Lake heatwaves under climate change.

Lake ecosystems, and the organisms that live within them, are vulnerable to temperature change1-5, including the increased occurrence of thermal extremes6. However, very little is known about lake heatwaves-periods of extreme warm lake surface water temperature-and how they may change under global warming. Here we use satellite observations and a numerical model to investigate changes in lake heatwaves for hundreds of lakes worldwide from 1901 to 2099. We show that lake heatwaves will become hotter and longer by the end of the twenty-first century. For the high-greenhouse-gas-emission scenario (Representative Concentration Pathway (RCP) 8.5), the average intensity of lake heatwaves, defined relative to the historical period (1970 to 1999), will increase from 3.7 ± 0.1 to 5.4 ± 0.8 degrees Celsius and their average duration will increase dramatically from 7.7 ± 0.4 to 95.5 ± 35.3 days. In the low-greenhouse-gas-emission RCP 2.6 scenario, heatwave intensity and duration will increase to 4.0 ± 0.2 degrees Celsius and 27.0 ± 7.6 days, respectively. Surface heatwaves are longer-lasting but less intense in deeper lakes (up to 60 metres deep) than in shallower lakes during both historic and future periods. As lakes warm during the twenty-first century7,8, their heatwaves will begin to extend across multiple seasons, with some lakes reaching a permanent heatwave state. Lake heatwaves are likely to exacerbate the adverse effects of long-term warming in lakes and exert widespread influence on their physical structure and chemical properties. Lake heatwaves could alter species composition by pushing aquatic species and ecosystems to the limits of their resilience. This in turn could threaten lake biodiversity9 and the key ecological and economic benefits that lakes provide to society.

External α-carbonic anhydrase and solute carrier 4 are required for bicarbonate uptake in a freshwater angiosperm.

The freshwater monocot Ottelia alismoides is the only known species to operate three CO2-concentrating mechanisms (CCMs): constitutive bicarbonate (HCO3-) use, C4 photosynthesis, and facultative Crassulacean acid metabolism, but the mechanism of HCO3- use is unknown. We found that the inhibitor of an anion exchange protein, 4,4'-diisothio-cyanatostilbene-2,2'-disulfonate (DIDS), prevented HCO3- use but also had a small effect on CO2 uptake. An inhibitor of external carbonic anhydrase (CA), acetazolamide (AZ), reduced the affinity for CO2 uptake but also prevented HCO3- use via an effect on the anion exchange protein. Analysis of mRNA transcripts identified a homologue of solute carrier 4 (SLC4) responsible for HCO3- transport, likely to be the target of DIDS, and a periplasmic α-carbonic anhydrase 1 (α-CA1). A model to quantify the contribution of the three different pathways involved in inorganic carbon uptake showed that passive CO2 diffusion dominates inorganic carbon uptake at high CO2 concentrations. However, as CO2 concentrations fall, two other pathways become predominant: conversion of HCO3- to CO2 at the plasmalemma by α-CA1 and transport of HCO3- across the plasmalemma by SLC4. These mechanisms allow access to a much larger proportion of the inorganic carbon pool and continued photosynthesis during periods of strong carbon depletion in productive ecosystems.

Structural basis for C4 photosynthesis without Kranz anatomy in leaves of the submerged freshwater plant Ottelia alismoides.

BACKGROUND AND AIMS: Ottelia alismoides (Hydrocharitaceae) is a freshwater macrophyte that, unusually, possesses three different CO2-concentrating mechanisms. Here we describe its leaf anatomy and chloroplast ultrastructure, how these are altered by CO2 concentration and how they may underlie C4 photosynthesis. METHODS: Light and transmission electron microscopy were used to study the anatomy of mature leaves of O. alismoides grown at high and low CO2 concentrations. Diel acid change and the activity of phosphoenolpyruvate carboxylase were measured to confirm that CAM activity and C4 photosynthesis were present. KEY RESULTS: When O. alismoides was grown at low CO2, the leaves performed both C4 and CAM photosynthesis whereas at high CO2 leaves used C4 photosynthesis. The leaf comprised an upper and lower layer of epidermal cells separated by a large air space occupying about 22 % of the leaf transverse-section area, and by mesophyll cells connecting the two epidermal layers. Kranz anatomy was absent. At low CO2, chloroplasts in the mesophyll cells were filled with starch even at the start of the photoperiod, while epidermal chloroplasts contained small starch grains. The number of chloroplasts in the epidermis was greater than in the mesophyll cells. At high CO2, the structure was unchanged but the thicknesses of the two epidermal layers, the air space, mesophyll and the transverse-section area of cells and air space were greater. CONCLUSIONS: Leaves of O. alismoides have epidermal and mesophyll cells that contain chloroplasts and large air spaces but lack Kranz anatomy. The high starch content of mesophyll cells suggests they may benefit from an internal source of CO2, for example via C4 metabolism, and are also sites of starch storage. The air spaces may help in the recycling of decarboxylated or respired CO2. The structural similarity of leaves at low and high CO2 is consistent with the constitutive nature of bicarbonate and C4 photosynthesis. There is sufficient structural diversity within the leaf of O. alismoides to support dual-cell C4 photosynthesis even though Kranz anatomy is absent.

Insights on the Functions and Ecophysiological Relevance of the Diverse Carbonic Anhydrases in Microalgae.

Carbonic anhydrases (CAs) exist in all kingdoms of life. They are metalloenzymes, often containing zinc, that catalyze the interconversion of bicarbonate and carbon dioxide-a ubiquitous reaction involved in a variety of cellular processes. So far, eight classes of apparently evolutionary unrelated CAs that are present in a large diversity of living organisms have been described. In this review, we focus on the diversity of CAs and their roles in photosynthetic microalgae. We describe their essential role in carbon dioxide-concentrating mechanisms and photosynthesis, their regulation, as well as their less studied roles in non-photosynthetic processes. We also discuss the presence in some microalgae, especially diatoms, of cambialistic CAs (i.e., CAs that can replace Zn by Co, Cd, or Fe) and, more recently, a CA that uses Mn as a metal cofactor, with potential ecological relevance in aquatic environments where trace metal concentrations are low. There has been a recent explosion of knowledge about this well-known enzyme with exciting future opportunities to answer outstanding questions using a range of different approaches.

Response of cyanobacteria and phytoplankton abundance to warming, extreme rainfall events and nutrient enrichment.

Cyanobacterial blooms are an increasing threat to water quality and global water security caused by the nutrient enrichment of freshwaters. There is also a broad consensus that blooms are increasing with global warming, but the impacts of other concomitant environmental changes, such as an increase in extreme rainfall events, may affect this response. One of the potential effects of high rainfall events on phytoplankton communities is greater loss of biomass through hydraulic flushing. Here we used a shallow lake mesocosm experiment to test the combined effects of: warming (ambient vs. +4°C increase), high rainfall (flushing) events (no events vs. seasonal events) and nutrient loading (eutrophic vs. hypertrophic) on total phytoplankton chlorophyll-a and cyanobacterial abundance and composition. Our hypotheses were that: (a) total phytoplankton and cyanobacterial abundance would be higher in heated mesocosms; (b) the stimulatory effects of warming on cyanobacterial abundance would be enhanced in higher nutrient mesocosms, resulting in a synergistic interaction; (c) the recovery of biomass from flushing induced losses would be quicker in heated and nutrient-enriched treatments, and during the growing season. The results supported the first and, in part, the third hypotheses: total phytoplankton and cyanobacterial abundance increased in heated mesocosms with an increase in common bloom-forming taxa-Microcystis spp. and Dolichospermum spp. Recovery from flushing was slowest in the winter, but unaffected by warming or higher nutrient loading. Contrary to the second hypothesis, an antagonistic interaction between warming and nutrient enrichment was detected for both cyanobacteria and chlorophyll-a demonstrating that ecological surprises can occur, dependent on the environmental context. While this study highlights the clear need to mitigate against global warming, oversimplification of global change effects on cyanobacteria should be avoided; stressor gradients and seasonal effects should be considered as important factors shaping the response.

Different CO2 acclimation strategies in juvenile and mature leaves of Ottelia alismoides.

The freshwater macrophyte, Ottelia alismoides, is a bicarbonate user performing C4 photosynthesis in the light, and crassulacean acid metabolism (CAM) when acclimated to low CO2. The regulation of the three mechanisms by CO2 concentration was studied in juvenile and mature leaves. For mature leaves, the ratios of phosphoenolpyruvate carboxylase (PEPC) to ribulose-bisphosphate carboxylase/oxygenase (Rubisco) are in the range of that of C4 plants regardless of CO2 concentration (1.5-2.5 at low CO2, 1.8-3.4 at high CO2). In contrast, results for juvenile leaves suggest that C4 is facultative and only present under low CO2. pH-drift experiments showed that both juvenile and mature leaves can use bicarbonate irrespective of CO2 concentration, but mature leaves have a significantly greater carbon-extracting ability than juvenile leaves at low CO2. At high CO2, neither juvenile nor mature leaves perform CAM as indicated by lack of diurnal acid fluctuation. However, CAM was present at low CO2, though the fluctuation of titratable acidity in juvenile leaves (15-17 µequiv g-1 FW) was slightly but significantly lower than in mature leaves (19-25 µequiv g-1 FW), implying that the capacity to perform CAM increases as leaves mature. The increased CAM activity is associated with elevated PEPC activity and large diel changes in starch content. These results show that in O. alismoides, carbon-dioxide concentrating mechanisms are more effective in mature compared to juvenile leaves, and C4 is facultative in juvenile leaves but constitutive in mature leaves.

Effects of multiple stressors on cyanobacteria abundance vary with lake type.

Blooms of cyanobacteria are a current threat to global water security that is expected to increase in the future because of increasing nutrient enrichment, increasing temperature and extreme precipitation in combination with prolonged drought. However, the responses to multiple stressors, such as those above, are often complex and there is contradictory evidence as to how they may interact. Here we used broad scale data from 494 lakes in central and northern Europe, to assess how cyanobacteria respond to nutrients (phosphorus), temperature and water retention time in different types of lakes. Eight lake types were examined based on factorial combinations of major factors that determine phytoplankton composition and sensitivity to nutrients: alkalinity (low and medium-high), colour (clear and humic) and mixing intensity (polymictic and stratified). In line with expectations, cyanobacteria increased with temperature and retention time in five of the eight lake types. Temperature effects were greatest in lake types situated at higher latitudes, suggesting that lakes currently not at risk could be affected by warming in the future. However, the sensitivity of cyanobacteria to temperature, retention time and phosphorus varied among lake types highlighting the complex responses of lakes to multiple stressors. For example, in polymictic, medium-high alkalinity, humic lakes cyanobacteria biovolume was positively explained by retention time and a synergy between TP and temperature, while in polymictic, medium-high alkalinity, clear lakes only retention time was identified as an explanatory variable. These results show that, although climate change will need to be accounted for when managing the risk of cyanobacteria in lakes, a "one-size fits-all" approach is not appropriate. When forecasting the response of cyanobacteria to future environmental change, including changes caused by climate and local management, it will be important to take this differential sensitivity of lakes into account.

Committing to place: the potential of open collaborations for trusted environmental governance.

Conventional modes of environmental governance, which typically exclude those stakeholders that are most directly linked to the specific place, frequently fail to have the desired impact. Using the example of lake water management in Loweswater, a small hamlet within the English Lake District, we consider the ways in which new "collectives" for local, bottom-up governance of water bodies can reframe problems in ways which both bind lay and professional people to place, and also recast the meaning of "solutions" in thought-provoking ways.

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.

Diversity of CO2-concentrating mechanisms and responses to CO2 concentration in marine and freshwater diatoms.

The presence of CO2-concentrating mechanisms (CCMs) is believed to be one of the characteristics that allows diatoms to thrive in many environments and to be major contributors to global productivity. Here, the type of CCM and the responses to variable CO2 concentration were studied in marine and freshwater diatoms. At 400 ppm, there was a large diversity in physiological and biochemical mechanisms among the species. While Phaeodactylum tricornutum mainly used HCO3-, Thalassiosira pseudonana mainly used CO2. Carbonic anhydrase was an important component of the CCM in all species and C4 metabolism was absent, even with T. weissflogii. For all species, at 20 000 ppm, the affinity for dissolved inorganic carbon was lower than at 400 ppm CO2 and the reliance on CO2 was higher. Despite the difference in availability of inorganic carbon in marine and fresh waters, there were only small differences in CCMs between species from the two environments, and Navicula pelliculosa behaved similarly when grown in the two environments. The results suggest that species-specific differences are great, and more important than environmental differences in determining the nature and effectiveness of the CCM in diatoms.

Responses of Ottelia alismoides, an aquatic plant with three CCMs, to variable CO2 and light.

Ottelia alismoides is a constitutive C4 plant and bicarbonate user, and has facultative crassulacean acid metabolism (CAM) at low CO2. Acclimation to a factorial combination of light and CO2 showed that the ratio of phosphoenolpyruvate carboxylase (PEPC) to ribulose-bisphosphate carboxylase/oxygenase (Rubisco) (>5) is in the range of that of C4 plants. This and short-term response experiments showed that the activity of PEPC and pyruvate phosphate dikinase (PPDK) was high even at the end of the night, consistent with night-time acid accumulation and daytime carbon fixation. The diel acidity change was maximal at high light and low CO2 at 17-25 µequiv g-1 FW. Decarboxylation proceeded at ~2-3 µequiv g-1 FW h-1, starting at the beginning of the photoperiod, but did not occur at high CO2; the rate was greater at high, compared with low light. There was an inverse relationship between starch formation and acidity loss. Acidity changes account for up to 21% of starch production and stimulate early morning photosynthesis, but night-time accumulation of acid traps <6% of respiratory carbon release. Ottelia alismoides is the only known species to operate CAM and C4 in the same tissue, and one of only two known aquatic species to operate CAM and bicarbonate use.

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.

Ecophysiology matters: linking inorganic carbon acquisition to ecological preference in four species of microalgae (Chlorophyceae).

The effect of CO2 supply is likely to play an important role in algal ecology. Since inorganic carbon (Ci ) acquisition strategies are very diverse among microalgae and Ci availability varies greatly within and among habitats, we hypothesized that Ci acquisition depends on the pH of their preferred natural environment (adaptation) and that the efficiency of Ci uptake is affected by CO2 availability (acclimation). To test this, four species of green algae originating from different habitats were studied. The pH-drift and Ci uptake kinetic experiments were used to characterize Ci acquisition strategies and their ability to acclimate to high and low CO2 conditions and high and low pH was evaluated. Results from pH drift experiments revealed that the acidophile and acidotolerant Chlamydomonas species were mainly restricted to CO2 , whereas the two neutrophiles were efficient bicarbonate users. CO2 compensation points in low CO2 -acclimated cultures ranged between 0.6 and 1.4 µM CO2 and acclimation to different culture pH and CO2 conditions suggested that CO2 concentrating mechanisms were present in most species. High CO2 acclimated cultures adapted rapidly to low CO2 condition during pH-drifts. Ci uptake kinetics at different pH values showed that the affinity for Ci was largely influenced by external pH, being highest under conditions where CO2 dominated the Ci pool. In conclusion, Ci acquisition was highly variable among four species of green algae and linked to growth pH preference, suggesting that there is a connection between Ci acquisition and ecological distribution.

When phenology matters: age-size truncation alters population response to trophic mismatch.

Climate-induced shifts in the timing of life-history events are a worldwide phenomenon, and these shifts can de-synchronize species interactions such as predator-prey relationships. In order to understand the ecological implications of altered seasonality, we need to consider how shifts in phenology interact with other agents of environmental change such as exploitation and disease spread, which commonly act to erode the demographic structure of wild populations. Using long-term observational data on the phenology and dynamics of a model predator-prey system (fish and zooplankton in Windermere, UK), we show that age-size truncation of the predator population alters the consequences of phenological mismatch for offspring survival and population abundance. Specifically, age-size truncation reduces intraspecific density regulation due to competition and cannibalism, and thereby amplifies the population sensitivity to climate-induced predator-prey asynchrony, which increases variability in predator abundance. High population variability poses major ecological and economic challenges as it can diminish sustainable harvest rates and increase the risk of population collapse. Our results stress the importance of maintaining within-population age-size diversity in order to buffer populations against phenological asynchrony, and highlight the need to consider interactive effects of environmental impacts if we are to understand and project complex ecological outcomes.

Glyceraldehyde-3-phosphate dehydrogenase is regulated by ferredoxin-NADP reductase in the diatom Asterionella formosa.

Diatoms are a widespread and ecologically important group of heterokont algae that contribute c. 20% to global productivity. Previous work has shown that regulation of their key Calvin cycle enzymes differs from that of the Plantae, and that in crude extracts, glyceraldehyde-3-phosphate dehydrogenase (GAPDH) can be inhibited by nicotinamide adenine dinucleotide phosphate reduced (NADPH) under oxidizing conditions. The freshwater diatom, Asterionella formosa, was studied using enzyme kinetics, chromatography, surface plasmon resonance, mass spectrometry and sequence analysis to determine the mechanism behind this GAPDH inhibition. GAPDH interacted with ferredoxin-nicotinamide adenine dinucleotide phosphate (NADP) reductase (FNR) from the primary phase of photosynthesis, and the small chloroplast protein, CP12. Sequences of copurified GAPDH and FNR were highly homologous with published sequences. However, the widespread ternary complex among GAPDH, phosphoribulokinase and CP12 was absent. Activity measurements under oxidizing conditions showed that NADPH can inhibit GAPDH-CP12 in the presence of FNR, explaining the earlier observed inhibition within crude extracts. Diatom plastids have a distinctive metabolism, including the lack of the oxidative pentose phosphate pathway, and so cannot produce NADPH in the dark. The observed down-regulation of GAPDH in the dark may allow NADPH to be rerouted towards other reductive processes contributing to their ecological success.

Biochemical and biophysical CO2 concentrating mechanisms in two species of freshwater macrophyte within the genus Ottelia (Hydrocharitaceae).

Two freshwater macrophytes, Ottelia alismoides and O. acuminata, were grown at low (mean 5 µmol L(-1)) and high (mean 400 µmol L(-1)) CO2 concentrations under natural conditions. The ratio of PEPC to RuBisCO activity was 1.8 in O. acuminata in both treatments. In O. alismoides, this ratio was 2.8 and 5.9 when grown at high and low CO2, respectively, as a result of a twofold increase in PEPC activity. The activity of PPDK was similar to, and changed with, PEPC (1.9-fold change). The activity of the decarboxylating NADP-malic enzyme (ME) was very low in both species, while NAD-ME activity was high and increased with PEPC activity in O. alismoides. These results suggest that O. alismoides might perform a type of C4 metabolism with NAD-ME decarboxylation, despite lacking Kranz anatomy. The C4-activity was still present at high CO2 suggesting that it could be constitutive. O. alismoides at low CO2 showed diel acidity variation of up to 34 µequiv g(-1) FW indicating that it may also operate a form of crassulacean acid metabolism (CAM). pH-drift experiments showed that both species were able to use bicarbonate. In O. acuminata, the kinetics of carbon uptake were altered by CO2 growth conditions, unlike in O. alismoides. Thus, the two species appear to regulate their carbon concentrating mechanisms differently in response to changing CO2. O. alismoides is potentially using three different concentrating mechanisms. The Hydrocharitaceae have many species with evidence for C4, CAM or some other metabolism involving organic acids, and are worthy of further study.

Evaluating the use of lake sedimentary DNA in palaeolimnology: A comparison with long-term microscopy-based monitoring of the phytoplankton community.

Palaeolimnological records provide valuable information about how phytoplankton respond to long-term drivers of environmental change. Traditional palaeolimnological tools such as microfossils and pigments are restricted to taxa that leave sub-fossil remains, and a method that can be applied to the wider community is required. Sedimentary DNA (sedDNA), extracted from lake sediment cores, shows promise in palaeolimnology, but validation against data from long-term monitoring of lake water is necessary to enable its development as a reliable record of past phytoplankton communities. To address this need, 18S rRNA gene amplicon sequencing was carried out on lake sediments from a core collected from Esthwaite Water (English Lake District) spanning ~105 years. This sedDNA record was compared with concurrent long-term microscopy-based monitoring of phytoplankton in the surface water. Broadly comparable trends were observed between the datasets, with respect to the diversity and relative abundance and occurrence of chlorophytes, dinoflagellates, ochrophytes and bacillariophytes. Up to 20% of genera were successfully captured using both methods, and sedDNA revealed a previously undetected community of phytoplankton. These results suggest that sedDNA can be used as an effective record of past phytoplankton communities, at least over timescales of <100 years. However, a substantial proportion of genera identified by microscopy were not detected using sedDNA, highlighting the current limitations of the technique that require further development such as reference database coverage. The taphonomic processes which may affect its reliability, such as the extent and rate of deposition and DNA degradation, also require further research.

Food web de-synchronization in England's largest lake: an assessment based on multiple phenological metrics.

Phenological changes have been observed globally for marine, freshwater and terrestrial species, and are an important element of the global biological 'fingerprint' of climate change. Differences in rates of change could desynchronize seasonal species interactions within a food web, threatening ecosystem functioning. Quantification of this risk is hampered by the rarity of long-term data for multiple interacting species from the same ecosystem and by the diversity of possible phenological metrics, which vary in their ecological relevance to food web interactions. We compare phenological change for phytoplankton (chlorophyll a), zooplankton (Daphnia) and fish (perch, Perca fluviatilis) in two basins of Windermere over 40 years and determine whether change has differed among trophic levels, while explicitly accounting for among-metric differences in rates of change. Though rates of change differed markedly among the nine metrics used, seasonal events shifted earlier for all metrics and trophic levels: zooplankton advanced most, and fish least, rapidly. Evidence of altered synchrony was found in both lake basins, when combining information from all phenological metrics. However, comparisons based on single metrics did not consistently detect this signal. A multimetric approach showed that across trophic levels, earlier phenological events have been associated with increasing water temperature. However, for phytoplankton and zooplankton, phenological change was also associated with changes in resource availability. Lower silicate, and higher phosphorus, concentrations were associated with earlier phytoplankton growth, and earlier phytoplankton growth was associated with earlier zooplankton growth. The developing trophic mismatch detected between the dominant fish species in Windermere and important zooplankton food resources may ultimately affect fish survival and portend significant impacts upon ecosystem functioning. We advocate that future studies on phenological synchrony combine data from multiple phenological metrics, to increase confidence in assessments of change and likely ecological consequences.

An intrinsically disordered protein, CP12: jack of all trades and master of the Calvin cycle.

Many proteins contain disordered regions under physiological conditions and lack specific three-dimensional structure. These are referred to as IDPs (intrinsically disordered proteins). CP12 is a chloroplast protein of approximately 80 amino acids and has a molecular mass of approximately 8.2-8.5 kDa. It is enriched in charged amino acids and has a small number of hydrophobic residues. It has a high proportion of disorder-promoting residues, but has at least two (often four) cysteine residues forming one (or two) disulfide bridge(s) under oxidizing conditions that confers some order. However, CP12 behaves like an IDP. It appears to be universally distributed in oxygenic photosynthetic organisms and has recently been detected in a cyanophage. The best studied role of CP12 is its regulation of the Calvin cycle responsible for CO2 assimilation. Oxidized CP12 forms a supramolecular complex with two key Calvin cycle enzymes, GAPDH (glyceraldehyde-3-phosphate dehydrogenase) and PRK (phosphoribulokinase), down-regulating their activity. Association-dissociation of this complex, induced by the redox state of CP12, allows the Calvin cycle to be inactive in the dark and active in the light. CP12 is promiscuous and interacts with other enzymes such as aldolase and malate dehydrogenase. It also plays other roles in plant metabolism such as protecting GAPDH from inactivation and scavenging metal ions such as copper and nickel, and it is also linked to stress responses. Thus CP12 seems to be involved in many functions in photosynthetic cells and behaves like a jack of all trades as well as being a master of the Calvin cycle.

Sediment amendment with Phoslock® in Clatto Reservoir (Dundee, UK): Investigating changes in sediment elemental composition and phosphorus fractionation.

Lanthanum-modified bentonite clay (Phoslock(®) is a lake remediation tool designed to strip dissolved phosphorus (P) from the water column and increase the sediment P-sorption capacity. This study investigated short term alterations in sediment elemental composition and sediment P-fractions based on sediment cores taken 2 days before and 28 days following the application of 24 t of Phoslock® to a 9 ha, man-made reservoir. Following the application, sediment lanthanum (La) content increased significantly (p < 0.05; n = 4) in the top 8 cm of the sediment, thereby theoretically increasing sediment P-binding capacity on the whole reservoir scale by 250 kg. Mass balance calculations were used to estimate the theoretical binding of release-sensitive P (P(mobile); sum of 'labile P', 'reductant-soluble P' and 'organic P' fraction) by La across the top 4 cm and 10 cm depth of sediment. The amended mass of La in the sediment had the potential to bind 42% of P(mobile) present in the top 4 cm or 17% of P(mobile) present in the top 10 cm. However, with the exception of a significant increase (p<0.05; n=4) in the 'residual P' fraction in the top 2 cm, sediment P-fractions, including P(mobile,) did not differ significantly following the Phoslock® application. Experimental P-adsorption studies indicated P-saturation values for Phoslock® of 21,670 mg P kg⁻¹ Phoslock®. Sequential extraction of P from saturated Phoslock® under laboratory conditions indicated that around 21% of P bound by Phoslock® was release-sensitive, while around 79% of bound P was unlikely to be released under reducing or common pH (5-9) conditions in shallow lakes. Applying Phoslock® is, therefore, likely to increase the P-sorption capacity of sediments under reducing conditions.