strongly induce the CCM to basal constitutive activity. There was no detectable growth cost of acclimation to low Ci across the entire range of irradiance and growth was nearly light saturated at 50 l mol photons·m^?2·s^?1. Cells acclimated to low Ci significantly re‐allocated macromolecular resources to support their CCM, while maintaining near homeostatis of metabolic flux per unit photosynthetic complex. Changing growth irradiance also drove re‐organization of the photosynthetic machinery to balance excitation flux and metabolic demands, but flux per complex varied widely across the range of tolerable growth irradiances. Across the range of growth irradiance, low Ci cells had significantly less phycocyanin than high Ci cells, which corresponded to a lower PSII absorbance capacity. Furthermore, low Ci cells maintained more PSI per cell^?1 than high Ci cells under high growth irradiance. Low Ci cells could therefore maintain more of their PSII reaction centers open at high growth irradiance than could high Ci cells, which experienced a significant PSII closure. Thus, acclimation to growth under high available Ci actually constrained acclimation to high light by restricting electron transport downstream from PSII in S. elongatus.     

DETECTION OF DIATOM XANTHOPHYLL CYCLE USING SPECTRAL REFLECTANCE1

Analysis of reflectance spectra was used to monitor the conversion of diadinoxanthin (DD) into diatoxanthin (DT) in two benthic diatom species, Amphora coffeaeformis (C. Agardh) Kütz. and Cylindrotheca closterium (Ehrenb.) J. C. Lewin et Reiman, cultured at high light (HL, 400 μmol · m^?2 · s^?1 PAR) and low light (LL, 25 μmol · m^?2 · s^?1 PAR). Cultures were exposed to saturating light for 32 min. HL cultures of both species showed higher (DT + DD) content, whereas LL cultures exhibited higher chl a and fucoxanthin content. DD to DT conversion, measured by HPLC, occurred mainly in the first 2 min (LL) or 5 min (HL) after exposure to saturating light. Nonphotochemical quenching (NPQ), measured by PAM fluorescence, showed the same pattern as DT/(DD + DT), resulting in a linear relationship between these parameters. Addition of dithiothreitol (DTT) blocked the conversion of DD into DT and significantly reduced NPQ induction. Reflectance spectra showed no obvious change after light exposure. However, second derivative spectra (δδ) showed a shift in reflectance from 487 to 508 nm, which was not present for DTT‐treated samples. Changes in δδ₄₈₇ were strongly correlated with changes in DD (r = 0.76), while changes in δδ₅₀₈ were strongly correlated with changes in DT (r = 0.94). The best index to estimate DD to DT conversion was δδ₅₀₈/δδ₆₃₀ (r = 0.87). This index was very sensitive to minute changes that occurred immediately after exposure to light and was species insensitive. Good relationships were observed between indices for xanthophyll cycle activation (DD to DT conversion and NPQ induction) and the second derivative spectra. With further in situ validation, this index may prove to be highly useful for investigation into aquatic global photoregulation mechanisms in diatom‐dominated samples.     

Stoichiometry of Photosystem I, Photosystem II, and Phycobilisomes in the Red Alga Porphyridium cruentum as a Function of Growth Irradiance

Cells of the red alga Porphyridium cruentum (ATCC 50161) exposed to increasing growth irradiance exhibited up to a three-fold reduction in photosystems I and II (PSI and PSII) and phycobilisomes but little change in the relative numbers of these components. Batch cultures of P. cruentum were grown under four photon flux densities of continuous white light; 6 (low light, LL), 35 (medium light, ML), 180 (high light, HL), and 280 (very high light, VHL) microeinsteins per square meter per second and sampled in the exponential phase of growth. Ratios of PSII to PSI ranged between 0.43 and 0.54. About three PSII centers per phycobilisome were found, regardless of growth irradiance. The phycoerythrin content of phycobilisomes decreased by about 25% for HL and VHL compared to LL and ML cultures. The unit sizes of PSI (chlorophyll/P₇₀₀) and PSII (chlorophyll/Q_A) decreased by about 20% with increase in photon flux density from 6 to 280 microeinsteins per square meter per second. A threefold reduction in cell content of chlorophyll at the higher photon flux densities was accompanied by a twofold reduction in β-carotene, and a drastic reduction in thylakoid membrane area. Cell content of zeaxanthin, the major carotenoid in P. cruentum, did not vary with growth irradiance, suggesting a role other than light-harvesting. HL cultures had a growth rate twice that of ML, eight times that of LL, and slightly greater than that of VHL cultures. Cell volume increased threefold from LL to VHL, but volume of the single chloroplast did not change. From this study it is evident that a relatively fixed stoichiometry of PSI, PSII, and phycobilisomes is maintained in the photosynthetic apparatus of this red alga over a wide range of growth irradiance.     

Acclimation to low light by C4 maize: implications for bundle sheath leakiness

C4 plants have a biochemical carbon concentrating mechanism (CCM) that increases CO₂ concentration around ribulose bisphosphate carboxylase oxygenase (Rubisco) in the bundle sheath (BS). Under limiting light, the activity of the CCM generally decreases, causing an increase in leakiness, (Φ), the ratio of CO₂ retrodiffusing from the BS relative to C4 carboxylation processes. Maize plants were grown under high and low light regimes (respectively HL, 600 versus LL, 100 μE m^?2 s^?1). Short‐term acclimation of Φ was compared from isotopic discrimination (Δ), gas exchange and photochemistry. Direct measurement of respiration in the light, and ATP production rate (J_ATP), allowed us use a novel approach to derive Φ, compared with the conventional fitting of measured and predicted Δ. HL grown plants responded to decreasing light intensities with the well‐documented increase in Φ. Conversely, LL plants showed a constant Φ, which has not been observed previously. We explain the pattern by two contrasting acclimation strategies: HL plants maintained a high CCM activity at LL, resulting in high CO₂ overcycling and increased Φ; LL plants acclimated by down‐regulating the CCM, effectively optimizing scarce ATP supply. This surprising plasticity may limit the impact of Φ‐dependent carbon losses in leaves becoming shaded within developing canopies.     

First description of the environmental niche of the epibenthic dinoflagellate species Coolia palmyrensis,C. malayensis,and C. tropicalis (Dinophyceae) from Eastern Australia

Environmental variables such as temperature, salinity, and irradiance are significant drivers of microalgal growth and distribution. Therefore, understanding how these variables influence fitness of potentially toxic microalgal species is particularly important. In this study, strains of the potentially harmful epibenthic dinoflagellate species Coolia palmyrensis, C. malayensis, and C. tropicalis were isolated from coastal shallow water habitats on the east coast of Australia and identified using the D1‐D3 region of the large subunit (LSU) ribosomal DNA (rDNA). To determine the environmental niche of each taxon, growth was measured across a gradient of temperature (15–30°C), salinity (20–38), and irradiance (10–200 μmol photons · m^?2 · s^?1). Specific growth rates of Coolia tropicalis were highest under warm temperatures (27°C), low salinities (ca. 23), and intermediate irradiance levels (150 μmol photons · m^?2 · s^?1), while C. malayensis showed the highest growth at moderate temperatures (24°C) and irradiance levels (150 μmol photons · m^?2 · s^?1) and growth rates were consistent across the range of salinity levels tested (20–38). Coolia palmyrensis had the highest growth rate of all species tested and favored moderate temperatures (24°C), oceanic salinity (35), and high irradiance (>200 μmol photons · m^?2 · s^?1). This is the first study to characterize the environmental niche of species from the benthic harmful algal bloom genus Coolia and provides important information to help define species distributions and inform risk management.     

PHOTOINHIBITION OF PSII IN EMILIANIA HUXLEYI (HAPTOPHYTA) UNDER HIGH LIGHT STRESS: THE ROLES OF PHOTOACCLIMATION,PHOTOPROTECTION, AND PHOTOREPAIR1

The response of the coccolithophorid Emiliania huxleyi (Lohmann) W. H. Hay et H. Mohler to acute exposure to high photon flux densities (PFD) was examined in terms of PSII photoinhibition, photoprotection, and photorepair. The time and light dependencies of these processes were characterized as a function of the photoacclimation state of the alga. Low‐light (LL) acclimated cells displayed a higher degree of photoinhibition, measured as decline in F_v/F_m, than high‐light (HL) acclimated cells. However, HL cultures were more susceptible to photodamage but also more capable of compensating for it by performing a faster repair cycle. The relation between gross photoinhibition (observed in the presence of an inhibitor of repair) and PFD to which the algae were exposed deviated from linearity at high PFD, which calls into question the universality of current concepts of photoinhibition in mechanistic models. The light dependence of the de‐epoxidation state (DPS) of the xanthophyll cycle (XC) pigments on the timescale of hours was the same in cells acclimated to LL and HL. However, HL cells were more efficient in realizing nonphotochemical quenching (NPQ) on short timescales, most likely due to a larger XC pool. LL cells displayed an increase in the PSII effective cross‐section (σ_PSII) as a result of photoinhibition, which was observed also in HL cells when net photoinhibition was induced by blocking the D1 repair cycle. The link between σ_PSII and photoinhibition suggests that the population of PSII reaction centers (RCIIs) of E. huxleyi shares a common antenna, according to a “lake” organization of the light‐harvesting complex.     

Growth,biochemical and enzymatic responses of thermal cyanobacterium Gloeocapsa sp. (Cyanophyceae) to temperature and irradiance

The present study describes a strain of Gloeocapsa sp. designated as Gacheva 2007/R‐06/1, originally isolated from a geothermal flow located in Rupite, Bulgaria. To evaluate whether this cyanobacterium is locally adapted to hot environment or has the ability to tolerate lower temperatures, its growth, biochemical composition, enzyme isoforms and activity of the main antioxidant enzymes and proteases were characterized under various temperatures and two irradiance levels. The strain was able to grow over the whole temperature range (15–40°C) under two different photon fluence densities – 132 μmol photons m^?2 s^?1 (unilateral, low light, LL) and 2 × 132 μmol photons m^?2 s^?1 (bilateral, high light, HL). The best growth occurred at either 34°C and LL or at 36°C and HL, but significant growth inhibition was noted at 15°C and 40°C. Low temperature treatment (15°C) resulted in higher levels of total protein and an increased activity of manganese superoxide dismutase (MnSOD) and glutathione reductase, as compared to optimum growth temperatures. After simultaneous exposure to 15°C and HL, increases in lipid content and activity of iron superoxide dismutase and catalase (CAT) were also observed. Cultivation of cells at 40°C enhanced MnSOD, CAT and peroxidase activities, regardless of irradiance level. Increased total protein content and protease activity at 40°C was only associated with the HL treatment. Overall, these results indicate that Gloeocapsa sp. strain Gacheva 2007/R‐06/1 used different strategies to enable cells to efficiently acclimate and withstand adverse low or high temperatures. This strain obviously tolerates a wide range of temperatures below its natural habitat temperature, and does not seem to be locally adapted to its original thermal regime. It behaved as a thermotolerant rather than a thermophilic cyanobacterium, which suggests its wider distribution in nature.     

EFFECTS OF SUBSTRATE RELIEF ON THE DISTRIBUTION OF PERIPHYTON IN LABORATORY STREAMS. II. INTERACTIONS WITH IRRADIANCE1

Laboratory streams were used in a 42-day experiment designed to investigate how the spatial and temporal distribution of lotic periphyton created by current flow over cobble-size substrates is a affected by irradiance. The streams contained 22.5 × 22.5 × 4 cm substrate blocks and were exposed to either 385, 90 or 20 μE·m^?2·s^?1. We monitored periphyton succession in fast current regimes on top of blocks and in slower current regimes on surfaces recessed between blocks. The absolute differences in AFDW algal biomass between top and recessed substrates were significantly affected by irradiance and time. At the end of the experiment, biomass in streams exposed to 385 μE·m^?2·s^?1. was approximately 2 and 8 times greater than in streams exposed to 90 and 20 μE·m^?2·s^?1, respectively. Differences in biomass were greater between irradiance levels than between top and recessed substrates within an irradiance level. Irradiance also had a greater effect than current regime on the taxonomic composition of assemblages. Oscillatoria agardhii Gomont and Navicula minima Grun. characterized assemblages at 20 μE·m^?2·s^?1, whereas Fragilaria vaucheriae (Kütz.), Nitzschia oregona Sov., Navicula arvensis Hust. and Stigeoclonium tenue (Ag.) Kütz. were more abundant at the two higher irradiances. Detrended correspondence analysis indicated that the rate of succession was relatively high for assemblages at high irradiance and in the slow current regimes between blocks. The results suggested that in natural streams, periphyton patches produced by large differences in irradiance should have a greater effect on periphyton heterogeneity than substrate-induced patches. Moreover, the heterogeneity of algal patches produced by hydrologic differences over a substrate is constrained by irradiance level.     

Characterizing the escape response of juvenile summer flounder Paralichthys dentatus to diel‐cycling hypoxia

Swimming speed, angular correlation and expected displacement were measured in juvenile summer flounder Paralichthys dentatus acclimated to either oxygen saturation (c. 7·8 mg O₂ l^?1; saturation‐acclimated fish) or diel‐cycling hypoxia (cycling between 11·0 and 2·0 mg O₂ l^?1) for 10 days and subsequently exposed to more severe diel‐cycling hypoxia (cycling between 7·0 and 0·4 mg O₂ l^?1). Saturation‐acclimated P. dentatus exhibited an active response to declining dissolved oxygen (DO) by increasing swimming speed, angular correlation and expected displacement to peak levels at 1·4 mg O₂ l^?1 that were 3·5, 5·5 and 4·2 fold, respectively, greater than those at DO saturation. Diel‐cycling hypoxia‐acclimated P. dentatus also exhibited an active response to declining DO, although it was relatively less pronounced. Diel‐cycling hypoxia‐acclimated P. dentatus swimming speed, however, still doubled as DO decreased from 7·0 to 2·8 mg O₂ l^?1. Diel‐cycling hypoxia‐acclimated P. dentatus did not recover as well from low DO exposure as did saturation‐acclimated fish. This was reflected in their relatively more random swimming (low angular correlation between successive moves) and poor maintenance of rank order between individuals during the recovery phase. Even saturation‐acclimated P. dentatus did not resume swimming at speeds observed at saturation until DO was 4·2 mg O₂ l^?1. Paralichthys dentatus were very sensitive to decreasing DO, even at DO levels that were not lethal or growth limiting. This sensitivity and their poor recovery may preclude juvenile P. dentatus from using highly productive nursery habitats affected by diel‐cycling hypoxia.