The relationship between the dissolved inorganic carbon concentration and growth rate in marine phytoplankton

Feasibility study of storing CO2 in the ocean by marine environmental impact assessment

Since the industrial revolution, which was accompanied with the use of fossil fuels as an energy source, the content of carbon dioxide (CO2) in the atmosphere has increased. To mitigate global warming, industries that utilize fossil fuels have continuously explored new approaches to reduce CO2 emissions and convert it to alternative fuels. The ocean is a vast source of absorbed CO2 on Earth, and various studies have been conducted on the use of the ocean to reduce global CO2. This study focused on reducing CO2 in the atmosphere by storing it as bicarbonate, a form of CO2 that exists in the ocean. The optimum condition for the conversion of CO2 into bicarbonate was investigated by considering the dissolved inorganic carbon (DIC; HCO3-, CO32-, H2CO3) concentration and pH. To confirm the biological impact of this conversion, biological impact experiments were conducted under various DIC concentrations using Skeletonema japonicum, a phytoplankton present in most areas of the sea. Based on the DIC concentration (2.09 mM) of the seawater, the DIC concentrations used in the Lab-scale experiment ranged from 2.5 mM to 18.75 mM, and the concentration with the highest conversion rate (< 6.38 mM) was applied in the pilot plant. Marine environmental impact modeling was performed to observe the effect of discharge to the ocean and its movement. The results revealed a slight growth inhibition of phytoplankton at DIC concentrations higher than the base concentration. Nevertheless, the change in the DIC concentration exerted no effect on the phytoplankton growth except at extremely high concentrations. Moreover, the high DIC concentration can be diluted by the ocean current flow rate, thus counterbalancing the growth inhibition effect. The results obtained in this study demonstrate the feasibility of CO2 storage in the form of DIC, and will be helpful for further development of CO2 mitigation.

Feeding in mixoplankton enhances phototrophy increasing bloom-induced pH changes with ocean acidification

Plankton phototrophy consumes CO₂, increasing seawater pH, while heterotrophy does the converse. Elevation of pH (>8.5) during coastal blooms becomes increasingly deleterious for plankton. Mixoplankton, which can be important bloom-formers, engage in both photoautotrophy and phagoheterotrophy; in theory, this activity could create a relatively stable pH environment for plankton growth. Using a systems biology modelling approach, we explored whether different mixoplankton functional groups could modulate the environmental pH compared to the extreme activities of phototrophic phytoplankton and heterotrophic zooplankton. Activities by most mixoplankton groups do not stabilize seawater pH. Through access to additional nutrient streams from internal recycling with phagotrophy, mixoplankton phototrophy is enhanced, elevating pH; this is especially so for constitutive and plastidic specialist non-constitutive mixoplankton. Mixoplankton blooms can exceed the size of phytoplankton blooms; the synergisms of mixoplankton physiology, accessing nutrition via phagotrophy as well as from inorganic sources, enhance or augment primary production rather than depressing it. Ocean acidification will thus enable larger coastal mixoplankton blooms to form before basification becomes detrimental. The dynamics of such bloom developments will depend on whether the mixoplankton are consuming heterotrophs and/or phototrophs and how the plankton community succession evolves.

Increasing diversity and biotic homogenization of lake plankton during recovery from acidification

Determining biodiversity responses to environmental change, such as acidification, is critical for ecosystem projections under future global change scenarios. Here, we analyzed three plankton communities of phytoplankton, crustaceans and rotifers in 28 lakes in the Adirondack Park, USA, during 1994-2012, and examined the spatiotemporal trends in their alpha and beta diversity during recovery from acidification. For all plankton assemblages, Shannon diversity increased towards recent years and high lake pH, and there was an increasing community dissimilarity with pH changes. The spatial mean Bray-Curtis dissimilarities across all lakes decreased over time for phytoplankton and rotifers leading to an increase in spatial homogenization. Such a homogenization cooccurred however with the overall increasing diversity in this region, which contrasts with the previous classic view that homogenization is mainly driven by loss of species and results in biodiversity loss. We further observed lower temporal mean beta diversity in low-pH lakes for crustaceans and rotifers, but not for phytoplankton. Generally, spatial and temporal mean beta diversity of the three taxonomic groups were primarily driven by lake-water ion variables, and rotifers were also constrained by nutrients and climate. Collectively, our results show how and why plankton community compositions vary over space along with acidification recovery, and further highlight the importance of spatiotemporal studies combined with long-term monitoring programs in assessing biodiversity change during the recovery of disturbed ecosystems.

Research Status and Trends of Underwater Photosynthesis

Underwater photosynthesis is the most important metabolic activity for submerged plants since it could utilize carbon fixation to replenish lost carbohydrates and improve internal aeration by producing O2. The present study used bibliometric methods to quantify the annual number of publications related to underwater photosynthesis. CiteSpace, as a visual analytic software for the literature, was employed to analyze the distribution of the subject categories, author collaborations, institution collaborations, international (regional) collaborations, and cocitation and keyword burst. The results show the basic characteristics of the literature, the main intellectual base, and the main research powers of underwater photosynthesis. Meanwhile, this paper revealed the research hotspots and trends of this field. This study provides an objective and comprehensive analysis of underwater photosynthesis from a bibliometric perspective. It is expected to provide reference information for scholars in related fields to refine the research direction, solve specific scientific problems, and assist scholars in seeking/establishing relevant collaborations in their areas of interest.

Elevated co2 has Differential Effects on Five Species of Microalgae from a Subtropical Freshwater Lake: Possible Implications for Phytoplankton Species Composition

Rising atmospheric CO₂ concentrations are predicted to have a significant impact on global phytoplankton populations. Of particular interest in freshwater systems are those species that produce toxins or impact water quality, though evidence for how these species, and many others, will respond is limited. This study investigated the effects of elevated CO₂ (1,000 ppm) relative to current atmospheric CO₂ partial pressures (400 ppm), on growth, cell size, carbon acquisition, and photophysiology of five freshwater phytoplankton species including a toxic cyanophyte, Raphidiopsis raciborskii, from Lake Wivenhoe, Australia. Effects of elevated CO₂ on growth rate varied between species; notably growth rate was considerably higher for Staurastrum sp. and significantly lower for Stichococcus sp. with a trend to lower growth rate for R. raciborskii. Surface area to volume ratio was significantly lower with elevated CO₂ , for all species except Cyclotella sp. Timing of maximum cell concentrations of those genera studied in monoculture occurred in the lake in order of CO₂ affinity when free CO₂ concentrations dropped below air equilibrium. The results presented here suggest that as atmospheric levels of CO₂ rise, R. raciborskii may become less of a problem to water quality, while some species of chlorophytes may become more dominant. This has implications for stakeholders of many freshwater systems.

Inorganic carbon concentrating mechanisms in free-living and symbiotic dinoflagellates and chromerids

Photosynthetic dinoflagellates are ecologically and biogeochemically important in marine and freshwater environments. However, surprisingly little is known of how this group acquires inorganic carbon or how these diverse processes evolved. Consequently, how CO₂ availability ultimately influences the success of dinoflagellates over space and time remains poorly resolved compared to other microalgal groups. Here we review the evidence. Photosynthetic core dinoflagellates have a Form II RuBisCO (replaced by Form IB or Form ID in derived dinoflagellates). The in vitro kinetics of the Form II RuBisCO from dinoflagellates are largely unknown, but dinoflagellates with Form II (and other) RuBisCOs have inorganic carbon concentrating mechanisms (CCMs), as indicated by in vivo internal inorganic C accumulation and affinity for external inorganic C. However, the location of the membrane(s) at which the essential active transport component(s) of the CCM occur(s) is (are) unresolved; isolation and characterization of functionally competent chloroplasts would help in this respect. Endosymbiotic Symbiodiniaceae (in Foraminifera, Acantharia, Radiolaria, Ciliata, Porifera, Acoela, Cnidaria, and Mollusca) obtain inorganic C by transport from seawater through host tissue. In corals this transport apparently provides an inorganic C concentration around the photobiont that obviates the need for photobiont CCM. This is not the case for tridacnid bivalves, medusae, or, possibly, Foraminifera. Overcoming these long-standing knowledge gaps relies on technical advances (e.g., the in vitro kinetics of Form II RuBisCO) that can functionally track the fate of inorganic C forms.

Light alters the responses of two marine diatoms to increased warming

In this study, we examined the effects of increased temperature (15, 20 and 25 °C) and different light levels (50, 200 μmol photons m^-2 s^-1) on two widely distributed diatoms, namely Phaeodactylum tricornutum and Thalassiosira weissflogii. Results showed that increasing light level counteracted the negative effects of high temperature on photosynthesis in both species, suggesting an antagonistic interaction between light and temperature. Contrary to the above results, light limitation diminished the temperature-sensitivity of carbonic anhydrase activity in two diatoms. We also observed species-specific responses of biomass, where increased temperature significantly decreased the biomass of P. tricornutum at both low and high light levels but showed no effects on T. weissflogii. Our study demonstrated that light can alter the physiological responses of diatoms to temperature but also revealed interspecific variations. We predict that in the future ocean with shallower upper mixed layer, T. weissflogii may be more competitive than P. tricornutum.

Dynamic CO2 and pH levels in coastal, estuarine, and inland waters: Theoretical and observed effects on harmful algal blooms

Rising concentrations of atmospheric CO₂ results in higher equilibrium concentrations of dissolved CO₂ in natural waters, with corresponding increases in hydrogen ion and bicarbonate concentrations and decreases in hydroxyl ion and carbonate concentrations. Superimposed on these climate change effects is the dynamic nature of carbon cycling in coastal zones, which can lead to seasonal and diel changes in pH and CO₂ concentrations that can exceed changes expected for open ocean ecosystems by the end of the century. Among harmful algae, i.e. some species and/or strains of Cyanobacteria, Dinophyceae, Prymnesiophyceae, Bacillariophyceae, and Ulvophyceae, the occurrence of a CO₂ concentrating mechanisms (CCMs) is the most frequent mechanism of inorganic carbon acquisition in natural waters in equilibrium with the present atmosphere (400 μmol CO₂  mol^-1 total gas), with varying phenotypic modification of the CCM. No data on CCMs are available for Raphidophyceae or the brown tide Pelagophyceae. Several HAB species and/or strains respond to increased CO₂ concentrations with increases in growth rate and/or cellular toxin content, however, others are unaffected. Beyond the effects of altered C concentrations and speciation on HABs, changes in pH in natural waters are likely to have profound effects on algal physiology. This review outlines the implications of changes in inorganic cycling for HABs in coastal zones, and reviews the knowns and unknowns with regard to how HABs can be expected to ocean acidification. We further point to the large regions of uncertainty with regard to this evolving field.

Responses of carbonic anhydrases and Rubisco to abrupt CO2 changes of seawater in two marine diatoms

Diatoms are experiencing striking fluctuations in seawater carbonate chemistry in the natural marine environment, especially in coastal seawaters. Here, we show that the diatoms Thalassiosira weissflogii and Phaeodactylum tricornutum, which utilize different carbon acquisition mechanisms, respond differently to short-term changes in seawater carbonate chemistry. Our results showed that T. weissflogii showed significantly higher photosynthetic oxygen evolution rates than that of P. tricornutum at low levels of CO₂ or HCO₃^-. This suggests that T. weissflogii had higher affinities for CO₂ or HCO₃^- when their concentrations were not sufficient to support saturated growth and photosynthesis. While the activity of Rubisco in P. tricornutum positively correlated with carbonic anhydrases (CA), we observed negative relationship between Rubisco and CA activity in the diatom T. weissflogii. These contrasting physiological responses of diatoms with varied carbon acquisition mechanisms indicate different abilities to cope up with abrupt changes in seawater carbonate chemistry. We propose that the ability to respond to varying carbonate chemistry may act as one determinant of the diatom distributions and phytoplankton community structures.

Carbon limitation of lake productivity

Phytoplankton productivity in lakes controls the rate of synthesis of organic matter that drives energy flow through the food webs and regulates the transparency and oxygen conditions in the water. Limitation of phytoplankton productivity and biomass by nutrients and light availability is an established paradigm for lake ecosystems, whereas invasion of atmospheric CO2 has been assumed to cover the high demands of dissolved inorganic carbon (DIC) during intense organic productivity. We challenge this paradigm, and show up to a 5-fold stimulation of phytoplankton productivity and biomass in outdoor mesocosms enriched with DIC, compared to mesocosms with lower DIC concentrations. High DIC supported phytoplankton productivity by direct algal uptake of bicarbonate, through the release of CO2 coupled to calcification and by inducing high pH that greatly enhances atmospheric CO2 invasion. Comparisons of 204 natural Danish lakes supported mesocosm experiments showing higher phytoplankton biomass and pH levels in hard water than soft water lakes for the same nutrient and light availabilities. The most productive lakes are nutrient-rich, hard water lakes that attain surface pHs of 10-11 and chemically enhance atmospheric CO2 uptake 10-15-fold. Our results will help understand natural variations of lake productivity along gradients in nutrients, DIC and pH.

Effect of light intensity and nutrients supply on microalgae cultivated in urban wastewater: Biomass production, lipids accumulation and settleability characteristics

Microalgae cultivation systems fed with wastewater as source of nutrients represents the principal sustainable condition to produce microalgal biomass to be converted conveniently to biofuels. In order to optimize microalgae growth and their lipid content, the effect of light intensity and nutrients load in real wastewater was investigated through batch microalgal cultivation tests. A microalgal polyculture was used as inoculum and grown for 10 days in batch at different conditions of light intensity (i.e. 20, 50 and 100 μmol s^-1m^-2) and nutrients concentration in wastewater. Experimental results showed that biomass productivity decreased for rich nutrients conditions and increased for high light intensities. The highest lipid mass content (29%) was found for high light intensity condition (100 μmol s^-1m^-2). Furthermore, microalgae settleability tests, conducted at the end of the cultivation time, resulted in the highest biomass recovery efficiency (72%) for low light intensity and nutrients supply conditions.

CO₂ Bubbling to Improve Algal Growth, Nutrient Removal, and Membrane Performance in an Algal Membrane Bioreactor

  Algae generally prefer CO2 through passive gas diffusion to HCO-3 or CO2-3, as uptake of carbonate species relies on active transport. In this study, the effects of CO2 bubbling on algal growth, nutrient uptake, lipid accumulation, and membrane fouling control were investigated in an algal membrane bioreactor (A-MBR). Bubbling with 10% CO2 in the A-MBR system increased algal specific oxygen production rate by 43 ± 5% and algal productivity by 39 ± 1%, even though there was abundant dissolved inorganic carbon available in the secondary wastewater effluent (about 3.6 mM). Meanwhile, nitrogen removal capacity increased from originally 2.6 ± 0.4 g/m3•d to 3.6 ± 0.4 g/m3•d through continuous CO2 bubbling. Furthermore, membrane fouling was significantly reduced in the A-MBR system with CO2 addition, likely because of reduced mineral precipitation on the membrane at lower pHs.

Decomposing the effects of ocean environments on predator-prey body-size relationships in food webs

Body-size relationships between predators and their prey are important in ecological studies because they reflect the structure and function of food webs. Inspired by studies on the impact of global warming on food webs, the effects of temperature on body-size relationships have been widely investigated; however, the impact of environmental factors on body-size relationships has not been fully evaluated because climate warming affects various ocean environments. Thus, here, we comprehensively investigated the effects of ocean environments and predator-prey body-size relationships by integrating a large-scale dataset of predator-prey body-size relationships in marine food webs with global oceanographic data. We showed that various oceanographic parameters influence prey size selection. In particular, oxygen concentration, primary production and salinity, in addition to temperature, significantly alter body-size relationships. Furthermore, we demonstrated that variability (seasonality) of ocean environments significantly affects body-size relationships. The effects of ocean environments on body-size relationships were generally remarkable for small body sizes, but were also significant for large body sizes and were relatively weak for intermediate body sizes, in the cases of temperature seasonality, oxygen concentration and salinity variability. These findings break down the complex effects of ocean environments on body-size relationships, advancing our understanding of how ocean environments influence the structure and functioning of food webs.

Acquisition and metabolism of carbon in the Ochrophyta other than diatoms

The acquisition and assimilation of inorganic C have been investigated in several of the 15 clades of the Ochrophyta other than diatoms, with biochemical, physiological and genomic data indicating significant mechanistic variation. Form ID Rubiscos in the Ochrophyta are characterized by a broad range of kinetics values. In spite of relatively high K0.5CO2 and low CO2 : O2 selectivity, diffusive entry of CO2 occurs in the Chrysophyceae and Synurophyceae. Eustigmatophyceae and Phaeophyceae, on the contrary, have CO2 concentrating mechanisms, usually involving the direct or indirect use of [Formula: see text] This variability is possibly due to the ecological contexts of the organism. In brown algae, C fixation generally takes place through a classical C3 metabolism, but there are some hints of the occurrence of C4 metabolism and low amplitude CAM in a few members of the Fucales. Genomic data show the presence of a number of potential C4 and CAM genes in Ochrophyta other than diatoms, but the other core functions of many of these genes give a very limited diagnostic value to their presence and are insufficient to conclude that C4 photosynthesis is present in these algae.This article is part of the themed issue 'The peculiar carbon metabolism in diatoms'.

Variation in growth rate, carbon assimilation, and photosynthetic efficiency in response to nitrogen source and concentration in phytoplankton isolated from upper San Francisco Bay

Six species of phytoplankton recently isolated from upper San Francisco Bay were tested for their sensitivity to growth inhibition by ammonium (NH4+ ), and for differences in growth rates according to inorganic nitrogen (N) growth source. The quantum yield of photosystem II (Fv /Fm ) was a sensitive indicator of NH4+ toxicity, manifested by a suppression of Fv /Fm in a dose-dependent manner. Two chlorophytes were the least sensitive to NH4+ inhibition, at concentrations of >3,000 μmoles NH4+  · L-1 , followed by two estuarine diatoms that were sensitive at concentrations >1,000 μmoles NH4+  · L-1 , followed lastly by two freshwater diatoms that were sensitive at concentrations between 200 and 500 μmoles NH4+  · L-1 . At non-inhibiting concentrations of NH4+ , the freshwater diatom species grew fastest, followed by the estuarine diatoms, while the chlorophytes grew slowest. Variations in growth rates with N source did not follow taxonomic divisions. Of the two chlorophytes, one grew significantly faster on nitrate (NO3- ), whereas the other grew significantly faster on NH4+ . All four diatoms tested grew faster on NH4+ compared with NO3- . We showed that in cases where growth rates were faster on NH4+ than they were on NO3- , the difference was not larger for chlorophytes compared with diatoms. This holds true for comparisons across a number of culture investigations suggesting that diatoms as a group will not be at a competitive disadvantage under natural conditions when NH4+ dominates the total N pool and they will also not have a growth advantage when NO3- is dominant, as long as N concentrations are sufficient.

Ecological imperatives for aquatic CO2-concentrating mechanisms

In aquatic environments, the concentration of inorganic carbon is spatially and temporally variable and CO2 can be substantially oversaturated or depleted. Depletion of CO2 plus low rates of diffusion cause inorganic carbon to be more limiting in aquatic than terrestrial environments, and the frequency of species with a CO2-concentrating mechanism (CCM), and their contribution to productivity, is correspondingly greater. Aquatic photoautotrophs may have biochemical or biophysical CCMs and exploit CO2 from the sediment or the atmosphere. Though partly constrained by phylogeny, CCM activity is related to environmental conditions. CCMs are absent or down-regulated when their increased energy costs, lower CO2 affinity, or altered mineral requirements outweigh their benefits. Aquatic CCMs are most widespread in environments with low CO2, high HCO3-, high pH, and high light. Freshwater species are generally less effective at inorganic carbon removal than marine species, but have a greater range of ability to remove carbon, matching the environmental variability in carbon availability. The diversity of CCMs in seagrasses and marine phytoplankton, and detailed mechanistic studies on larger aquatic photoautotrophs are understudied. Strengthening the links between ecology and CCMs will increase our understanding of the mechanisms underlying ecological success and will place mechanistic studies in a clearer ecological context.

The relative availability of inorganic carbon and inorganic nitrogen influences the response of the dinoflagellate Protoceratium reticulatum to elevated CO2

This work originates from three facts: (i) changes in CO₂ availability influence metabolic processes in algal cells; (ii) Spatial and temporal variations of nitrogen availability cause repercussions on phytoplankton physiology; (iii) Growth and cell composition are dependent on the stoichiometry of nutritional resources. In this study, we assess whether the impact of rising pCO₂ is influenced by N availability, through the impact that it would have on the C/N stoichiometry, in conditions of N sufficiency. Our experiments used the dinoflagellate Protoceratium reticulatum, which we cultured under three CO₂ regimes (400, 1,000, and 5,000 ppmv, pH of 8.1) and either variable (the NO₃^- concentration was always 2.5 mmol · L^-1 ) or constant (NO₃^- concentration varied to maintain the same C_i /NO₃^- ratio at all pCO₂ ) C_i /NO₃^- ratio. Regardless of N availability, cells had higher specific growth rates, but lower cell dry weight and C and N quotas, at elevated CO₂ . The carbohydrate pool size and the C/N was unaltered in all treatments. The lipid content only decreased at high pCO₂ at constant C_i /NO₃^- ratio. In the variable C_i /NO₃^- conditions, the relative abundance of Rubisco (and other proteins) also changed; this did not occur at constant C_i /NO₃^- . Thus, the biomass quality of P. reticulatum for grazers was affected by the C_i /NO₃^- ratio in the environment and not only by the pCO₂ , both with respect to the size of the main organic pools and the composition of the expressed proteome.

Micro-scale variability enhances trophic transfer and potentially sustains biodiversity in plankton ecosystems

We develop moment closure approximations to represent micro-scale spatial variability in the concentrations of nutrients (N), phytoplankton (P) and zooplankton (Z) in an NPZ model, which we apply to examine the impact of different levels of micro-scale variability on both ecosystem dynamics and trophic transfer. Accounting explicitly for both the mean-field and fluctuating components of each prognostic variable in the NPZ model yields different dynamics for the mean-field concentrations, as well as lower phytoplankton biomass and greater zooplankton biomass, compared to the conventional NPZ model without micro-scale variability. The biomass of zooplankton consistently increases with increasing total micro-scale variability, and a minimum threshold of such variability is required for the existence of stable steady state solutions in the NPZ closure model. Compared to the conventional NPZ model, the domain of parameter space over which stable solutions exist is larger than for the NPZ closure model, and this stable domain widens with increasing total variability. The latter result suggests that natural systems with greater micro-scale variability may have the potential to sustain greater biodiversity. We find that with the NPZ closure model: (1) the stability domains increases with micro-scale variability, (2) increase of the level of total micro-scale variability enhances trophic transfer, i.e. increases the biomass of zooplankton, and (3) the coefficient of variation (CV_P) of phytoplankton increases with micro-scale variability.

The nature of the CO2 -concentrating mechanisms in a marine diatom, Thalassiosira pseudonana

Diatoms are widespread in aquatic ecosystems where they may be limited by the supply of inorganic carbon. Their carbon dioxide-concentrating mechanisms (CCMs) involving transporters and carbonic anhydrases (CAs) are well known, but the contribution of a biochemical CCM involving C4 metabolism is contentious. The CCM(s) present in the marine-centric diatom, Thalassiosira pseudonana, were studied in cells exposed to high or low concentrations of CO2 , using a range of approaches. At low CO2 , cells possessed a CCM based on active uptake of CO2 (70% contribution) and bicarbonate, while at high CO2 , cells were restricted to CO2 . CA was highly and rapidly activated on transfer to low CO2 and played a key role because inhibition of external CA produced uptake kinetics similar to cells grown at high CO2 . The activities of phosphoenolpyruvate (PEP) carboxylase (PEPC) and the PEP-regenerating enzyme, pyruvate phosphate dikinase (PPDK), were lower in cells grown at low than at high CO2 . The ratios of PEPC and PPDK to ribulose bisphosphate carboxylase were substantially lower than 1, even at low CO2 . Our data suggest that the kinetic properties of this species results from a biophysical CCM and not from C4 type metabolism.

Effect of Ocean Acidification and pH Fluctuations on the Growth and Development of Coralline Algal Recruits, and an Associated Benthic Algal Assemblage

Coralline algae are susceptible to the changes in the seawater carbonate system associated with ocean acidification (OA). However, the coastal environments in which corallines grow are subject to large daily pH fluctuations which may affect their responses to OA. Here, we followed the growth and development of the juvenile coralline alga Arthrocardia corymbosa, which had recruited into experimental conditions during a prior experiment, using a novel OA laboratory culture system to simulate the pH fluctuations observed within a kelp forest. Microscopic life history stages are considered more susceptible to environmental stress than adult stages; we compared the responses of newly recruited A. corymbosa to static and fluctuating seawater pH with those of their field-collected parents. Recruits were cultivated for 16 weeks under static pH 8.05 and 7.65, representing ambient and 4× preindustrial pCO2 concentrations, respectively, and two fluctuating pH treatments of daily [Formula: see text] (daytime pH = 8.45, night-time pH = 7.65) and daily [Formula: see text] (daytime pH = 8.05, night-time pH = 7.25). Positive growth rates of new recruits were recorded in all treatments, and were highest under static pH 8.05 and lowest under fluctuating pH 7.65. This pattern was similar to the adults' response, except that adults had zero growth under fluctuating pH 7.65. The % dry weight of MgCO3 in calcite of the juveniles was reduced from 10% at pH 8.05 to 8% at pH 7.65, but there was no effect of pH fluctuation. A wide range of fleshy macroalgae and at least 6 species of benthic diatoms recruited across all experimental treatments, from cryptic spores associated with the adult A. corymbosa. There was no effect of experimental treatment on the growth of the benthic diatoms. On the community level, pH-sensitive species may survive lower pH in the presence of diatoms and fleshy macroalgae, whose high metabolic activity may raise the pH of the local microhabitat.

Enhancing microalgal photosynthesis and productivity in wastewater treatment high rate algal ponds for biofuel production

With microalgal biofuels currently receiving much attention, there has been renewed interest in the combined use of high rate algal ponds (HRAP) for wastewater treatment and biofuel production. This combined use of HRAPs is considered to be an economically feasible option for biofuel production, however, increased microalgal productivity and nutrient removal together with reduced capital costs are needed before it can be commercially viable. Despite HRAPs being an established technology, microalgal photosynthesis and productivity is still limited in these ponds and is well below the theoretical maximum. This paper critically evaluates the parameters that limit microalgal light absorption and photosynthesis in wastewater HRAPs and examines biological, chemical and physical options for improving light absorption and utilisation, with the view of enhancing biomass production and nutrient removal.

The effects of CO₂ addition along a pH gradient on wastewater microalgal photo-physiology, biomass production and nutrient removal

Carbon limitation in domestic wastewater high rate algal ponds is thought to constrain microalgal photo-physiology and productivity, particularly in summer. This paper investigates the effects of CO₂ addition along a pH gradient on the performance of wastewater microalgae in high rate algal mesocosms. Performance was measured in terms of light absorption, electron transport rate, photosynthetic efficiency, biomass production and nutrient removal efficiency. Light absorption by the microalgae increased by up to 128% with increasing CO₂ supply, while a reduction in the package effect meant that there was less internal self-shading thereby increasing the efficiency of light absorption. CO₂ augmentation increased the maximum rate of both electron transport and photosynthesis by up to 256%. This led to increased biomass, with the highest yield occurring at the highest dissolved inorganic carbon/lowest pH combination tested (pH 6.5), with a doubling of chlorophyll-a (Chl-a) biomass while total microalgal biovolume increased by 660% in Micractinium bornhemiense and by 260% in Pediastrum boryanum dominated cultures. Increased microalgal biomass did not off-set the reduction in ammonia volatilisation in the control and overall nutrient removal was lower with CO₂ than without. Microalgal nutrient removal efficiency decreased as pH decreased and may have been related to decreased Chl-a per cell. This experiment demonstrated that CO₂ augmentation increased microalgal biomass in two distinct communities, however, care must be taken when interpreting results from standard biomass measurements with respect to CO₂ augmentation.

Effects of Ocean Acidification on Temperate Coastal Marine Ecosystems and Fisheries in the Northeast Pacific

As the oceans absorb anthropogenic CO₂ they become more acidic, a problem termed ocean acidification (OA). Since this increase in CO₂ is occurring rapidly, OA may have profound implications for marine ecosystems. In the temperate northeast Pacific, fisheries play key economic and cultural roles and provide significant employment, especially in rural areas. In British Columbia (BC), sport (recreational) fishing generates more income than commercial fishing (including the expanding aquaculture industry). Salmon (fished recreationally and farmed) and Pacific Halibut are responsible for the majority of fishery-related income. This region naturally has relatively acidic (low pH) waters due to ocean circulation, and so may be particularly vulnerable to OA. We have analyzed available data to provide a current description of the marine ecosystem, focusing on vertical distributions of commercially harvested groups in BC in the context of local carbon and pH conditions. We then evaluated the potential impact of OA on this temperate marine system using currently available studies. Our results highlight significant knowledge gaps. Above trophic levels 2–3 (where most local fishery-income is generated), little is known about the direct impact of OA, and more importantly about the combined impact of multi-stressors, like temperature, that are also changing as our climate changes. There is evidence that OA may have indirect negative impacts on finfish through changes at lower trophic levels and in habitats. In particular, OA may lead to increased fish-killing algal blooms that can affect the lucrative salmon aquaculture industry. On the other hand, some species of locally farmed shellfish have been well-studied and exhibit significant negative direct impacts associated with OA, especially at the larval stage. We summarize the direct and indirect impacts of OA on all groups of marine organisms in this region and provide conclusions, ordered by immediacy and certainty.

Variation in elemental stoichiometry of the marine diatom Thalassiosira weissflogii (Bacillariophyceae) in response to combined nutrient stress and changes in carbonate chemistry

The combined consequences of the multi-stressors of pH and nutrient availability upon the growth of a marine diatom were investigated. Thalassiosira weissflogii was grown in N- or P-limited batch culture in sealed systems, with pH commencing at 8.2 ("extant" conditions) or 7.6 ("ocean acidification" [OA] conditions), and then pH was allowed to either drift with growth, or was held fixed. Results indicated that within the pH range tested, the stability of environmental pH rather than its value (i.e., OA vs. extant) fundamentally influenced biomass accumul-ation and C:N:P stoichiometry. Despite large changes in total alkalinity in the fixed pH systems, final biomass production was consistently greater in these systems than that in drifting pH systems. In drift systems, pH increased to exceed pH 9.5, a level of alkalinity that was inhibitory to growth. No statis-tically significant differences between pH treatments were measured for N:C, P:C or N:P ratios during nutrient-replete growth, although the diatom expre-ssed greater plasticity in P:C and N:P ratios than in N:C during this growth phase. During nutrient-deplete conditions, the capacity for uncoupled carbon fixa-tion at fixed pH was considerably greater than that measured in drift pH systems, leading to strong contrasts in C:N:P stoichiometry between these treatments. Whether environmental pH was stable or drifted directly influenced the extent of physiological stress. In contrast, few distinctions could be drawn between "extant" versus "OA" conditions for cell physiology.

Observations and models of highly intermittent phytoplankton distributions

The measurement of phytoplankton distributions in ocean ecosystems provides the basis for elucidating the influences of physical processes on plankton dynamics. Technological advances allow for measurement of phytoplankton data to greater resolution, displaying high spatial variability. In conventional mathematical models, the mean value of the measured variable is approximated to compare with the model output, which may misinterpret the reality of planktonic ecosystems, especially at the microscale level. To consider intermittency of variables, in this work, a new modelling approach to the planktonic ecosystem is applied, called the closure approach. Using this approach for a simple nutrient-phytoplankton model, we have shown how consideration of the fluctuating parts of model variables can affect system dynamics. Also, we have found a critical value of variance of overall fluctuating terms below which the conventional non-closure model and the mean value from the closure model exhibit the same result. This analysis gives an idea about the importance of the fluctuating parts of model variables and about when to use the closure approach. Comparisons of plot of mean versus standard deviation of phytoplankton at different depths, obtained using this new approach with real observations, give this approach good conformity.

Impact of dissolved inorganic carbon concentrations and pH on growth of the chemolithoautotrophic epsilonproteobacterium Sulfurimonas gotlandica GD1T

Epsilonproteobacteria have been found globally distributed in marine anoxic/sulfidic areas mediating relevant transformations within the sulfur and nitrogen cycles. In the Baltic Sea redox zones, chemoautotrophic epsilonproteobacteria mainly belong to the Sulfurimonas gotlandica GD17 cluster for which recently a representative strain, S. gotlandica GD1(T), could be established as a model organism. In this study, the potential effects of changes in dissolved inorganic carbon (DIC) and pH on S. gotlandica GD1(T) were examined. Bacterial cell abundance within a broad range of DIC concentrations and pH values were monitored and substrate utilization was determined. The results showed that the DIC saturation concentration for achieving maximal cell numbers was already reached at 800 μmol L(-1), which is well below in situ DIC levels. The pH optimum was between 6.6 and 8.0. Within a pH range of 6.6-7.1 there was no significant difference in substrate utilization; however, at lower pH values maximum cell abundance decreased sharply and cell-specific substrate consumption increased.

Multiparametric analyses reveal the pH-dependence of silicon biomineralization in diatoms

Diatoms, the major contributors of the global biogenic silica cycle in modern oceans, account for about 40% of global marine primary productivity. They are an important component of the biological pump in the ocean, and their assemblage can be used as useful climate proxies; it is therefore critical to better understand the changes induced by environmental pH on their physiology, silicification capability and morphology. Here, we show that external pH influences cell growth of the ubiquitous diatom Thalassiosira weissflogii, and modifies intracellular silicic acid and biogenic silica contents per cell. Measurements at the single-cell level reveal that extracellular pH modifications lead to intracellular acidosis. To further understand how variations of the acid-base balance affect silicon metabolism and theca formation, we developed novel imaging techniques to measure the dynamics of valve formation. We demonstrate that the kinetics of valve morphogenesis, at least in the early stages, depends on pH. Analytical modeling results suggest that acidic conditions alter the dynamics of the expansion of the vesicles within which silica polymerization occurs, and probably its internal pH. Morphological analysis of valve patterns reveals that acidification also reduces the dimension of the nanometric pores present on the valves, and concurrently overall valve porosity. Variations in the valve silica network seem to be more correlated to the dynamics and the regulation of the morphogenesis process than the silicon incorporation rate. These multiparametric analyses from single-cell to cell-population levels demonstrate that several higher-level processes are sensitive to the acid-base balance in diatoms, and its regulation is a key factor for the control of pattern formation and silicon metabolism.

Independent colimitation for carbon dioxide and inorganic phosphorus

Simultaneous limitation of plant growth by two or more nutrients is increasingly acknowledged as a common phenomenon in nature, but its cellular mechanisms are far from understood. We investigated the uptake kinetics of CO(2) and phosphorus of the algae Chlamydomonas acidophila in response to growth at limiting conditions of CO(2) and phosphorus. In addition, we fitted the data to four different Monod-type models: one assuming Liebigs Law of the minimum, one assuming that the affinity for the uptake of one nutrient is not influenced by the supply of the other (independent colimitation) and two where the uptake affinity for one nutrient depends on the supply of the other (dependent colimitation). In addition we asked whether the physiological response under colimitation differs from that under single nutrient limitation.We found no negative correlation between the affinities for uptake of the two nutrients, thereby rejecting a dependent colimitation. Kinetic data were supported by a better model fit assuming independent uptake of colimiting nutrients than when assuming Liebigs Law of the minimum or a dependent colimitation. Results show that cell nutrient homeostasis regulated nutrient acquisition which resulted in a trade-off in the maximum uptake rates of CO(2) and phosphorus, possibly driven by space limitation on the cell membrane for porters for the different nutrients. Hence, the response to colimitation deviated from that to a single nutrient limitation. In conclusion, responses to single nutrient limitation cannot be extrapolated to situations where multiple nutrients are limiting, which calls for colimitation experiments and models to properly predict growth responses to a changing natural environment. These deviations from single nutrient limitation response under colimiting conditions and independent colimitation may also hold for other nutrients in algae and in higher plants.

The response of Thalassiosira pseudonana to long-term exposure to increased CO2 and decreased pH

The effect of ocean acidification conditions has been investigated in cultures of the diatom Thalassiosira pseudonana CCMP1335. Expected end-of-the-century pCO(2) (aq) concentrations of 760 µatm (equivalent to pH 7.8) were compared with present-day condition (380 µatm CO(2), pH 8.1). Batch culture pH changed rapidly because of CO(2) (aq) assimilation and pH targets of 7.8 and 8.1 could not be sustained. Long-term (∼100 generation) pH-auxostat, continuous cultures could be maintained at target pH when cell density was kept low (<2×10(5) cells mL(-1)). After 3 months continuous culture, the C:N ratio was slightly decreased under high CO(2) conditions and red fluorescence per cell was slightly increased. However, no change was detected in photosynthetic efficiency (F(v)/F(m)) or functional cross section of PS II (σ(PSII)). Elevated pCO(2) has been predicted to be beneficial to diatoms due to reduced cost of carbon concentration mechanisms. There was reduced transcription of one putative δ-carbonic anhydrase (CA-4) after 3 months growth at increased CO(2) but 3 other δ-CAs and the small subunit of RUBISCO showed no change. There was no evidence of adaptation or clade selection of T. pseudonana after ∼100 generations at elevated CO(2). On the basis of this long-term culture, pH change of this magnitude in the future ocean may have little effect on T. pseudonana in the absence of genetic adaption.

Integration of microalgae cultivation with industrial waste remediation for biofuel and bioenergy production: opportunities and limitations

There is currently a renewed interest in developing microalgae as a source of renewable energy and fuel. Microalgae hold great potential as a source of biomass for the production of energy and fungible liquid transportation fuels. However, the technologies required for large-scale cultivation, processing, and conversion of microalgal biomass to energy products are underdeveloped. Microalgae offer several advantages over traditional 'first-generation' biofuels crops like corn: these include superior biomass productivity, the ability to grow on poor-quality land unsuitable for agriculture, and the potential for sustainable growth by extracting macro- and micronutrients from wastewater and industrial flue-stack emissions. Integrating microalgal cultivation with municipal wastewater treatment and industrial CO(2) emissions from coal-fired power plants is a potential strategy to produce large quantities of biomass, and represents an opportunity to develop, test, and optimize the necessary technologies to make microalgal biofuels more cost-effective and efficient. However, many constraints on the eventual deployment of this technology must be taken into consideration and mitigating strategies developed before large scale microalgal cultivation can become a reality. As a strategy for CO(2) biomitigation from industrial point source emitters, microalgal cultivation can be limited by the availability of land, light, and other nutrients like N and P. Effective removal of N and P from municipal wastewater is limited by the processing capacity of available microalgal cultivation systems. Strategies to mitigate against the constraints are discussed.

Response of the calcifying coccolithophore Emiliania huxleyi to low pH/high pCO2: from physiology to molecular level

The emergence of ocean acidification as a significant threat to calcifying organisms in marine ecosystems creates a pressing need to understand the physiological and molecular mechanisms by which calcification is affected by environmental parameters. We report here, for the first time, changes in gene expression induced by variations in pH/pCO2 in the widespread and abundant coccolithophore Emiliania huxleyi. Batch cultures were subjected to increased partial pressure of CO2 (pCO2; i.e. decreased pH), and the changes in expression of four functional gene classes directly or indirectly related to calcification were investigated. Increased pCO2 did not affect the calcification rate and only carbonic anhydrase transcripts exhibited a significant down-regulation. Our observation that elevated pCO2 induces only limited changes in the transcription of several transporters of calcium and bicarbonate gives new significant elements to understand cellular mechanisms underlying the early response of E. huxleyi to CO2-driven ocean acidification.

Inorganic carbon acquisition by eukaryotic algae: four current questions

The phylogenetically and morphologically diverse eukaryotic algae are typically oxygenic photolithotrophs. They have a diversity of incompletely understood mechanisms of inorganic carbon acquisition: this article reviews four areas where investigations continue. The first topic is diffusive CO(2) entry. Most eukaryotic algae, like all cyanobacteria, have inorganic carbon concentrating mechanisms (CCMs). The ancestral condition was presumably the absence of a CCM, i.e. diffusive CO(2) entry, as found in a small minority of eukaryotic algae today; however, it is likely that, as is found in several cases, this condition is due to a loss of a CCM. There are a number of algae which are in various respects intermediate between diffusive CO(2) entry and occurrence of a CCM: further study is needed on this aspect. A second topic is the nature of cyanelles and their role in inorganic carbon assimilation. The cyanelles (plastids) of the euglyphid amoeba Paulinella have been acquired relatively recently by endosymbiosis with genetic integration of an α-cyanobacterium with a Form 1A Rubisco. The α-carboxysomes in the cyanelles are presumably involved in a CCM, but further investigation is needed.Also called cyanelles are the plastids of glaucocystophycean algae, but is it now clear that these were derived from the β-cyanobacterial ancestor of all plastids other than that of Paulinella. The resemblances of the central body of the cyanelles of glaucocystophycean algae to carboxysomes may not reflect derivation from cyanobacterial β-carboxysomes; although it is clear that these algae have CCMs but these are now well characterized. The other two topics concern CCMs in other eukaryotic algae; these CCMs arose polyphyletically and independently of the cyanobacterial CCMs. It is generally believed that eukaryotic algal, like cyanobacterial, CCMs are based on active transport of an inorganic carbon species and/or protons, and they have C(3) biochemistry. This is the case for the organism considered as the third topic, i.e. Chlamydomonas reinhardtii, the eukaryotic alga with the best understood CCM. This CCM involves HCO(3)(-) conversion to CO(2) in the thylakoid lumen so the external inorganic carbon must cross four membranes in series with a final CO(2) effux from the thylakoid. More remains to be investigated about this CCM. The final topic is that of the occurrence of C(4)-like metabolism in the CCMs of marine diatoms. Different conclusions have been reached depending on the organism investigated and the techniques used, and several aspects require further study.

Adaptation, extinction and global change

We discuss three interlinked issues: the natural pace of environmental change and adaptation, the likelihood that a population will adapt to a potentially lethal change, and adaptation to elevated CO2, the prime mover of global change. Environmental variability is governed by power laws showing that ln difference in conditions increases with ln elapsed time at a rate of 0.3-0.4. This leads to strong but fluctuating selection in many natural populations.The effect of repeated adverse change on mean fitness depends on its frequency rather than its severity. If the depression of mean fitness leads to population decline, however, severe stress may cause extinction. Evolutionary rescue from extinction requires abundant genetic variation or a high mutation supply rate, and thus a large population size. Although natural populations can sustain quite intense selection, they often fail to adapt to anthropogenic stresses such as pollution and acidification and instead become extinct.Experimental selection lines of algae show no specific adaptation to elevated CO2, but instead lose their carbon-concentrating mechanism through mutational degradation. This is likely to reduce the effectiveness of the oceanic carbon pump. Elevated CO2 is also likely to lead to changes in phytoplankton community composition, although it is not yet clear what these will be. We emphasize the importance of experimental evolution in understanding and predicting the biological response to global change. This will be one of the main tasks of evolutionary biologists in the coming decade.

Expression and inhibition of the carboxylating and decarboxylating enzymes in the photosynthetic C4 pathway of marine diatoms

There is evidence that the CO(2)-concentrating mechanism in the marine diatom Thalassiosira weissflogii operates as a type of single-cell C(4) photosynthesis. In quantitative-PCR assays, we observed 2- to 4-fold up-regulation of two phosphoenolpyruvate carboxylase (PEPC) gene transcripts in Thalassiosira pseudonana cells adapted to low pCO(2), but did not detect such regulation in Phaeodactylum tricornutum grown under similar conditions. Transcripts encoding phosphoenolpyruvate carboxykinase did not appear to be regulated by pCO(2) in either diatom. In T. pseudonana and T. weissflogii, net CO(2) fixation was blocked by 3,3-dichloro-2-(dihydroxyphosphinoyl-methyl)-propenoate (a specific inhibitor of PEPC), but was restored by about 50% and 80%, respectively, by addition of millimolar concentrations of KHCO(3). In T. pseudonana, T. weissflogii, and P. tricornutum, rates of net O(2) evolution were reduced by an average of 67%, 55%, and 62%, respectively, in the presence of 20 microm quercetin, also an inhibitor of PEPC. Quercetin promoted net CO(2) leakage from inhibited cells to levels in excess of the equilibrium CO(2) concentration, suggesting that a fraction of the HCO(3)(-) taken up is fated to leak back into the medium as CO(2) when PEPC activity is blocked. In parallel to these experiments, in vitro assays on crude extracts of T. pseudonana demonstrated mean inhibition of 65% of PEPC activity by quercetin. In the presence of 5 mm 3-mercaptopicolinic acid (3-MPA), a classic inhibitor of phosphoenolpyruvate carboxykinase, photosynthetic O(2) evolution was reduced by 90% in T. pseudonana. In T. weissflogii and P. tricornutum, 5 mm 3-MPA totally inhibited net CO(2) fixation and O(2) evolution. Neither quercetin nor 3-MPA had a significant inhibitory effect on photosynthetic O(2) evolution or CO(2) uptake in the marine chlorophyte isolates Chlamydomonas sp. or Dunaliella tertiolecta. Our evidence supports the idea that C(4)-based CO(2)-concentrating mechanisms are generally distributed in diatoms. This conclusion is discussed within the context of the evolutionary success of diatoms.

C3 and C4 pathways of photosynthetic carbon assimilation in marine diatoms are under genetic, not environmental, control

Marine diatoms are responsible for up to 20% of global CO(2) fixation. Their photosynthetic efficiency is enhanced by concentrating CO(2) around Rubisco, diminishing photorespiration, but the mechanism is yet to be resolved. Diatoms have been regarded as C(3) photosynthesizers, but recent metabolic labeling and genome sequencing data suggest that they perform C(4) photosynthesis. We studied the pathways of photosynthetic carbon assimilation in two diatoms by short-term metabolic (14)C labeling. In Thalassiosira weissflogii, both C3 (glycerate-P and triose-P) and C4 (mainly malate) compounds were major initial (2-5 s) products, whereas Thalassiosira pseudonana produced mainly C3 and C6 (hexose-P) compounds. The data provide evidence of C(3)-C(4) intermediate photosynthesis in T. weissflogii, but exclusively C(3) photosynthesis in T. pseudonana. The labeling patterns were the same for cells grown at near-ambient (380 microL L(-1)) and low (100 microL L(-1)) CO(2) concentrations. The lack of environmental modulation of carbon assimilatory pathways was supported in T. pseudonana by measurements of gene transcript and protein abundances of C(4)-metabolic enzymes (phosphoenolpyruvate carboxylase and phosphoenolpyruvate carboxykinase) and Rubisco. This study suggests that the photosynthetic pathways of diatoms are diverse, and may involve combined CO(2)-concentrating mechanisms. Furthermore, it emphasizes the requirement for metabolic and functional genetic and enzymic analyses before accepting the presence of C(4)-metabolic enzymes as evidence for C(4) photosynthesis.

Chlorophyll content and fluorescence responses cannot be used to gauge reliably phytoplankton biomass, nutrient status or growth rate

To consider the relationship between chlorophyll a (Chl a) content and phytoplankton growth and nutrient status, four phytoplankton species were grown in nitrogen (N)-limited [and, for one species, phosphorus (P)-limited] culture and measurements were made of CNP biomass, in vivo and in vitro Chl a content, the ratio of variable to maximum fluorescence (FV/FM) and the performance index for photosynthesis, PIABS (a derivative of the O-J-I-P analysis of photosystem II functionality). Interspecies differences plus the development of intraspecies differences during nutrient stress produced c. 10-fold variations in Chl : C. Estimates of C from in vivo Chl content were better than those from extracted Chl content, as the decline in Chl : C during nutrient stress was offset in part by increased Chl fluorescence. FV/FM was not a robust indicator of nutrient status or relative growth rate. Responses of FV/FM in cells re-fed the limiting nutrient showed no consistent pattern with which to gauge nutrient status. PIABS showed some promise as an indicator of nutrient status and relative growth rate. Chl a content and fluorescence parameters do not deserve the unquestioned status they usually enjoy as indicators of biomass and physiological status.

Analysis of expressed sequence tags from calcifying cells of marine coccolithophorid (Emiliania huxleyi)

An expressed sequence tag (EST) approach was used to investigate gene expression in the unicelluar marine alga Emiliania huxleyi. We randomly selected 3000 EST sequences from a cDNA library of transcripts expressed under conditions promoting coccolithogenesis. Cluster analysis and contig assembly resulted in a unigene set of approximately 1523 ESTs. Only 36% of the unique sequences exhibited significant homology to sequences in GenBank. Of particular interest were the numerous transcripts with homology to sequences associated with sexual reproduction and calcium homeostasis in other unicellular and multicellular organisms. The majority of ESTs (64%) had little or no significant sequence homology to entries in GenBank, suggesting a potential for further novel gene discovery. The catalog of ESTs reported herein represents a significant increase in the limited sequence information currently available for E. huxleyi and should make the coccolithophorid more accessible to powerful genomics and postgenomics technologies.