Mol Photons Research Articles

Photosynthetic characteristics of charophytes from tropical lotic ecosystems

SUMMARY Responses of photosynthetic rates, determined by oxygen evolution using the light and dark bottles technique, to different temperatures, irradiances, pH, and diurnal rhythm were analyzed under laboratory conditions in four charophyte species ( Chara braunii Gmelin , C. guairensis R. Bicudo , Nitella subglomerata A. Braun and Nitella sp.) from lotic habitats in southeastern Brazil. Parameters derived from the photosynthesis versus irradiance curves indicated affinity to low irradiances for all algae tested. Some degree of photoinhibition, [ β = -(0.30‐0.13) mg O2 g ‐1 dry weight h ‐1 ( µ mol photons m ‐2 s ‐1 ) ‐1 ], low light compensation points (I c = 4‐20 µ mol photons m ‐2 s ‐1 ) were found for all species analyzed, as well as low values of light saturation parameter (I k ) and saturation (I s ) 29‐130 and 92‐169 µ mol photons m ‐2 s ‐1 , respectively. Photoacclimation was observed in two populations of N. subglomerata collected from sites with different irradiances, consisting of variations in photosynthetic parameters (higher values of α , and lower of I k and maximum photosynthetic rate, P max , in the population under lower irradiance). The highest photosynthetic rates for Chara species were observed at 10‐15 ° C, while for Nitella the highest photosynthetic rate was observed at 20‐25 ° C, despite the lack of significant differences among most levels tested. Rates of dark respiration significantly increase with temperature, with the highest values at 25 ° C. The results from pH experiments showed highest photosynthetic rates under pH 4.0 for all algae, suggesting higher affinity for inorganic carbon in the form of carbon dioxide, except in one population of N. subglomerata , with similar rates under the three levels, suggesting indistinct use of bicarbonate and carbon dioxide. Diurnal changes in photosynthetic rates revealed a general pattern for most algae tested, which was characterized by two peaks: the first (higher) during the morning (07.00‐11.00) and the second (lower) in the afternoon (14.00‐17.00). This suggests an endogenous rhythm determining the daily variations in photosynthetic rates.

Effect of elevated CO2 on the community metabolism of an experimental coral reef

The effect of elevated pCO2 on the metabolism of a coral reef community dominated by macroalgae has been investigated utilizing the large 2650 m3 coral reef mesocosm at the Biosphere‐2 facility near Tucson, Arizona. The carbonate chemistry of the water was manipulated to simulate present‐day and a doubled CO2 future condition. Each experiment consisted of a 1–2 month preconditioning period followed by a 7–9 day observational period. The pCO2 was 404 ± 63 μatm during the present‐day pCO2 experiment and 658 ± 59 μatm during the elevated pCO2 experiment. Nutrient levels were low and typical of natural reefs waters (NO3− 0.5–0.9 μM, NH4+ 0.4 μM, PO43− 0.07–0.09 μM). The temperature and salinity of the water were held constant at 26.5 ± 0.2°C and 34.4 ± 0.2 ppt. Photosynthetically available irradiance was 10 ± 2 during the present‐day experiment and 7.4 ± 0.5 mol photons m−2 d−1 during the elevated pCO2 experiment. The primary producer biomass in the mesocosm was dominated by four species of macroalgae; Haptilon cubense, Amphiroa fragillisima, Gelidiopsis intricata and Chondria dasyphylla. Algal biomass was 10.4 mol C m−2 during the present‐day and 8.7 mol C m−2 and during the elevated pCO2 experiments. As previously observed, the increase in pCO2 resulted in a decrease in calcification from 0.041 ± 0.007 to 0.006 ± 0.003 mol CaCO3 m−2 d−1. Net community production (NCP) and dark respiration did not change in response to elevated pCO2. Light respiration measured by a new radiocarbon isotope dilution method exceeded dark respiration by a factor of 1.2 ± 0.3 to 2.1 ± 0.4 on a daily basis and by 2.2 ± 0.6 to 3.9 ± 0.8 on an hourly basis. The 1.8‐fold increase with increasing pCO2 indicates that the enhanced respiration in the light was not due to photorespiration. Gross production (GPP) computed as the sum of NCP plus daily respiration (light + dark) increased significantly (0.24 ± 0.03 vs. 0.32 ± 0.04 mol C m−2 d−1). However, the conventional calculation of GPP based on the assumption that respiration in the light proceeds at the same rate as the dark underestimated the true rate of GPP by 41–100% and completely missed the increased rate of carbon cycling due to elevated pCO2. We conclude that under natural, undisturbed, nutrient‐limited conditions elevated CO2 depresses calcification, stimulates the rate of turnover of organic carbon, particularly in the light, but has no effect on net organic production. The hypothesis that an increase pCO2 would produce an increase in net production that would counterbalance the effect of decreasing saturation state on calcification is not supported by these data.

The Stable Carbon Isotope Ratio of Biogenic Emissions of Isoprene and the Potential Use of Stable Isotope Ratio Measurements to Study Photochemical Processing of Isoprene in the Atmosphere

A technique was developed that allows the determination of the stable carbon isotope ratio of isoprene in air. The method was used for a limited number of ambient measurements as well as laboratory studies of isoprene emitted from Velvet Bean (Mucana pruriens L. var. utilis), including the light and temperature dependence. The mean stable carbon isotope ratio (δ 13C) of isoprene emitted from Velvet Bean (Mucana pruriens L. var. utilis) for all our measurements is –27.7‰ ± 2.0‰ (standard deviation for 23 data points). Our results indicate a small dependence of the stable carbon isotope ratios on leaf temperature and photosynthetic photon flux density (PPFD). The light dependence is 0.0026 ± 0.0012‰/(μ mol of photons m−2 s−1) for the studied range from 400 to 1700 μ mol of photons m−2 s−1. The temperature dependence is 0.16 ± 0.09‰/K. On average, the emitted isoprene is 2.6 ± 0.9‰ lighter than the leaf carbon. An uncertainty analysis of the possibility to use stable carbon isotope ratio measurements of isoprene for estimates of its mean photochemical age suggests that meaningful results can be obtained. This is supported by the results of a small number of measurements of the stable carbon isotope composition of ambient isoprene at different locations. The results range from approximately –29‰ to –16‰. They are consistent with vegetation emissions of isoprene that is slightly depleted in 13C relative to the plant material and enrichment of 13C in the atmosphere due to isotope fractionation associated with the reaction with OH-radicals. The stable carbon isotope ratio of ambient isoprene at locations directly influenced by isoprene emissions is very close to the values we found in our emission studies, whereas at sites located remote from isoprene emitting vegetation we find substantial enrichment of 13C. This suggests that stable carbon isotope ratio measurements will be a valuable, quantitative method to determine the extent of photochemical processing of isoprene in ambient air.

Effects of light and soil water availability on leaf photosynthesis and growth of Arisaema heterophyllum, a riparian forest understorey plant.

The effects of soil-water availability on leaf light acclimation and whole-plant carbon gain were examined in Arisaema heterophyllum Blume, a riparian deciduous forest understorey plant. Photosynthesis, above-ground morphology and ramet biomass accumulation (relative growth rate: RGR of a corm for a full leaf life-span) were measured on plants raised under three light treatments combined with two soil water conditions. The two higher light treatments during growth (high: max. 550 micro mol photons m(-2) s(-1); medium: 150 micro mol photons m(-2) s(-1)) resulted in a twofold increase in RGRs, 30% higher photosynthetic capacities and 20% less photosynthetic low-light use efficiency than those under a low light condition (50 micro mol photons m(-2) s(-1)). Leaf area was the smallest and leaf mass area ratio was the largest under the high light treatment. Water stress decreased both photosynthetic rate and leaf area and, hence, RGR in all the light regimes. However, water stress did not alter the general patterns of physiological and morphological responses to different light regimes. We estimated that higher photosynthetic low-light use efficiency and larger leaf area in the low light leaf would lead to a threefold carbon gain as compared with the high light leaf under simulated low light conditions. Both experimental and simulation results suggest that the physiological and morphological acclimations tend to be beneficial to carbon gain when light availability is low, whereas they favor increased water use efficiency when light availability is sufficiently high.

Contrasting effects of UV-A and UV-B on photosynthesis and photoprotection of beta-carotene in two Dunaliella spp.

Photosynthetic and antioxidant responses following exposure to either ultraviolet-A or ultraviolet-B were contrasted in two species of the unicellular green alga, DUNALIELLA: Species selection was based on the ability of Dunaliella bardawil (UTEX 2538) to accumulate inter-thylakoid beta-carotene when subjected to environmental stress while Dunaliella salina (UTEX 200) lacks this ability. Cells were cultured in high and low levels of visible light (150 and 35 micro mol photons m(-2 )s(-1), respectively) and then either ultraviolet-A (320-400 nm) or ultraviolet-B (290-320 nm) was added to visible light for 24-h exposure. A potassium chromate solution was found to be an ideal screen for removal of ultraviolet-A and ultraviolet-C from ultraviolet-B radiation. There were no significant changes in photosynthetic or antioxidant parameters following exposure to ultraviolet-B. Ultraviolet-A exposure significantly decreased photosynthetic parameters (>70% decrease in Fv/Fm and the ratio of light-limited to light-saturated photosynthesis in low beta-carotene cells) and resulted in 50% increases in ascorbate peroxidase activity and ascorbate concentrations. The results suggest exposure to ultraviolet-A (but not ultraviolet-B) directly affects photosynthesis, observed as a loss of photosystem II electron transport efficiency and increased radical formation. This research indicates that the accumulated beta-carotene in D. bardawil prevents UV-related photosynthetic damage through blue-light/ultraviolet-A absorption (supported by trends observed for antioxidant enzyme responses).

A novel stress-inducible antioxidant enzyme identified from the resurrection plant Xerophyta viscosa Baker.

A cDNA corresponding to 1-Cys peroxiredoxin, an evolutionarily conserved thiol-specific antioxidant enzyme, was isolated from Xerophyta viscosa Baker, a resurrection plant indigenous to Southern Africa and belonging to the family Velloziaceae. The cDNA, designated XvPer1, contains an open reading frame that encodes a polypeptide of 219 residues with a predicted molecular weight of 24.2 kDa. The XvPer1 polypeptide shows significant sequence identity (approx. 70%) to other recently identified plant 1-Cys peroxiredoxins and relatively high levels of sequence similarity (approx. 40%) to non-plant 1-Cys peroxiredoxins. The XvPer1 cDNA contains a putative polyadenylation site. As for all 1-Cys peroxiredoxins identified to date, the amino acid sequence proposed to constitute the active site of the enzyme, PVCTTE, is highly conserved in XvPer1. It also contains a putative bipartite nuclear localization signal. Southern blot analysis revealed that there is a single copy of XvPer1 in the X. viscosa genome. All angiosperm 1-Cys peroxiredoxins described to date are seed-specific and absent in vegetative tissues even under stress conditions; therefore, XvPer1 is unique in that it is expressed in the vegetative tissues of X. viscosa. The XvPer1 transcript was absent in fully hydrated X. viscosa tissue but levels increased in tissues subjected to abiotic stresses such as dehydration, heat (42 degrees C), high light intensity (1,500 micro mol photons m(-2) s(-1)) and when treated with abscisic acid (100 micro M ABA) and sodium chloride (100 mM NaCl). Western blot analyses correlated with the patterns of expression of XvPer1 transcripts under different stress conditions. Immunofluorescence analyses revealed that XvPer1 is localized in the nucleus of dehydrated X. viscosa leaf cells. These results suggest that XvPer1 is a stress-inducible gene, which may function to protect nucleic acids within the nucleus against oxidative injury.

The North Atlantic spring phytoplankton bloom and Sverdrup's critical depth hypothesis.

More than 50 years ago, Harald Sverdrup developed a simple model for the necessary conditions leading to the spring bloom of phytoplankton. Although this model has been used extensively across a variety of aquatic ecosystems, its application requires knowledge of community compensation irradiance (IC), the light level where photosynthetic and ecosystem community loss processes balance. However, reported IC values have varied by an order of magnitude. Here, IC estimates are determined using satellite and hydrographic data sets consistent with the assumptions in Sverdrup's 1953 critical depth hypothesis. Retrieved values of IC are approximately uniform throughout much of the North Atlantic with a mean value of 1.3 mol photons meter-2 day-1. These community-based IC determinations are roughly twice typical values found for phytoplankton alone indicating that phytoplankton account for approximately one-half of community ecosystem losses. This work also suggests that important aspects of heterotrophic community dynamics can be assessed using satellite observations.

Effects of light availability on growth, architecture and nutrient content of the seagrass Zostera noltii Hornem

The growth vs. irradiance response of the seagrass Zostera noltii from Cadiz Bay Natural Park (southwestern Spain) was characterised. Plants were exposed along 14 days to different light treatments (1%, 7%, 42% and 100% surface irradiance, SI), using shade screens in an outdoor mesocosm. Growth at 100% SI (1.6 mg DW plant −1 day −1) was lower than that at 42% SI (2.4 mg DW plant −1 day −1), suggesting photoinhibition. The minimum light requirement estimated was 0.8 mol photons m −2 day −1 (2% SI). Light availability affected the pattern of plant development and the overall plant growth. The contribution of the apical shoots to the aboveground production was nearly constant (c.a. 1.13 cm plant −1 day −1) regardless of the light level (except at 1% SI). In contrast, recruitment and growth of lateral shoots arising from the main rhizome axes accounted for the observed differences in aboveground growth. Rhizome branching was only observed at 42% SI. The possibility of a light threshold for rhizome branching could explain the seasonality of shoot recruitment, as well as the observed decrease in shoot density along depth (or light) gradients in seagrass meadows. Carbon demands at low irradiances (1% and 7% SI) were partially met by mobilization of carbohydrate reserves (sucrose in belowground and starch in aboveground parts). Plant nitrogen content decreased with increasing light, especially in belowground parts, reaching critical levels for growth.

Studies on the Tropical Agarophyte Gracilaria cornea J. Agardh (Rhodophyta, Gracilariales) from Yucatán, Mexico. I. Seasonal Physiological and Biochemical Responses

Photosynthetic performance, photosynthetic pigments, carbohydrate and protein content were measured monthly in Gracilaria cornea collected over 1 year in Yucatan, Mexico. The algae showed a marked seasonality in photosynthetic capacity and biochemical constituents. The photosynthetic rates (4 to 11 mg O 2 g d wt - 1 h - 1 ) were higher during the cold (November-February) and dry (March-June) seasons compared to the wet (June-October) season. Compensation and saturation irradiances were low (17-37 and 72-155 pmol photons m - 2 s - 1 ). Thus, although the populations occurred in shallow (2-4 m) water, the low light acclimation reflects the effect of high turbidity, sedimentation on the branches, and low irradiance values that were continuously recorded (1.97 to 8.85 mol photons m - 2 d - 1 ) throughout the year. The low concentrations of chlorophyll a (0.25 to 0.49 mg g d wt - 1 ) and phycoerythrin (0.03 to 0.25 mg g d wt - 1 ) reflect the oligotrophic waters as shown by concentrations of dissolved inorganic nitrogen (9.1 pmol L - 1 ) and dissolved reactive phosphate (0.5 μmol L - 1 ). Total soluble carbohydrate levels increased from July to October reaching 56 % (dry weight basis) coinciding with high water temperatures. Protein levels ranged from 5.1 % in February to 3.0 % (dry weight basis) in August with no significant differences between months. Thus, the growth period for Gracilaria cornea appears to be temperature limited, occurring mainly during the dry seasons.

The influence of nutrients on growth and photosynthesis of seedlings of two rainforest dipterocarp species.

This study aims to determine the influence of nutrient additions in the form of nitrogen (N), phosphorus (P) and potassium (K) on the photosynthetic physiology, morphology and growth of Dryobalanops lanceolata Burck and Shorea johorensis Foxw., two co-existing dipterocarp species in the understorey of secondary forest. Analysis of the light environment revealed that the total daily irradiance in the understorey was 4.57 ± 0.70 mol photons m-2 d-1, approximately 20% higher than that which could be expected to occur in the primary forest understorey. Around 50% of total irradiance occurred at fluxes over 400 μmol photons m-2 s-1, and this was received over 7% of the daylight period. Both D. lanceolata and S. johorensis grew over the 16-month study, but only S. johorensis increased growth in response to nutrient additions. Neither species altered biomass partitioning in response to additional nutrients. Both species also showed increased rates of photosynthetic induction (T0.5Asat) in response to nutrient additions, but only S. johorensis showed an increase in light-saturated rates of photosynthetic capacity (Asat), and the chlorophyll content of leaves. We argue that additional nutrients increased the capacity of S. johorensis to utilise the sporadic burst of high irradiance in sunflecks. Analysis of the nutrient composition of leaves from age cohorts up to > 16 months showed that additional nutrients increased the P and K concentration in D. lanceolata leaves, and the P and N content of S. johorensis leaves. This suggests that neither P nor K were limiting the growth of D. lanceolata in this secondary forest environment, but that P and/or N may limit growth of S. johorensis. Our data highlights differences in the ecology of two co-existing dipterocarp species. We argue that D. lanceolata shows characteristics consistent with a low resource-demanding species that may have a competitive advantage over S. johorensis in low resource environments, while S. johorensis shows traits better suited to higher resource availability. Thus, there are distinct species-specific differences in the ability to respond to nutrients that depend on the irradiance environment, and which may have a role in niche partitioning among the dipterocarps.

Photosynthesis, primary production and phytoplankton growth rates in Gerlache and Bransfield Straits during Austral summer: cruise FRUELA 95

Bio-opticalparameters, primary production, and phytopl ankton growth rates were determined in the southern part of the Bellingshausen Sea, the western part of the Bransfield Strait, and the Gerlache Strait, during December 1995 and early January 1996. Three water bodies were present at the surface (o100 m); Bellingshausen warm waters (BWW), Bellingshausen cold waters (BCW), and transitional Weddell waters (TWW), which were separated by a thermal front. High chlorophyll concentrations were found in the upper mixed layer (UML) of the BCW waters (5.5774.04 mg m � 3 ), with intermediate values in BWW (2.8571.24 mg m � 3 ) and the lowest values in the TWW (1.5370.94 mg m � 3 ). The broadband photosynthetic parameters ðP B ; a B and EKPARÞ were highest in BWW and lowest in TWW, except for EKPAR; which did not show significant differences between water bodies. Spectralbio-opticalparameters ðaph; f max and EKPURÞ showed a similar distribution to chlorophyll concentrations except EKPUR; which was highest in BWW. A comparison between light-saturation parameters and water-column irradiance suggests that photosynthesis was not light-limited in the mixed layer. However, transitory situations exist where photosynthesis could be close to limitation at the bottom of the photic layer when high chlorophyll concentrations were present in the surface layers. The high maximum quantum yields (0.07370.032 molC (mol photons) � 1 ) also suggest that iron limitation of photosynthesis should not be occurring in the region during the cruise. The mean primary production rates were 1.1170.68 g Cm � 2 d � 1 in the TWW, 2.2870.98 g C m � 2 d � 1 in the BCW, and 2.6870.94 g C � 2 d � 1 in the BWW. The Gerlache Strait and frontal zones were the most productive, with values analogous to those of upwelling areas. Carbon-specific growth rates in UML (0.4370.16 d � 1 ) were similar to those of temperate seas, neither nutrient- nor light-limited. Growth rates in the UML of BCW (0.5470.20 d � 1 ) were higher than those of TWW (0.3770.08 d � 1 ) and BWW (0.3570.09 d � 1 ). The results indicate that factors as microzooplankton grazing or sinking should control the standing

Photosynthetic efficiency of Dunaliella tertiolecta under short light/dark cycles

Inside photobioreactors algae are exposed to light/dark fluctuations. In this study the marine green alga Dunaliella tertiolecta was cultivated under short light/dark cycles of 3/3 s, 94/94 ms and 31/156 ms as found in medium and short light-path reactors. The photon flux density (PFD) during the light period was 440–455 μmol m −2 s −1 and, under the 31/156 ms cycle only, 1025 μmol m −2 s −1. The photosynthetic efficiency was determined and expressed as the biomass yield on light energy in gram protein produced per mol of photons absorbed. The yield under the 94/94 ms cycle was higher than the yield under continuous light of 440–455 μmol m −2 s −1. Apparently a light integration effect occurs under the 94/94 ms cycle together with an increase of the photosynthetic efficiency. On the contrary, at the 3/3 s cycle the efficiency decreased. Also under the 31/156 ms cycle, with 1025 μmol m −2 s −1 in the light period, the yield was lower than under continuous light. The results obtained under the 31/156 ms light/dark cycle are discussed with respect to the performance of short light-path flat panel photobioreactors operated at high biomass densities.

Remote Oxidation of Organic Compounds by UV-Irradiated TiO2 via the Gas Phase

The remote oxidation of organic materials via the gas phase was studied in detail. A TiO2-coated glass plate was faced to an organic film separated by a small gap (50 μm to 2.2 mm), and the TiO2 was irradiated with UV light. As a result, aromatic and aliphatic substances were oxygenated and decomposed to generate CO2 by active oxygen species that were generated at the TiO2 surface and transported in the gas phase. The amount of CO2 generated during the irradiation with 1 mol of photons was estimated to be 2 × 10-4 mol for polystyrene oxidation and 3 × 10-5 mol for polyethylene oxidation. The active oxygen species that oxidizes the organic substrates is most likely to be HO•. Strong evidence for this is the result that the organic materials were oxidized by chemically generated HO• in a similar way.

STUDIES ON THE LIFE CYCLE OF PORPHYRA DIOICA AND PORPHYRA UMBILICALIS FROM PORTUGAL

Pereira, R.1,2, Sousa‐Pinto, I.1,2 & Yarish, C.3 1Centre for Marine and Environmental Research, Rua do Campo Alegre, 823, 4150‐180 Porto, PORTUGAL; 2Botany Dept. of the University of Porto; 3University of Connecticut, Dept. of Ecology and Evolutionary Biology, 1 University Place, Stamford CT 06901‐2315, USAPorphyra is a well known genus, as it is one of the most important maricultured seaweeds. It has an annual retail value of US $1.8 billion principally grown in Japan, Korea and China. In the past 10 years, expanding markets in North America and Europe have put new demands on the import of the red alga. After some attempts trying to grow Asian species, efforts are now centered on domestication of native species, both in North America and Europe. With our current understanding of this genus, we believe that their are at least six Porphyra species along the Portuguese coastline. We have initially identified two species, P. dioica and P. umbilicalis, as potentially promising for aquaculture development. We are studying the life cycles and phenology of these species, with special note on morphological variability and on the environmental control of each phase of the life cycle. Porphyra dioica carpospores germinate best at 15‐20°C and 25‐75 micro mol photons m‐2 s‐1. Best conditions for conchocelis vegetative growth and the development of conchosporangia are at 15° C and 25‐75 micro mol photons m‐2 s‐1. P. dioica conchocelis growth appears to be under temperature controlled, rather than by photoperiod. For P.umbilicalis, 15° C and 25 micro mol photons m‐2 s‐1 are the best conditions to obtain conchocelis and for formation of conchosporangia. At these conditions carpospore survival rate is about 33.4 %. Conchospores are released at 10 and 15° C and at 25‐75 micro mol m‐2 s‐1. For both species, formation of conchosporangia seems to be more related with time and culture conditions, rather than with size or development of the conchocelis.

INFLUENCE OF SALINITY AND TEMPERATURE ON GROWTH AND PHOTOSYNTHESIS IN THE EXTREMOPHILIC CHLOROPHYTE, NANNOCHLORIS SP.

Major, K. M. & Henley, W. J.Department of Botany, Oklahoma State University, Stillwater, OK 74078‐3013 USAPreliminary data suggest Nannochloris sp., isolated from the Great Salt Plains National Wildlife Refuge, is a true extremophile. This alga is able to withstand salinities ranging from 0 to 150 ç and temperatures up to 45°C. To test the hypothesis that acclimation to high salinity confers tolerance to high temperature, experimental cultures were acclimated to salinities of 25 and 100 ç and/or temperatures of 23 and 38°C; irradiance (500 mol photons m‐2 s‐1) was saturating for both growth and photosynthesis. Cells acclimated to low salt and low temperature exhibited high photosynthetic performance in terms of both light‐saturated photosynthesis (Pmax; 45.0 fmol O2 cell‐1 h‐1) and light‐harvesting efficiency (0.103 fmol O2 cell‐1 h‐1/mol photons m‐2 s‐1). However, high‐salinity cells exhibited values for net Pmax (18.1 fmol O2 cell‐1 h‐1), (0.107 fmol O2 cell‐1 h‐1/mol photons m‐2 s‐1) and growth rates (ca. 0.4 d‐1) that were equal to, or higher than, those of low‐salinity cells when acclimated to high temperature. Both the amount of light required to achieve net photosynthesis (Ic) and that required to achieve light‐saturated photosynthesis (Ik) were lower in high‐salinity cells than those exhibited by low‐salinity cells grown at high temperature; reductions in Ic and Ik were primarily due to increases in light‐harvesting efficiency. We propose that an increase in growth temperature might release Nannochloris sp. from energy constraints associated with osmolyte production and low‐temperature effects on enzyme activity. These data are consistent with effects of short‐term temperature stress on Chl a fluorescence kinetics in this alga.

NDP‐sugars, floridoside and floridean starch levels in relation to activities of UDP‐glucose pyrophosphorylase and UDP‐glucose‐4‐epimerase in Solieria chorda l is (Rhodophyceae) under experimental conditions

SUMMARYUnder limited nutrient availability (i.e. unenriched sea‐water) and under 75 mol photons m–2 s–1 irradiance 12:12 LD, thalli of Solieria chordalis J. Agardh accumulated floridean starch and floridoside. When they were transferred into nutrient‐enriched seawater (150 umol L−1 NO31‐ and 7 umol L−1 P043i at 35 umol photons m−2 s−1 in irradiance 12:12 LD, starch and floridoside levels decreased. The main nucleotide diphosphate (NDP) sugars (i.e. UDP‐glucose, UDP‐galactose and ADP‐glucose) and the activities of UDP‐glucose pyrophosphorylase [Enzyme Code (EC) 2.7.7.9] and UDP‐glucose‐4‐epimerase (EC 5.1.3.2) were measured under these controlled culture conditions. Both UDP‐glucose and UDP‐galactose in the thal l i increased under conditions known to favor the accumulation of floridean starch and floridoside, whereas they decreased under conditions leading to floridean starch and floridoside breakdown. On the other hand, ADP‐glucose level only varied slightly. Although UDP‐glucose pyrophosphorylase activity rose under conditions of floridean starch synthesis, little variation was observed in UDP‐glucose‐4‐epimerase activity. These results suggest a possible enzymatic regulation of the NDP‐sugar and carbohydrate pool in which UDP‐glucose pyrophosphorylase would play a major role.

Effects of periodic, low UVA radiation on cell characteristics and oxidative stress in the marine planktonic diatom Ditylum brightwellii

A continuous culture experiment was conducted with the marine diatom Ditylum brighiwellii to assess the photo-oxidative effects of transition from dim light to high Light at three successive levels. The focus was on temporary stress by ambient WA at the water surface, via a simulation of the periodic ascent of diatoms caused by vertical mixing. Within a 14 h dim- light period, cells were exposed daily to 4 h of: (A) moderate PAR (100 mu mol photon m(-2) s(-1)), (B) high PAR (400 mu mol photon m(-2) s(-1)) and (C) high PAR (400-700 nm) plus 67 pmol photon m(-2) s(-1) (2.2 W m(-2)) UVA. Cell shape, life cycle and sinking rates were not affected by high PAR per se (mode B). The daily 4 h high-irradiance mode C, with additional UVA (WA:PAR = 0.016), caused an increase in cell diameter, a decrease in length axes, and a production of vegetative resting stages. Superoxide dismutase (SOD) activities, oxidized glutathione (GSSG) pools and malondialdehyde (MDA) contents increased as well, which indicates that this biologically effective WA dose (0.78 W m(-2)) promoted active oxygen production, oxidative stress and lipid peroxidation. Sinking rates in WA-exposed D. brightwellii were higher than in those grown without UVA. A close correlation between sinking rates and MDA contents suggests that UVA-induced lipid peroxidation has altered the membrane functions which regulate buoyancy in D. brightwellii. Although UVA-induced formation of resting cells will contribute to accelerated sinking, it is not evident that this can be considered as a stress avoidance mechanism under adverse light (WA) conditions. D. brightwellii proved to be sensitive to transitions from shade to natural surface irradiance conditions. In coastal waters, such large diatom species may experience oxidative stress after a few hours' residence near the water surface at ambient UVA levels. [KEYWORDS: cell size; Ditylum brightwellii; glutathione redox;lipid peroxidation; oxidative stress; planktonic diatom; resting stage; sinking rate; superoxide; ultraviolet-A Ultraviolet-b radiation; lipid-peroxidation; chlorella-vulgaris; photosynthetic pigment; superoxide-dismutase; thylakoid membranes; antioxidant systems; maize seedlings; sinking rates; phytoplankton]

Dependence of calcification on light and carbonate ion concentration for the hermatypic coral Porites compressa

Temperature, light and aragonite saturation state of seawater are important determinants of the global distribution of coral reefs. Saturation state of surface seawater is decreasing in response to increases in atmospheric pCO 2 , causing concern for a global reduction in the rates ol reef accretion. In this study we address the influences of light and saturation state on skeletal growth of the hermatypic coral Porites compressa. We conducted 2 experiments using coral nubbins; one in a controlled laboratory with Biosphere 2 ocean water and the other in the 711 m 2 Biosphere 2 ocean itself. The laboratory experiment consisted of 3 light treatments (PAR = 698, 150, 81 μmol photons m -2 s -1 ) and 2 chemical treatments (pCO 2 = 199 μatm, Ω = 5.05, CO 3 2- = 355 μmol kg -1 ; and pCO 2 = 448 μatm, Ω = 2.48, CO 3 2- = 183 μmol kg -1 ). In the Biosphere 2 ocean, coral nubbins were deployed at 4 depths (6, 4, 2 and 0.5 m) which corresponded to different light conditions (PAR from 2.7 to 29.5 mol photons m -2 d -1 ) during 3 different chemical states, mimicking the conditions of the Last Glacial Maximum (LGM), the present day and the year 2100 (Y2100). Calcification rate changed with light following the typical hyperbolic tangent function. Calcification rate was positively correlated with saturation state, which ranged from 5 (LGM) to 2.4 (Y2100). From the glacial experimental conditions (Ω = 5.05, pCO 2 = 186) to those of the future (Ω = 2.25, pCO 2 = 641), calcification dropped by 30%; from present day conditions (Ω = 3.64, pCO 2 = 336) to those of the future, calcification dropped by 11%, This decrease in calcification rate occurred at all light levels, indicating that rising CO 2 will impact corals living at all depths.

ACCLIMATION OF PALMARIA PALMATA (RHODOPHYTA) TO LIGHT INTENSITY: COMPARISON BETWEEN ARTIFICIAL AND NATURAL LIGHT FIELDS

The acclimation of the photosynthetic apparatus of Palmaria palmata (L.) to light intensity was examined in the field and under laboratory conditions. Algae from 3 different shore levels and from laboratory cultures adapted to 6 different photon flux densities were compared. This was done on the basis of light doses, which were delivered by different light regimes in the field and in the laboratory. Laboratory samples were adjusted to constant photon flux densities between 7 and 569 μmol photons·m−2·s−1 in a 16:8 light:dark photoperiod. Under field conditions the daily amplitudes reached up to approximately 2000 μmol photons·m−2·s−1 within a natural daily light course. Over the course of 14 days the light doses resulting from those different regimes are similar for both treatments. An increasing growth rate per day with increasing light doses was observed in the laboratory. Growth was saturated at 113 mol photons·m−2·14 d−1. Light saturation points (Ek) of photosynthesis increased with increasing light doses for both field and laboratory samples, and all Ek values were significantly related to the growth light dose. A correlation between fresh weight‐related lutein content and growth light dose was found for laboratory samples only, whereas the lutein:chlorophyll a (chl a) ratio was strongly correlated with Ek for laboratory and field samples. The content of chl a and phycoerythrin (PE) per fresh weight decreased significantly with increasing light doses under field conditions. Simultaneously, the PE:chl a ratio increased, whereas this ratio was not influenced by laboratory treatments. The correspondence of Ek values for field and laboratory treatments indicated that they were affected mainly by light dose. However, the variability in pigmentation was mainly dependent on temporal variability in light intensity (the amplitude of variations in incident light).

Photosynthetic utilisation of carbon and light by two tropical seagrass species as measured in situ

In situ measurements of seagrass photosynthesis in relation to inorganic carbon (Ci) availability, increased pH and an inhibitor of extracellular carbonic anhydrase were made using an underwater pulse amplitude modulated (PAM) fluorometer. By combining the instrument with a specially designed Perspex chamber, we were able to alter the water surrounding a leaf without removing it from the growing plant. Responses to Ci within the chamber showed that subtidal plants of the seagrasses Cymodocea serrulata and Halophila ovalis had photosynthetic rates that were limited by the ambient Ci concentration depending on the irradiance that was available during short-term photosynthesis–irradiance trials. Relative electron transport rates (RETRs) at light saturation (up to 500 μ mol photons m−2 s−1) increased by 66–100% when the Ci concentration was increased from ca. 2.2 to 6.2 mM. On the other hand, intertidal plants of the same species exhibited a much lesser limitation of photosynthesis by Ci at any irradiance (up to 1500 μ mol photons m−2 s−1). Both species were able to use HCO−3 efficiently, and there was stronger evidence for direct uptake of HCO−3 rather than extracellular dehydration of HCO−3 to CO2 prior to Ci uptake. Subtidally, H. ovalis and C. serrulata grew to 10 and 12 m, respectively, where ambient irradiances were approximately 16 and 11% of those at the surface. Maximum RETRs (at light saturation) were lower for these deep-growing plants than for the intertidally growing ones. For both species, the onset of light saturation of photosynthesis (Ek) occurred at approximately 100 μ mol photons m−2 s−1 for the deep water populations, which was four and two times lower than for the shallow populations of C. serrulata and H. ovalis, respectively. This, and the differences in maximal photosynthetic rates (RETRmax), reflects an acclimation of the deep-growing populations to the lower light environment. The results presented here show that photosynthesis, as measured in situ, was limited by the availability of Ci for the deeper growing plants in Zanzibar, while the intertidally growing plants photosynthesised at close to Ci saturation. The latter result is contrary to previous conclusions regarding Ci limitations for these intertidal plants, and, in general, our findings highlight the need for performing similar experiments in situ rather than under laboratory conditions.