Low CO2 Availability Research Articles

Phosphorus enrichment and carbon depletion contribute to high Microcystis biomass and microcystin concentrations in Ugandan lakes

We investigated the factors influencing cyanobacterial biomass and microcystin (MC) concentrations in several Ugandan lakes from September 2008 to February 2009. We characterized thermal structure, light availability, nutrient concentrations, chlorophyll a, phytoplankton δ13C (as an indicator of CO2 limitation), and phytoplankton community composition and abundance as well as MC concentrations. We used these data to test several hypotheses based on previous research in temperate lakes regarding the factors that encourage high cyanobacterial biomass and MC concentrations. Site characteristics that appeared to favor high cyanobacterial biomass (especially Microcystis) included: high total phosphorus concentrations, low total nitrogen to total phosphorus (TN : TP) ratios, and possibly low CO2 availability. Light availability, total nitrogen concentrations, and thermal structure of the water column were not related to cyanobacterial biomass. MC concentrations were strongly related to Microcystis biomass (and were not related to the biomass of any other cyanobacterial taxa), which was positively correlated with total phosphorus and chlorophyll a concentrations. MC cell content may be moderated by CO2 availability, with MC cell quotas tending to be lower where the potential for C‐limitation of photosynthesis was higher. In these phosphorus‐rich tropical lakes, the shallowest study sites were most conducive to the development of large standing crops of Microcystis and high MC concentrations. The environmental conditions that appear to favor high cyanobacterial biomass and MC concentrations in our Ugandan study lakes are similar to what has been observed for temperate lakes.

Algal and aquatic plant carbon concentrating mechanisms in relation to environmental change

Carbon dioxide concentrating mechanisms (also known as inorganic carbon concentrating mechanisms; both abbreviated as CCMs) presumably evolved under conditions of low CO(2) availability. However, the timing of their origin is unclear since there are no sound estimates from molecular clocks, and even if there were, there are no proxies for the functioning of CCMs. Accordingly, we cannot use previous episodes of high CO(2) (e.g. the Palaeocene-Eocene Thermal Maximum) to indicate how organisms with CCMs responded. Present and predicted environmental change in terms of increased CO(2) and temperature are leading to increased CO(2) and HCO(3)(-) and decreased CO(3)(2-) and pH in surface seawater, as well as decreasing the depth of the upper mixed layer and increasing the degree of isolation of this layer with respect to nutrient flux from deeper waters. The outcome of these forcing factors is to increase the availability of inorganic carbon, photosynthetic active radiation (PAR) and ultraviolet B radiation (UVB) to aquatic photolithotrophs and to decrease the supply of the nutrients (combined) nitrogen and phosphorus and of any non-aeolian iron. The influence of these variations on CCM expression has been examined to varying degrees as acclimation by extant organisms. Increased PAR increases CCM expression in terms of CO(2) affinity, whilst increased UVB has a range of effects in the organisms examined; little relevant information is available on increased temperature. Decreased combined nitrogen supply generally increases CO(2) affinity, decreased iron availability increases CO(2) affinity, and decreased phosphorus supply has varying effects on the organisms examined. There are few data sets showing interactions amongst the observed changes, and even less information on genetic (adaptation) changes in response to the forcing factors. In freshwaters, changes in phytoplankton species composition may alter with environmental change with consequences for frequency of species with or without CCMs. The information available permits less predictive power as to the effect of the forcing factors on CCM expression than for their overall effects on growth. CCMs are currently not part of models as to how global environmental change has altered, and is likely to further alter, algal and aquatic plant primary productivity.

El metabolismo ácido de las crasuláceas: diversidad, fisiología ambiental y productividad

México posee una gran diversidad de especies con el Metabolismo Ácido de las Crasuláceas (CAM). Las especies con este metabolismo pueden crecer en sitios donde la disponibilidad de agua es infrecuente, como son las zonas áridas y semiáridas y las copas de los árboles, o como hidrofitas en sitios con poca disponibilidad de CO2. Esta revisión presenta aspectos metodológicos, evolutivos, ecológicos y fisiológicos de este tipo de fotosíntesis. Además, se presentan datos de estudios recientes relacionados con la influencia de las condiciones ambientales en el ciclo diario de la fotosíntesis CAM. Finalmente, se presenta una reflexión sobre la falta de estudios en la fisiología de plantas CAM en México a pesar de su enorme diversidad.

CO2‐CONCENTRATING MECHANISMS OF THE POTENTIALLY TOXIC DINOFLAGELLATE PROTOCERATIUM RETICULATUM (DINOPHYCEAE, GONYAULACALES)1

The low CO2 concentration in seawater poses severe restrictions on photosynthesis, especially on those species with form II RUBISCO. We found that the potentially toxic dinoflagellate Protoceratium reticulatum Clap. et J. Lachm. possesses a form II RUBISCO. To cast some light on the mechanisms this organism undergoes to cope with low CO2 availability, we compared cells grown at atmospheric (370 ppm) and high (5000 ppm) CO2 concentrations, with respect to a number of physiological parameters related to dissolved inorganic carbon (DIC) acquisition and assimilation. The photosynthetic affinity for DIC was about one order of magnitude lower in cells cultivated at high [CO2]. End‐point pH‐drift experiments suggest that P. reticulatum was not able to efficiently use HCO3− under our growth conditions. Only internal carbonic anhydrase (CA) activity was detected, and its activity decreased by about 60% in cells cultured at high [CO2]. Antibodies raised against a variety of algal CAs were used for Western blot analysis: P. reticulatum extracts only cross‐reacted with anti‐ß‐CA sera, and the amount of immunoreactive protein decreased in cells grown at high [CO2]. No pyrenoids were observed under all growth conditions. Our data indicate that P. reticulatum has an inducible carbon‐concentrating mechanism (CCM) that operates in the absence of pyrenoids and with little intracellular CO2 accumulation. Calculations on the impact of the CA activity to photosynthetic growth [CO2] suggest that it is an essential component of the CCM of P. reticulatum and is necessary to sustain the photosynthetic rates observed at ambient CO2.

Diversity and plasticity of C4 photosynthesis in Eleocharis (Cyperaceae).

Eleocharis contains many amphibious species, and displays diversity of photosynthetic mechanism (C3, C4 or C3-C4 intermediates). A unique feature of Eleocharis is the plasticity in the photosynthetic mechanism of some species in response to the environment. In this study, we have examined the culm anatomy and photosynthetic property of several Eleocharis species grown terrestrially and the changes in the newly produced culms over a short period time frame after switching from terrestrial to submerged condition. Eleocharis baldwinii (Torrey) Chapman is C4-like in terrestrial habitat, exhibiting O2 inhibition of photosynthesis with Rubisco expressed in both mesophyll and bundle sheath cells and PEPC strictly in the mesophyll cells, but switches to C3-C4 intermediacy when submerged. In addition to Eleocharis vivipara Link type 1 (which switches from C4-like to C3), two other photosynthetic types examined in this study were shown to have different responses to growth in either terrestrial or submerged conditions. E. vivipara type 2 is a typical C4 plant in the terrestrial habitat, but becomes a C3-C4 intermediate under submerged conditions. Further, terrestrially, E. vivipara type 3 is a C3-C4 intermediate, but when submerged the δ13C value increases to -6.7‰, indicating its use of bicarbonate as a major carbon source. The submerged form of this plant exhibited about three times higher photosynthetic O2 evolution rate, compared to the C3 species Eleocharis erythropoda Steudel. These Eleocharis species possess different molecular switches for regulating C4 gene expression in response to environmental stimuli both between different species, and in E. vivipara among different populations. The apparent expression of a bicarbonate transport system by E. vivipara type 3 while submerged represents a unique adaptation to low CO2 availability.

Anaerobic fermentation of glycerol byEscherichia coli: A new platform for metabolic engineering

The worldwide surplus of glycerol generated as inevitable byproduct of biodiesel fuel and oleochemical production is resulting in the shutdown of traditional glycerol-producing/refining plants and new applications are needed for this now abundant carbon source. In this article we report our finding that Escherichia coli can ferment glycerol in a pH-dependent manner. We hypothesize that glycerol fermentation is linked to the availability of CO(2), which under acidic conditions is produced by the oxidation of formate by the enzyme formate hydrogen lyase (FHL). In agreement with this hypothesis, glycerol fermentation was severely impaired by blocking the activity of FHL. We demonstrated that, unlike CO(2), hydrogen (the other product of FHL-mediated formate oxidation) had a negative impact on cell growth and glycerol fermentation. In addition, supplementation of the medium with CO(2) partially restored the ability of an FHL-deficient strain to ferment glycerol. High pH resulted in low CO(2) generation (low activity of FHL) and availability (most CO(2) is converted to bicarbonate), and consequently very inefficient fermentation of glycerol. Most of the fermented glycerol was recovered in the reduced compounds ethanol and succinate (93% of the product mixture), which reflects the highly reduced state of glycerol and confirms the fermentative nature of this process. Since glycerol is a cheap, abundant, and highly reduced carbon source, our findings should enable the development of an E. coli-based platform for the anaerobic production of reduced chemicals from glycerol at yields higher than those obtained from common sugars, such as glucose.

Isoprenoids content and photosynthetic limitations in rosemary and spearmint plants under water stress

Abstract Water scarcity is one of the most detrimental effects on crop yield. We studied the effect of water stress on carbon acquisition and allocation to monoterpenes of commercial importance in spearmint and rosemary plants. Increasing levels of water stress reduced photosynthesis and plant biomass. Under increasing water-stress levels photosynthesis was limited by low CO2 availability due to reduced stomatal and mesophyll conductance, while photochemical and biochemical processes were not affected. At the balsamic period, significantly higher amounts of monoterpenes per unit mass were formed in water-stressed leaves with respect to irrigated controls. However, because of plant biomass reduction, water stress reduced the total yield of monoterpenes. Appropriate irrigation strategies may, therefore, potentially improve yield of monoterpenes in field grown spearmint and rosemary crops.

Adaptações de plantas submersas à absorção do carbono inorgânico

No presente trabalho são discutidos alguns aspectos teóricos dos mecanismos e adaptações empregados pela vegetação submersa para maximizar o aproveitamento do carbono inorgânico na água. O tipo de estratégia utilizada pelas macrófitas aquáticas submersas deve-se a diferenças genéticas entre as espécies e também às condições ambientais predominantes. Vários mecanismos fisiológicos e morfológicos, como a utilização do metabolismo C4, do ácido das crassuláceas (CAM), a utilização do bicarbonato (HCO3-), a utilização do CO2 da água intersticial do sedimento e o desenvolvimento de folhas aéreas foram considerados as principais adaptações para evitar a limitação do carbono no ambiente aquático. De relevância ecológica, a utilização destas diferentes estratégias pode compensar baixas ofertas de CO2 às taxas fotossintéticas de várias espécies submersas e suprimir a fotorrespiração por garantir altas concentrações intracelulares de CO2. Assim, estes mecanismos são responsáveis, em parte, pelo sucesso das macrófitas aquáticas submersas em ambientes oligotróficos, com baixas concentrações de CO2.

Short- and long-term effects of irradiance and CO2 availability on carbon fixation by two marine diatoms

Unbuffered and nutrient-replete dilute batch cultures of Skeletonema costatum Cleve and Phaeodactylum tricornutum Bohlin were grown at high and low CO2 availability conditions and two incident irradiances, 150 and 30 µmol photons·m–2·s–1. Long-term combined effects of such light and CO2 availability conditions on carbon fixation rates of both diatoms were compared. At saturating light, P. tricornutum showed higher photosynthetic rates than S. costatum at both CO2 conditions. However, under subsaturating light, carbon fixation rates of P. tricornutum were higher than observed for S. costatum only at low CO2. Skeletonema costatum showed a strong reduction in photosynthetic rates only when both resources, irradiance and CO2, were low. Short-term alterations of light and CO2 availability on carbon fixation showed that the response of S. costatum differed considerably from long-term trends: the short-term reduction in CO2 availability at both light levels resulted in a considerable decrease in the maximum photosynthetic rates. This effect was much less noticeable in P. tricornutum. The results show that, at saturating light, both diatoms maintain maximum photosynthetic rates under low CO2 levels, but only P. tricornutum is well adapted to rapid changes in this resource. This capacity of adaptation seems to be light dependent, since light limitation altered the responses of both diatoms to low CO2 availability conditions.Key words: CO2, 14C fixation, irradiance, Phaeodactylum tricornutum, Skeletonema costatum.

CO2-concentrating mechanisms in Egeria densa, a submersed aquatic plant.

Egeria densa is an aquatic higher plant which has developed different mechanisms to deal with photosynthesis under conditions of low CO2 availability. On the one hand it shows leaf pH-polarity, which has been proposed to be used for bicarbonate utilization. In this way, at high light intensities and low dissolved carbon concentration, this species generates a low pH at the adaxial leaf surface. This acidification shifts the equilibrium HCO3-/CO2 towards CO2, which enters the cell by passive diffusion. By this means, E. densa increases the concentration of CO2 available for photosynthesis inside the cells, when this gas is limiting. On the other hand, under stress conditions resulting from high temperature and high light intensities, it shows a biochemical adaptation with the induction of a C4-like mechanism but without Kranz anatomy. Transfer from low to high temperature and light conditions induces increased levels of phosphoenolpyruvate carboxylase (PEPC, EC 4.1.1.31) and NADP-malic enzyme (NADP-ME, EC 1.1.1.40), both key enzymes participating in the Hatch-Slack cycle in plants with C4 metabolism. Moreover, one PEPC isoform, whose synthesis is induced by high temperature and light, is phosphorylated in the light, and changes in kinetic and regulatory properties are correlated with changes in the phosphorylation state of this enzyme. In the present review, we describe these two processes in this submersed angiosperm that appear to help it perform photosynthesis under conditions of extreme temperatures and high light intensities.

Effects of drought on light-energy dissipation mechanisms in high-light-acclimated, field-grown grapevines.

The response of several light-energy dissipation mechanisms to water shortage was analysed in a 10-year study in field-grown, high-light-acclimated grapevines, and compared with those of greenhouse-grown, low-light-acclimated grapevines. Dissipation mechanisms, except leaf photochemistry, differ among cultivars and acclimate to the prevailing light conditions during growth. However, no additional acclimation to drought was observed. The dependence of the dissipation responses on stomatal conductance suggests that low CO2 availability in the chloroplasts during drought triggers variations in the energy dissipation pattern. In irrigated grapevines under high light, more than 50% of total absorbed energy is thermally dissipated. There is evidence that implicates the xanthophyll cycle as the main thermal dissipation processes. CO2 assimilation is the most important photochemical pathway of dissipation in irrigated plants, but is replaced by photorespiration when CO2 assimilation declines under mild drought. Under moderate to severe drought, both photosynthesis and photorespiration decline, and thermal dissipation increases to account for up to 90% of total dissipation. Involvement of other processes in light dissipation is minimal in grapevines. Even in severely-stressed leaves, the incidence of photoinhibition is very low, indicating that safe dissipation of absorbed energy is very effective in grapevines.

Interactions between light and CO2 enhance the growth of Riccia fluitans

We grew the amphibious liverwort, Riccia fluitans, at saturating nitrogen and phosphorus concentrations in a cross-factorial design under three levels of light and three levels of CO2 making a matrix of nine treatments. Under these conditions, the relative growth rate (RGR) ranged from 0.011 d−1 at low light and CO2 availability to 0.138 d−1 at high light and CO2 with a significant positive interaction between light and CO2 on the RGR. After the growth experiments, a range of photosynthetic parameters were measured and in particular the maximum net photosynthesis (NPmax) showed a strong acclimation to light and CO2 availability. NPmax decreased significantly with increasing light intensities but increased with increasing CO2 concentration. Surprisingly, no positive correlation between the dark respiration (R) and the RGR was found. Rather, a strong positive correlation between NPmax and R was present and thus a positive correlation between R and RGR cannot be obtained since NPmax and RGR did not develop in parallel with increasing light and CO2 availability. The CO2 compensation point for photosynthesis was also strongly affected by the availability of light and CO2. The CO2 compensation point was very low a high light and low CO2 and increased at low light and high CO2 and there were significant interactions between light and CO2 on the CO2 compensation point throughout the entire experimental matrix. The observed responses to changes in light and CO2 availability and the interactions between the two will alleviate CO2 limitations in dense buoyant mats where the light is typically high. On the other hand, these interactions will also allow penetration to greater depths where light is scarce because the higher CO2 near the bottom can increase the light use efficiency.

Influence of nitrogen to phosphorus supply ratios and physicochemical conditions on cyanobacteria and phytoplankton species composition in the Experimental Lakes Area, Canada

To test the hypothesis that N:P supply ratios influence phytoplankton species composition, and particularly that cyanobacteria are favored by a low ratio, mesocosms at one pelagic and two littoral sites within the Experimental Lakes Area, Ontario, were fertilized for 10-12 weeks with similar amounts of P but different amounts of N. Total N:P supply (LN:LP) ratios (fertilizer plus natural inputs) ranged from 8:1 to 50:1. Nitrogen deficiency was detected in all mesocosms with LN:LP ratios <17:1, but N2-fixing Anabaena gained dominance only in the low-N:P pelagic mesocosms, and only in late summer. Cryptophytes and (or) chlorophytes dominated littoral mesocosms at all N:P ratios, while Pseudoanabaena catenata, a nonheterocystous cyanobacterium, was the late-summer dominant in pelagic mesocosms with LN:LP ratios >17:1. Canonical correlation analysis related cyanobacterial dominance to high P and low CO2 availability. Low light intensities and low N:P ratios also favored heterocystous (but not nonheterocystous) cyanobacteria. Total phytoplankton biomass and productivity increased with LN:LP ratio, while periphyton growth was maximal at low N:P ratios. Nitrogen limitation of phytoplankton may encourage blooms of N2 fixers or drive productivity down to the sediment surface where N is more available.

Bicarbonate uptake and utilization in marine macroalgae

Several macroalgae from the Swedish West Coast (11 green, 5 red and 11 brown algal species) and two Ulva spp. from the Mediterranean were tested for mechanisms of HCO3 − utilization. In all except one (Palmaria palmata), photosynthesis supported by inorganic carbon (Ci) at pH 8.7 was rapidly and substantially (>50%) inhibited by acetazolamide (AZ), an inhibitor of extracellular carbonic anhydrase activity. This suggests that extracellular dehydration followed by uptake of the CO2 formed is an important component of the HCO3 − utilization mechanism(s) of these algae. In representatives of most of the green algal genera, e.g. Enteromorpha, Ulva, Chaetomorpha and Monostroma, a specific inhibitor response could be induced. This response, a sensitivity to the anion exchange inhibitor 4,4′-diisothiocyanatostilbene-2,2′-disulphonate (DIDS) in the presence of AZ, is characteristic of an additional mechanism of HCO3 − utilization, a direct import of HCO3 − across the plasma membrane, which has been previously described only for some Ulva spp. and Enteromorpha intestinalis. The DIDS sensitivity could be induced in several ways, all involving a potential shortage of the supply of CO2 for photosynthesis: through photosynthesis at high pH, through growth in strong light and nutrient-rich seawater, or through photosynthesis in the presence of AZ. Induction of photosynthesis at high pH (which is a specific feature of green macroalgae) was not always accompanied by a development of this specific DIDS sensitivity ; it occurred in Chaetomorpha melagonium but not in C. linum. For Ulva lactuca cultivated in natural seawater (in continuous moderate irradiance), induction of DIDS sensitivity by high pH treatment was successful all year round, except during a period in late August. The results thus suggest that green macroalgae capable of developing photosynthesis at high pH are also capable of developing a DIDS-sensitive mechanism for HCO3 − uptake, but that some additional conditions (other than low CO2 availability) must be fulfilled. The ability to dehydrate extracellular HCO3 − still remained after induction of the direct HCO3 − uptake, and the two mechanisms for HCO3 − utilization may operate independently of one another. In fact the response to DIDS was more pronounced after the addition of AZ. The possibility that the results are related to a plasticity of the HCO3 − utilizing mechanisms of green macroalgae is discussed. Neither red nor brown macroalgae showed any sensitivity to DIDS in the presence of AZ.

Bicarbonate uptake and utilization in marine macroalgae

Several macroalgae from the Swedish West Coast (11 green, 5 red and 11 brown algal species) and two Ulva spp. from the Mediterranean were tested for mechanisms of HCO3 − utilization. In all except one (Palmaria palmata), photosynthesis supported by inorganic carbon (Ci) at pH 8.7 was rapidly and substantially (>50%) inhibited by acetazolamide (AZ), an inhibitor of extracellular carbonic anhydrase activity. This suggests that extracellular dehydration followed by uptake of the CO2 formed is an important component of the HCO3 − utilization mechanism(s) of these algae. In representatives of most of the green algal genera, e.g. Enteromorpha, Ulva, Chaetomorpha and Monostroma, a specific inhibitor response could be induced. This response, a sensitivity to the anion exchange inhibitor 4,4′-diisothiocyanatostilbene- 2,2′-disulphonate (DIDS) in the presence of AZ, is characteristic of an additional mechanism of HCO3 − utilization, a direct import of HCO3 − across the plasma membrane, which has been previously described only for some Ulva spp. and Enteromorpha intestinalis. The DIDS sensitivity could be induced in several ways, all involving a potential shortage of the supply of CO2 for photosynthesis: through photosynthesis at high pH, through growth in strong light and nutrient-rich seawater, or through photosynthesis in the presence of AZ. Induction of photosynthesis at high pH (which is a specific feature of green macroalgae) was not always accompanied by a development of this specific DIDS sensitivity ; it occurred in Chaetomorpha melagonium but not in C. linum. For Ulva lactuca cultivated in natural seawater (in continuous moderate irradiance), induction of DIDS sensitivity by high pH treatment was successful all year round, except during a period in late August. The results thus suggest that green macroalgae capable of developing photosynthesis at high pH are also capable of developing a DIDS-sensitive mechanism for HCO3 − uptake, but that some additional conditions (other than low CO2 availability) must be fulfilled. The ability to dehydrate extracellular HCO3 − still remained after induction of the direct HCO3 − uptake, and the two mechanisms for HCO3 − utilization may operate independently of one another. In fact the response to DIDS was more pronounced after the addition of AZ. The possibility that the results are related to a plasticity of the HCO3 − utilizing mechanisms of green macroalgae is discussed. Neither red nor brown macroalgae showed any sensitivity to DIDS in the presence of AZ.

Bicarbonate utilization and pH polarity. The response of photosynthetic electron transport to carbon limitation in Potamogeton lucens leaves

By the process of pH polarity, several submersed angiosperms can use bicarbonate as carbon source for photosynthesis. Under conditions of relatively high light intensity and low CO2 availability, the pH of the apoplast and unstirred layer becomes acid at one side of the leaf and alkaline at the other. In the acid region, bicarbonate is converted into CO2, which diffuses into the leaf where it is fixed. Previous experiments on the light-dependent reduction of extracellular electron acceptors led to the hypothesis of redox regulation. Under conditions of high light and low CO2, excess reducing power in the chloroplast was supposed to be shuttled to the cytoplasm where it can upregulate the plasma membrane proton pump, leading to activation of polarity. Chlorophyll a fluorescence is an indicator for photosynthetic electron transport, the energization of thylakoids, and the reoxidation of chloroplast NADPH. It was used therefore to test redox regulation in vivo in Potamogeton lucens L. leaves. The fluoresence parameter, qP, an indicator for photochemical quenching and NADPH reoxidation, appeared to be rather insensitive to the inorganic carbon concentration and to the presence or absence of polarity. In contrast, qN, an indicator for non-photochemical quenching related to thylakoid energization, photoinhibition, and state transitions, increased under conditions of low CO2 - high light and polarity. Taken together the data show polarity to be an effective mechanism to make bicarbonate accessible as carbon source and seem to agree with the idea of redox regulation of pH polarity.Key words: bicarbonate utilization, chlorophyll a fluoresence, pH polarity, redox regulation, Potamogeton lucens, submerged aquatic macrophyte.

Changes in inorganic carbon uptake during the progression of a dinoflagellate bloom in a lake ecosystem

The physiological, biochemical, and genetic aspects of inorganic (Ci) carbon uptake in aquatic plants and algae have been studied extensively. Yet, to date, few studies examined these questions on dominant phytoplankton populations in their natural environment. Lake Kinneret, Israel, provides a good example of a system in which changes in CO2availability play a vital role in the ecophysiology of inorganic carbon uptake and in the population dynamics during the annual bloom of the dinoflagellate Peridinium gatunense Nygaard. In this study we investigated whether the availability of CO2(aq)limited growth rates and primary productivity of in situ populations of P. gatunense and focused on the role of adaptive mechanisms for Ciuptake. At the onset of the bloom, when epilimnetic pH was low ( = 8) and Ciconcentrations were high ( = 2.5 mM), carbonic anhydrase activity and cellular affinity to CO2(aq)were comparatively low. At this time photosynthetic rates, quantum yields, and in situ growth rates were high. As P. gatunense biomass increased, inorganic carbon decreased by 40%, while CO2(aq)concentrations declined 50-fold to values less than 2 µM. The algae adapted by acquiring a CO2-concentrating mechanism indicated by (i) intracellular Ci-concentrations higher by a factor of 5-70 relative to the ambient Ci; (ii) levels of carbonic anhydrase activity higher by 5- to 50-fold than those at the beginning of the bloom; and (iii) enhanced affinity for Ciand CO2(aq)3- and 40-fold higher, respectively, than affinities at the start of the bloom. These mechanistic changes in carbon uptake were reflected in declining photosynthetic rates and quantum yields as well as in the carbon isotopic composition with lower fractionation (13C enrichment) of the algae as the bloom progressed. Finally, despite induction of adaptive uptake mechanisms to low CO2availability; scarcity of other nutrients combined with low CO2concentrations, increased temperatures, and increased turbulence cause a decline in in situ growth rates and the collapse of the dinoflagellate biomass.Key words: dinoflagellates, inorganic carbon uptake, CCM, carbonic anhydrase, Peridinium gatunense.

Dissection of Oxidative Stress Tolerance Using Transgenic Plants.

Environmental stress is the major limiting factor in plant productivity. Much of the injury to plants caused by stress exposure is associated with oxidative damage at the cellular level. Widespread losses of forests and crops due to ozone pollution provide a highly visible example of oxidative stress (see Tingey et al., 1993, for a review), but less obvious losses caused by oxidative damage associated with periods of cold or drought also take their toll in the accumulation of incremental setbacks during a growing season. The role of ROIs in plant stress damage is indicated by the increased production of ROIs and the increased oxidative damage in tissues during stress. In plants, the highly energetic reactions of photosynthesis and an abundant oxygen supply make the chloroplast a particularly rich source of ROIs. High light intensity can lead to excess reduction of PSI so that CO2 fixation cannot keep pace and NADP+ pools are reduced. Under these conditions, 02 can compete for electrons from PSI, leading to the generation of ROIs through the Mehler reaction. When CO2 fixation is limited by environmental conditions such as cold temperatures or low CO2 availability (closed stomata), excess PSI reduction and increased ROI production can occur even at moderate light intensities. Efficient removal of ROIs from chloroplasts is critical, since H202 concentrations as low as 10 ptM can inhibit photosynthesis by 50% (Kaiser, 1979). Although the toxicity of *?2and H202 themselves is relatively low, their metal-dependent conversion to the highly toxic -OH via the Haber-Weiss reaction is thought to be responsible for the majority of the biological damage associated with these molecules. Antioxidant systems of plant chloroplasts include enzymes such as SOD and APX, and nonenzymatic components such as ascorbic acid and glutathione. The proposed ROI scavenging pathway of chloroplasts is shown in Figure 1 (Asada, 1994). Superoxide radicals are produced by the reduction of molecular oxygen at PSI via the Mehler reaction. This ?2- is rapidly dismuted to H202 by SOD that is associated with the thylakoid. The H202 produced is

The effect of environmental variables on release of extracellular organic carbon by freshwater macrophytes

The14C-technique was used to investigate the effect of oxygen, inorganic carbon and light intensity on the release of extracellular organic carbon (EOC) by three submerged macrophytes. At low CO2 availability and low light intensities the release expressed as a percentage of the total carbon fixation was higher than at more optimal photosynthetic conditions. Oxygen did not influence the release of EOC. As the rates of release generally followed the rates of photosynthesis, it was concluded that release is closely coupled to photosynthesis. Using gel-filtration to separate EOC it was shown that the tested environmental variables did not influence the molecular weight distribution. The release of EOC was low in all species (0.1 to 4% of the total carbon fixation). Due to the low values it is suggested that release of EOC is of no quantitative importance to the carbon budget of the plants.

Transient peaks in the delayed luminescence from Scenedesmus obtusiusculus induced by phosphorus starvation and carbon dioxide deficiency

Cells of the unicellular green alga Scenedesmus obtusiusculus (Chod.) were starved of phosphorus for 24, 48, 72 and 96 h, and the decay kinetics of the delayed luminescence from the differently starved cells was monitored for several minutes. Cells starved for 24 h showed similar delayed luminescence decay kinetics and accumulated output of photons as control cells after excitation with white light. Two transient peaks (with several components) in the decay kinetics of delayed luminescence were observed after 48 h of phosphorus starvation but not after 72 or 96 h. The amplitude of the transient peaks varied depending on the length of the excitation period with white light and on the length of the dark period preceding light excitation. High CO2 availability induced no transient peak, whereas low CO2 availability induced a high transient peak. Transient peaks could not be induced by excitation with light of 660 or 680 nm and only a single transient peak developed using 700 nm light. The kinetics of the delayed luminescence was changed, and the accumulated output of photons was decreased when the pH of the medium was changed from 7.2 to 9.5, both in cells starved for phosphorus for 96 h and in controls. The data indicate that a complicated metabolic pattern is involved in the mechanisms giving rise to the observed transient peaks in the delayed luminescence. The main factors may be a reduction in the translocation of trioses from chloroplasts, a concomitant reduction in Calvin cycle activities and changes in the amount of ATP and reducing agents available.