Maximum Electron Transport Research Articles

Effects of iron deficiency on the growth and photosynthesis of three bloom‐forming cyanobacterial species isolated from Lake Taihu

SUMMARYCyanobacterial blooms are found in many freshwater ecosystems around the world, but the effect of environmental factors on their growth and the proportion of species still require more investigation. In this study, the physiological responses of bloom‐forming cyanobacteria M icrocystis aeruginosa FACHB912, M icrocystis flos‐aquae FACHB1028 and P seudanabaena sp. FACHB1282 to iron deficiency were investigated. Their specific growth rates were found to decrease as the available iron concentration decreased. At low available iron concentrations of 1 × 10−7 M (pFe 21.3) and 5 × 10−8 M (pFe 21.6), M . aeruginosa had the lowest specific growth rate among three studied species. The cell sizes of M . flos‐aquae and Pseudanabaena sp. were significantly smaller under the lowest iron concentration. The chlorophyll a content of the three species decreased at the lowest iron concentration. The maximal relative electron transport rate, photosynthetic efficiency, and light‐saturation parameter of M . aeruginosa were lower than the other two cyanobacteria at pFe 21.3. Therefore, M . aeruginosa was the least able to adapt to iron deficiency. Under iron deficiency, the functional absorption cross‐section of PSII and electron transport rate on the acceptor side of PSII decreased in M . aeruginosa, while the connectivity factor between individual photosynthetic units increased in M . flos‐aquae, and the electron transport rate on the acceptor side of PSII and between PSII and PSI decreased in P seudanabaena sp. The ability to store iron was highest in M . flos‐aquae, followed by P seudanabaena sp. and M . aeruginosa. Thus, these results provide necessary information for detecting the role of iron in the succession of cyanobacterial species in Lake Taihu, the third largest freshwater lake in China, because all three species were isolated from this lake.

Genetic variation in photosynthetic performance and tolerance to osmotic stress (desiccation, freezing, hyposalinity) in the rocky littoral foundation species Fucus vesiculosus (Fucales, Phaeophyceae).

Genetic diversity may play an analogous role to species diversity, as it can contribute to ecosystem function and stability, and provision of ecosystem services. In the Baltic Sea, perennial algal beds are often comprised of only Fucus vesiculosus and the amount of genetic variation in fitness-related traits (i.e., the ability of the alga to photosynthesize or withstand stress) will thus determine the alga's local persistence in a changing environment. To study genetic variation in the crucial traits behind persistence we grew replicate vegetative branches that came from the same genotype in common gardens. We quantified osmotic stress tolerance and recovery responses by exposing branches to desiccation, freezing, and hyposalinity regimens. Our results show that genetic variation among genotypes was apparent for some photosynthetic parameters (maximal electron transport rate, saturation irradiance for electron transport, nonphotochemical quenching) and growth. Algae tolerated freezing (1,440min at -2.5°C) and hyposalinity (1,560min at 2.5) well, but did not recover from desiccation (70min at 12°C, causing ~94% water loss). Furthermore, we found very little if any evidence on genetic variation in tolerance to these stressors. Our results suggest that low salinity and cold winters in the northern marginal populations selected for hyposalinity and freezing tolerant genotypes, possibly eroding genetic variation in tolerance, but that tolerance to harsh desiccation has been lost, likely due to relaxed selection. The overall availability of genetic variation in fitness related traits might be supportive for F.vesiculosus during adaptation to gradual changes of its environment.

Dynamic summer solar radiation in Antarctic coastal ecosystems and its effects on photosyn thesis of the endemic Antarctic brown macroalga Desmarestia menziesii (Phaeophyceae)

Ultraviolet (UV) radiation is a significant stress factor that harms life in both terrestrial and aquatic ecosystems in Antarctica. In summer (January–February), ground-level solar radiation regimes at the Antarctic Carlini Station (62°14'S, 58°40'W) on King George Island (South Shetland Islands) can be highly variable, depending on the presence of clouds. Spectrally-resolved underwater radiation regimes were measured at three study sites in the inner and outer Potter Cove nearby Carlini Station. The clear waters at Penon de Pesca allowed PAR, UV-A and UV-B radiation to penetrate deeply into the water column, expressed by z1% (i. e. 1%-depths) at 23–25 m, 20–22 m and 13–16 m, respectively, as well as by low attenuation coefficients of downward radiation (Kd). In contrast, turbid waters in the inner Potter Cove and at Penon Uno reduced the penetration of these three wavebands significantly. The photo-physiological mechanisms allowing macroalgae to acclimate to the incident gradients of PAR and UV radiation at Penon de Pesca were further elaborated by assessing photo-physiological data on the brown macroalga Desmarestia menziesii (Phaeophyceae), exposed to a PAR range between 15 and 130 μ mol photons m-2 s-1, either in the presence or absence of UV radiation (10.3 W m -2 UV-A and 0.73 W m -2 UV-B). PAM-fluorometry-based measurements revealed a similar decrease of the optimum quantum yield of photosystem II (PSII) under both PAR and UV radiation and a stronger effect of PAR over UV radiation in the regulation of maximum photosynthetic electron transport rate (ETRmax) as well as the photosynthetic light saturation (Ek). The down-regulation of the photochemistry of PSII by PAR as well as the reduction in the photosynthetic electron transport capacity (i.e.ETRmax) indicate important photoprotective mechanisms allowing D. menziesii to response effectively to a combination of PAR and UV stress in their upper subtidal habitat.

Retrieval of seasonal Rubisco-limited photosynthetic capacity at global FLUXNET sites from hyperspectral satellite remote sensing: Impact on carbon modelling

Process-based ecophysiological models, which simulate carbon exchange at the land-surface, are powerful and indispensable tools for understanding how vegetation behaves under present and future climate. However, these models are necessarily complex, containing numerous biophysical parameters which are often poorly defined. The current study develops a novel retrieval of Rubisco-limited top-of-canopy photosynthetic capacity (i.e. maximum carboxylation rate, Vcmax25,toc), which is one of the most critical parameters in the calculation of Gross Primary Productivity (GPP). The retrieval combines standard remote sensing satellite products of Leaf Area Index (LAI), from the Moderate Resolution Imaging Spectroradiometer (MODIS), with a hyperspectral index of total canopy chlorophyll concentration from the MEdium Resolution Imaging Spectrometer (MERIS). Monthly values of Vcmax25,toc are determined over a 9 year period for 296 global FLUXNET sites (catalogue made available online) and 8 Plant Functional Types (PFTs). PFT averages agree favourably with compilations of field-based measurements. However, according to a Monte Carlo analysis, our method is still currently subject to large systematic uncertainties (25–30%), much of which arises from the empirical relationship between maximum electron transport and leaf chlorophyll content. For all 8 PFTs, except tropical broadleaf forest, Vcmax25,toc varies considerably across the season (generally a factor of 1.6). Similarly, variability between sites of the same PFT is significant (interquartile range is 40% of the median). This suggests an important additional role for satellites in the spatial and temporal parameterisation of carbon models. Inclusion of this temporal and spatial variability in a process-based ecophysiological model produces, respectively, an impact of 11% and 12% on simulated GPP.

Effects of laser irradiation on a bloom forming cyanobacterium Microcystis aeruginosa.

Effects of laser irradiation on photosystem II (PS II) photochemical efficiencies, growth, and other physiological responses of Microcystis aeruginosa were investigated in this study. Results indicate that laser irradiation (wavelengths 405, 450, 532, and 650nm) could effectively inhibit maximal PS II quantum yield (Fv/Fm) and maximal electron transport rates (ETRmax) of M. aeruginosa, while saturating light levels (Ek) of M. aeruginosa did not change significantly. Among the four tested wavelengths, 650nm laser (red light) showed the highest inhibitory efficiency. Following 650nm laser irradiation, the growth of M. aeruginosa was significantly suppressed, and contents of cellular photosynthetic pigments (chlorophyll a, carotenoid, phycocyanin, and allophycocyanin) decreased as irradiation dose increased. Meanwhile, laser irradiation enhanced the enzyme activities of superoxide dismutase (SOD) and peroxidase (POD) in M. aeruginosa cells. Lower irradiation doses did not change the intracellular microcystin contents, but higher dose irradiation (>1284Jcm-2) caused the release of microcystin into the culture medium. Transmission electron microscope examination showed that the ultrastructure of M. aeruginosa cells was destructed progressively following laser irradiation. Effects of laser irradiation on M. aeruginosa may be a combination of photochemical, electromagnetic, and thermal effects.

Photosynthetic Performance of Ocimum sanctum Morphotypes in a Semiarid Region

ABSTRACTComparative ecophysiological studies by measurements of photosystem II (PSII) chlorophyll a fluorescence, pigment composition, and proline content were used in the synecological assessments of two Ocimum sanctum (holy basil) morphotypes, viz., Sri Tulsi and Krishna Tulsi, to compare their overall “adaptiveness.” The effective quantum yield of PSII (ΔF/Fm’sat) and maximum apparent electron transport rate (ETRmax) was 30% lower in Krishna Tulsi than in Sri Tulsi plants. Chlorophyll and carotenoid content in Sri Tulsi were twofold higher than in Krishna Tulsi. Results indicated that differences in photosynthetic efficiency observed in Sri Tulsi may determine ecological dominance.

Photosynthetic UV stress tolerance of the Antarctic snow alga Chlorella sp. modified by enhanced temperature?

Photosynthetic characteristics and the effect of UV radiation and elevated temperature measured were studied in Chlorella sp. isolated from a snow microalgal community at King George Island, Maritime Antarctica through the chlorophyll florescence (rapid light curves and maximum quantum yield, respectively). The environmental context was monitored through measurements of spectral depth profiles of solar radiation (down to 40 cm) in the snowpack as well as a through continuous recording of temperature and PAR using dataloggers located at different depths (0–30 cm) within the snow column. The photochemistry of Chlorella sp. was affected by UV radiation in a 12-h laboratory exposure under all studied temperatures (5, 10, 15, 20 °C): the algae exposed to PAR + UV-A radiation were inhibited by 5.8 % whilst PAR + UV-A + UV-B radiation decreased Fv/Fm by 15.8 %. In both treatments the 12-h recovery after UV exposure was almost complete (80–100 %). Electron transport based P-I curve parameters maximal electron transport rate (ETRmax), photosynthetic efficiency (α) and the saturating irradiance (Ek) no varied in response to different temperatures. Results revealed that Chlorella sp. not only shows high photosynthetic efficiency at ambient conditions, but also exhibits tolerance to solar radiation under higher temperatures and possessing a capacity for recovery after inhibition of photosynthesis by UV radiation.

Responses of photosynthesis and nitrogen assimilation in the green-tide macroalga Ulva prolifera to desiccation

The Yellow Sea in China has experienced annual large-scale green tides since 2007. Ulva prolifera, the dominant causative species, originating from Pyropia yezoensis aquaculture rafts, experiences periodic tidal emersion in the intertidal nursery. It was proposed that physiological adaptations of U. prolifera may enable it to survive the harsh intertidal environment and contribute to subsequent blooms. In this paper, we measured the responses of photosynthesis and nitrogen assimilation of U. prolifera during desiccation of 0–6 h and subsequent rehydration for 0–24 h. The results suggested that both PSII photosynthesis and nitrogen assimilation were significantly reduced at the onset of desiccation. These reductions were reflected by decreases in the maximum quantum yield (Fv/Fm), effective photochemical quantum yield (YII), maximal electron transport rate (ETRmax), light utilization efficiency (α), NO3-N and NH4-N uptake rates, tissue nitrogen concentration (TN), and nitrate reductase activity (NRA). Desiccation temporarily lowered PSII photosynthesis, but rates recovered after algae were submerged for 24 h. Assimilation of nitrogen was also reduced during exposure to air. Although both NO3-N and NH4-N uptake rate recovered within 12 h upon rehydration, TN and NRA were not completed within 24 h. U. prolifera therefore appears well adapted to survive prolonged periods of desiccation, as it experiences growing in Pyropia farms along the Chinese Jiangsu coast. These traits, along with its ability to grow fast, may explain its success and one of the mechanisms behind the formation of the extreme blooms in the Yellow Sea.

Rhizosphere bacteria containing 1-aminocyclopropane-1- carboxylate deaminase increase growth and photosynthesis of pea plants under salt stress by limiting Na+ accumulation.

Although plant salt tolerance has been improved by soil inoculation with rhizobacteria containing the enzyme 1-aminocyclopropane-1-carboxylate (ACC) deaminase (which metabolises ACC, the immediate precursor of the phytohormone ethylene), it is not always clear whether ion homeostasis and plant water relations are affected. When pea (Pisum sativum L. cv. Alderman) was grown with 70 and 130mM NaCl, the ACC-deaminase containing rhizobacterium Variovorax paradoxus 5C-2 increased total biomass by 25 and 54% respectively. Nutrient flow modelling showed that V. paradoxus 5C-2 increased K uptake and root to shoot K flow, but decreased Na flow and increased Na deposition in roots. Thus, shoot K+:Na+ ratio increased following V. paradoxus 5C-2 inoculation. At 70 and 130mM NaCl, rhizobacterial inoculation decreased stomatal resistance by 14 and 31% and decreased xylem balancing pressure by 7 and 21% respectively. Furthermore, rhizobacterial inoculation improved photosynthetic efficiency (Fv/Fm) by 12 and 19% and increased maximal electron transport rate (ETR) by 18 and 22% at 70 and 130mM NaCl respectively. Thus V. paradoxus 5C-2 mitigates salt stress by improving water relations, ion homeostasis and photosynthesis of pea plants, and may provide an economic means of promoting growth of plants exposed to salt stress.

Interactive Effect of UVR and Phosphorus on the Coastal Phytoplankton Community of the Western Mediterranean Sea: Unravelling Eco-Physiological Mechanisms.

Some of the most important effects of global change on coastal marine systems include increasing nutrient inputs and higher levels of ultraviolet radiation (UVR, 280–400 nm), which could affect primary producers, a key trophic link to the functioning of marine food webs. However, interactive effects of both factors on the phytoplankton community have not been assessed for the Mediterranean Sea. An in situ factorial experiment, with two levels of ultraviolet solar radiation (UVR+PAR vs. PAR) and nutrients (control vs. P-enriched), was performed to evaluate single and UVR×P effects on metabolic, enzymatic, stoichiometric and structural phytoplanktonic variables. While most phytoplankton variables were not affected by UVR, dissolved phosphatase (APAEX) and algal P content increased in the presence of UVR, which was interpreted as an acclimation mechanism of algae to oligotrophic marine waters. Synergistic UVR×P interactive effects were positive on photosynthetic variables (i.e., maximal electron transport rate, ETRmax), but negative on primary production and phytoplankton biomass because the pulse of P unmasked the inhibitory effect of UVR. This unmasking effect might be related to greater photodamage caused by an excess of electron flux after a P pulse (higher ETRmax) without an efficient release of carbon as the mechanism to dissipate the reducing power of photosynthetic electron transport.

Physiological Responses of a Model Marine Diatom to Fast pH Changes: Special Implications of Coastal Water Acidification.

Diatoms and other phytoplankton in coastal waters experience rapid pH changes in milieu due to high biological activities and/or upwelled CO2-rich waters. While CO2 concentrating mechanisms (CCMs) are employed by all diatoms tested to counter low CO2 availability in seawater, little is known how this mechanism responds to fast pH changes. In the present study, the model diatom Thalassiosira pseudonana was acclimated for 20 generations to low pH (7.81) at an elevated CO2 of 1000 μatm (HC) or to high pH (8.18) at ambient CO2 levels of 390 μatm (LC), then its physiological characteristics were investigated as cells were shifted from HC to LC or vice versa. The maximal electron transport rate (ETRmax) in the HC-acclimated cells was immediately reduced by decreased CO2 availability, showing much lower values compared to that of the LC-acclimated cells. However, the cells showed a high capacity to regain their photochemical performance regardless of the growth CO2 levels, with their ETRmax values recovering to initial levels in about 100 min. This result indicates that this diatom might modulate its CCMs quickly to maintain a steady state supply of CO2, which is required for sustaining photosynthesis. In addition, active uptake of CO2 could play a fundamental role during the induction of CCMs under CO2 limitation, since the cells maintained high ETR even when both intracellular and periplasmic carbonic anhydrases were inhibited. It is concluded that efficient regulation of the CCM is one of the key strategies for diatoms to survive in fast changing pH environment, e.g. for the tested species, which is a dominant species in coastal waters where highly fluctuating pH is observed.

Synergistic algicidal effect and mechanism of two diketopiperazines produced by Chryseobacterium sp. strain GLY-1106 on the harmful bloom-forming Microcystis aeruginosa.

A potent algicidal bacterium isolated from Lake Taihu, Chryseobacterium sp. strain GLY-1106, produces two algicidal compounds: 1106-A (cyclo(4-OH-Pro-Leu)) and 1106-B (cyclo(Pro-Leu)). Both diketopiperazines showed strong algicidal activities against Microcystis aeruginosa, the dominant bloom-forming cyanobacterium in Lake Taihu. Interestingly, these two algicidal compounds functioned synergistically. Compared with individual treatment, combined treatment with cyclo(4-OH-Pro-Leu) and cyclo(Pro-Leu) significantly enhanced algicidal activity, accelerated the increase in intracellular reactive oxygen species (ROS) levels in M. aeruginosa, and further decreased the activities of antioxidases, effective quantum yield and maximal electron transport rate of M. aeruginosa. The results also showed that the algicidal characteristics of cyclo(4-OH-Pro-Leu) are distinct from those of cyclo(Pro-Leu). Cyclo(4-OH-Pro-Leu) mainly interrupted the flux of electron transport in the cyanobacterial photosynthetic system, whereas cyclo(Pro-Leu) mainly inhibited the activity of cyanobacterial intracellular antioxidases. A possible algicidal mechanism for the synergism between cyclo(4-OH-Pro-Leu) and cyclo(Pro-Leu) is proposed, which is in accordance with their distinct algicidal characteristics in individual and combined treatment. These findings suggest that synergism between algicidal compounds might be used as an effective strategy for the future control of Microcystis blooms.

Dynamics of the photosystem II photochemical activity in the developing <em>Brassica nigra</em> L. seeds

Dynamics of the PS II activity in the pod walls, seed coat and embryo cotyledons at early and middle maturation stages of Brassica nigra L. has been studied using the Junior-PAM fluorometer (Heinz Walz Gmbh, Germany). The maximum quantum yield Fv/Fm, effective quantum yield Y(II) and coefficient of photochemical fluorescence quenching qP have been evaluated. We demonstrated maturation-dependent fluctuation of the PS II activity in the different part of seeds: Fv/Fm, Y(II) and qP decreased in the pod walls and seed coat, but increased in the cotyledons of embryo. During transition from early to middle stage of maturation, the maximal electron transport rate of PS II in the cotyledons increased and reached the maximum at higher level of photosynthetic active radiation. Improving the efficiency of PS II in the developing cotyledons of the embryo can be attributed to adaptation of the chloroplasts to a higher light probably due to the increase of light transmission through the seed coat and pericarp at later stages of seed maturation. Refs 29. Figs. 3.

Shading tolerance in rice is related to better light harvesting and use efficiency and grain filling rate during grain filling period

Low radiation caused by industrial development and environment change has become a limitation in crop production in China. Field experiments were conducted to investigate the effect of shading stress on photosynthetic and chlorophyll fluorescence characters, as well as grain yield and yield components of rice from 2010 to 2012, in Sichuan, China. Rice cv. IIyou 498, Gangyou 188, Dexiang 4103, Gangyou 527, and Chuanxiang 9838 were selected and shaded during grain filling period, which resulted in 53% shading environment (achieved with white cloth above the rice canopy). Shading increased the flag leaf chlorophyll (Chl) content and maximum efficiency of PSII photochemistry under dark-adaption (Fv/Fm), but decreased the net photosynthetic rate, electron transport rate (ETR), saturation irradiance (PARsat), and maximum electron transport (Jmax), which resulted in a sharply falling grain yield mainly due to reduced spikelet filling and grain weight. The shading tolerances were different in different rice varieties. Maximum net photosynthetic rate (Pmax), light saturation point (LSP), quantum yield of PSII (ΦPSII), and photochemical quenching (qP) of Gangyou 188 were reduced under shading. However, IIyou 498 had its adaptive capacity to shading enhanced by increasing the Chl content to improve light-harvesting potential as well as increased Pmax, LSP, Fv/Fm, and ΦPSII. In addition, there was reduced non-photochemical quenching (NPQ), while maintaining a more stable ETR, PARsat, and Jmax to enhance light-use efficiency and reduce the dissipation of light energy, which relieved damage to spikelet filling and resulted in a smaller reduction in grain yield of IIyou 498. Therefore, the improvement in light-harvesting and -use capability and increased spikelet filling under shading stress might be important characteristics for plant breeders. For example, IIyou 498 can be selected as a shade-tolerant variety.

Effects of iron availability on competition between Microcystis and Pseudanabaena or Chlorella species

Iron availability has been suggested to be one of the key factors limiting the formation of Microcystis sp. blooms in eutrophic lakes. However, the significance of competition of Microcystis sp. with other phytoplankton species under iron limitation is still unclear. Here, the competitive ability of Microcystis aeruginosa is reported from mixed cultures with Pseudanabaena sp. or Chlorella pyrenoidosa at two iron concentrations (pFe 21.4 and 20.3). In monoculture, low iron conditions caused a smaller decrease in maximal relative electron transport rate, PSI/PSII fluorescence ratio and phycocyanin content in M. aeruginosa than in Pseudanabaena sp. The same iron concentration resulted in a significant decrease in chlorophyll a content, PSI/PSII fluorescence ratio, and maximal PSII photochemical yield in M. aeruginosa but there was no change in these factors in C. pyrenoidosa. All three species were more vulnerable to photoinhibition under low iron conditions. Iron limitation also decreased their cellular volume and increased their functional absorption cross-section of PSII. When mix-cultured with Pseudanabaena sp. for 6 days, M. aeruginosa cell abundance decreased by 18.4% under high iron conditions but increased by 23.7% under low iron conditions. When mix-cultured with C. pyrenoidosa, M. aeruginosa cell abundance showed decreased percentages in both low (25.7%) and high (13.8%) iron conditions. In comparison to monoculture, C. pyrenoidosa showed significantly increased growth rate when mix-cultured with M. aeruginosa under low iron conditions. In such low iron conditions, Chrome Azurol-S (CAS)- positive molecules were secreted by M. aeruginosa, which were likely to promote the growth of C. pyrenoidosa. In summary, M. aeruginosa had a competitive advantage compared with Pseudanabaena sp. and a disadvantage compared with C. pyrenoidosa under low iron or iron-limited conditions.

Effects of natural flavonoids on photosynthetic activity and cell integrity in Microcystis aeruginosa.

Flavonoids are natural polyphenolic compounds produced by many aquatic plants and released in their environments. In this study, the effects of several aquatic flavonoids on cyanobacterial Microcystis aeruginosa, especially in relation to the cell growth, photosynthetic activity, cell morphology, and cell membrane integrity, were investigated. Significant growth inhibition was observed when the cyanobacteria were exposed to three flavonoids, namely, 5,4'-dihydroxyflavone (DHF), apigenin, and luteolin. Luteolin reduced the effective quantum yield, photosynthetic efficiency, and maximal electron transport rate by 70%, 59% and 44%, respectively, whereas 5,4'-DHF and apigenin slightly affected these parameters, which implies that luteolin disrupts the photosynthetic system. Moreover, 5,4'-DHF and apigenin compromised the membrane integrity, and induced membrane depolarization in 52% and 38%, and permeabilization in 30% and 44% of the cells, respectively. The 5,4'-DHF and apigenin showed more pronounced effects on M. aeruginosa morphology and membrane integrity, compared to the luteolin. These results suggest that flavonoids could have significant effects on growth and physiological functions in cyanobacterial species.

Effects of salinity stress on the growth and chlorophyll fluorescence of Phaeodactylum tricornutum and Chaetoceros gracilis (Bacillariophyceae)

Abstract In this study, we investigated changes of fluorescence parameters, specific growth rate, and relative chlorophyll content (the ratio of the chlorophyll content after salinity exposure to that of the same samples at inoculation) in the two diatoms Phaeodactylum tricornutum Bohlin and Chaetoceros gracilis Schuett, during exposure of the cultures to different salinities (1–90 psu for P. tricornutum and 1–70 psu for C. gracilis) for different times (12, 24, and 48 h). The results indicated that the maximal photochemical efficiency of PSII (Fv/Fm), maximal relative electron transport rate (rETRmax), and photochemical quenching (qP) decreased significantly with the degree of salinity stress and increased with increasing exposure times in the two diatoms. For P. tricornutum, the values of Fv/Fm at salinities of 20–40 psu were always higher than those in other salinity treatments, rETRmax reached maximal values at salinities of 20–30 psu, and qP peaked at salinities of 10–20 psu. For C. gracilis, the values of Fv/Fm at salinities of 5–40 psu were always higher than those at 1 psu or at 50–70 psu. The highest values of rETRmax were also observed at salinities of 5–40 psu, whereas qP decreased slowly with increasing salinity from 10 to 60 psu. The changes in non-photochemical quenching (NPQ) were different in the two diatoms. For P. tricornutum, there were no significant differences below 60 psu for all three exposure times. Above 60 psu, however, NPQ in P. tricornutum increased with salinity up to 80 psu and decreased at 90 psu. For C. gracilis, NPQ decreased with increasing salinity from 1 to 20 psu, but increased at salinities of 20–70 psu during the 12- and 24-h periods, and was always low after 48 h. The specific growth rate of the two diatoms was inhibited at both low and high salinities, and decreased with increasing exposure times. The relative chlorophyll content of the two diatoms showed a similar trend to each other, with lower chlorophyll content occurring at extremely low and high salinities. These results indicated that extremely low and high salinity stress negatively influenced the chlorophyll fluorescence parameters and growth of these two diatoms.

Short-term ecophysiological and biochemical responses of Cystoseira tamariscifolia and Ellisolandia elongata to environmental changes

Short-term ecophysiological and biochemical responses of Cystoseira tamariscifolia and Ellisolandia elongata to changes in solar irradiance and nutrient levels were analyzed in situ in oligotrophic coastal waters by transferring macroalgae collected at 0.5 and 2.0 m depth and exposing them to 2 irradiance levels (100 and 70% of surface irradiance) and nutrient conditions (nutrient-enriched and non-enriched). Both species were affected by changes in irradiance and nutrient levels. Few interactive effects between these 2 physical stressors were found, suggesting major additive effects on both species. C. tamariscifolia collected at 0.5 m and exposed to 70% irradiance had the highest maximal electron transport rate (ETRmax), saturated irradiance (EkETR) and chl a content and the lowest antioxidant activity. Under the same conditions, E. elongata had increased EkETR, antheraxanthin and β-carotene content. At 100% irradiance, C. tamariscifolia collected at 2.0 m had higher maximal quantum yield (Fv/Fm), photosynthetic efficiency (αETR), ETRmax, maximal non-photochemical quenching (NPQmax), saturation irradiance for NPQ (EkNPQ), and antheraxanthin and polyphenol content increased, whereas in E. elongata only αETR increased. In nutrient-enriched conditions, phenolic compounds, several carotenoids and N con- tent increased in C. tamariscifolia at both depths. E. elongata from 2.0 m depth at 100% irradiance and nutrient-enriched conditions showed increased N content and total mycosporine-like amino acids (MAAs). Our results show rapid photophysiological responses of C. tamariscifolia to varia- tions in in situ irradiance and nutrient conditions, suggesting efficient photoacclimation to envi- ronmental changes. In E. elongata, Fv/Fm and ETRmax did not change in the transplant experiment; in contrast, N content, pigment and MAAs (biochemical variables) changed. The responses of these macroalgae to nutrient enrichment indicate oligotrophic conditions at the study site and environmental stress.

Effects of linear alkylbenzene sulfonate on the growth and toxin production of Microcystis aeruginosa isolated from Lake Dianchi.

The exogenous organic pollutant linear alkylbenzene sulfonate (LAS) pollution and Microcystis bloom are two common phenomena in eutrophic lakes, but the effects of LAS alone on Microcystis remain unclear. In the present study, we investigated the effects of LAS on the growth, photochemical efficiency, and microcystin production of Microcystis aeruginosa in the laboratory. Results showed that low LAS (≤10mg/L) concentrations improved the growth of M. aeruginosa (12days of exposure). High LAS (20mg/L) concentrations inhibited the growth of M. aeruginosa on the first 8days of exposure; afterward, growth progressed. After 12days of exposure, the concentrations of chlorophyll a in algal cells were not significantly affected by any of LAS concentrations (0.05 to 20mg/L) in the present study; by contrast, carotenoid and protein concentrations were significantly inhibited when LAS concentrations reached as high as 20mg/L. After 6 and 12days of exposure, low LAS (≤10mg/L) concentrations enhanced the maximal photochemical efficiency (Fv/Fm) and the maximal electron transport rate (ETRmax) of M. aeruginosa. Furthermore, LAS increased the microcystin production of M. aeruginosa. Extracellular and intracellular microcystin contents were significantly increased after M. aeruginosa was exposed to high LAS concentrations. Our results indicated that LAS in eutrophic lakes may increase the risk of Microcystis bloom and microcystin production.

Photosynthetic efficiency, desiccation tolerance and ultrastructure in two phylogenetically distinct strains of alpine Zygnema sp. (Zygnematophyceae, Streptophyta): role of pre-akinete formation.

Two newly isolated strains of green algae from alpine regions were compared physiologically at different culture ages (1, 6, 9 and 15 months). The strains of Zygnema sp. were from different altitudes (‘Saalach’ (S), 440 m above sea level (a.s.l.), SAG 2419 and ‘Elmau-Alm’ (E-A), 1,500 m a.s.l., SAG 2418). Phylogenetic analysis of rbcL sequences grouped the strains into different major subclades of the genus. The mean diameters of the cells were 23.2 μm (Zygnema S) and 18.7 μm (Zygnema E-A) but were reduced significantly with culture age. The photophysiological response between the strains differed significantly; Zygnema S had a maximal relative electron transport rate (rETR max) of 103.4 μmol electrons m−2 s−1, Zygnema E-A only 61.7 μmol electrons m−2 s−1, and decreased significantly with culture age. Both strains showed a low-light adaption and the absence of strong photoinhibition up to 2,000 μmol photons m−2 s−1. Photosynthetic oxygen production showed similar results (P max Zygnema S, 527.2 μmol O2 h−1 mg−1 chlorophyll (chl.) a, Zygnema E-A, 390.7 μmol O2 h−1 mg−1 chl. a); the temperature optimum was at 35 °C for Zygnema S and 30 °C for Zygnema E-A. Increasing culture age moreover leads to the formation of pre-akinetes, which accumulate storage products as revealed by light and transmission electron microscopy. Desiccation at 84 % relative air humidity (RH) lead to a reduction of the effective quantum yield of photosystem II (PSII) (ΔFv/Fm′) to zero between 90 to 120 min (Zygnema S) and between 30 to 60 min (Zygnema E-A), depending on the culture age. A partial recovery of ΔFv/Fm′ was only observed in older cultures. We conclude that pre-akinetes are crucial for the aeroterrestrial lifestyle of Zygnema.