Energy costs of carbon dioxide concentrating mechanisms in aquatic organisms

Minimum energy (as photon) costs are predicted for core reactions of photosynthesis, for photorespiratory metabolism in algae lacking CO₂ concentrating mechanisms (CCMs) and for various types of CCMs; in algae, with CCMs; allowance was made for leakage of CO₂ from the internal pool. These predicted values are just compatible with the minimum measured photon costs of photosynthesis in microalgae and macroalgae lacking or expressing CCMs. More energy-expensive photorespiration, for example for organisms using Rubiscos with lower CO₂–O₂ selectivity coefficients, would be less readily accommodated within the lowest measured photon costs of photosynthesis by algae lacking CCMs. The same applies to the cases of CCMs with higher energy costs of active transport of protons or inorganic carbon species, or greater allowance for significant leakage from the accumulated intracellular pool of CO₂. High energetic efficiency can involve a higher concentration of catalyst to achieve a given rate of reaction, adding to the resource costs of growth. There are no obvious mechanistic interpretations of the occurrence of CCMs algae adapted to low light and low temperatures using the rationales adopted for the occurrence of C₄ photosynthesis in terrestrial flowering plants. There is an exception for cyanobacteria with low-selectivity Form IA or IB Rubiscos, and those dinoflagellates with low-selectivity Form II Rubiscos, for which very few natural environments have high enough CO₂:O₂ ratios to allow photosynthesis in the absence of CCMs.     

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 CO₂ 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 CO₂ 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 CO₂ 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 C₃ 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 HCO₃ ^? conversion to CO₂ in the thylakoid lumen so the external inorganic carbon must cross four membranes in series with a final CO₂ effux from the thylakoid. More remains to be investigated about this CCM. The final topic is that of the occurrence of C₄-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.     

Evolutionary bursts in Euphorbia (Euphorbiaceae) are linked with photosynthetic pathway

The mid‐Cenozoic decline of atmospheric CO₂ levels that promoted global climate change was critical to shaping contemporary arid ecosystems. Within angiosperms, two CO₂‐concentrating mechanisms (CCMs)—crassulacean acid metabolism (CAM) and C₄—evolved from the C₃ photosynthetic pathway, enabling more efficient whole‐plant function in such environments. Many angiosperm clades with CCMs are thought to have diversified rapidly due to Miocene aridification, but links between this climate change, CCM evolution, and increased net diversification rates (r) remain to be further understood. Euphorbia (～2000 species) includes a diversity of CAM‐using stem succulents, plus a single species‐rich C₄ subclade. We used ancestral state reconstructions with a dated molecular phylogeny to reveal that CCMs independently evolved 17–22 times in Euphorbia, principally from the Miocene onwards. Analyses assessing among‐lineage variation in r identified eight Euphorbia subclades with significantly increased r, six of which have a close temporal relationship with a lineage‐corresponding CCM origin. Our trait‐dependent diversification analysis indicated that r of Euphorbia CCM lineages is approximately threefold greater than C₃ lineages. Overall, these results suggest that CCM evolution in Euphorbia was likely an adaptive strategy that enabled the occupation of increased arid niche space accompanying Miocene expansion of arid ecosystems. These opportunities evidently facilitated recent, replicated bursts of diversification in Euphorbia.     

Energy crosstalk between photosynthesis and the algal CO2-concentrating mechanisms

Microalgal photosynthesis is responsible for nearly half of the CO₂ annually captured by Earth’s ecosystems. In aquatic environments where the CO₂ availability is low, the CO₂-fixing efficiency of microalgae greatly relies on mechanisms – called CO_2-concentrating mechanisms (CCMs) – for concentrating CO₂ at the catalytic site of the CO₂-fixing enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco). While the transport of inorganic carbon (C_i) across membrane bilayers against a concentration gradient consumes part of the chemical energy generated by photosynthesis, the bioenergetics and cellular mechanisms involved are only beginning to be elucidated. Here, we review the current knowledge relating to the energy requirement of CCMs in the light of recent advances in photosynthesis regulatory mechanisms and the spatial organization of CCM components.     

Role of carboxysomes in cyanobacterial CO2 assimilation: CO2 concentrating mechanisms and metabolon implications

Many carbon-fixing organisms have evolved CO₂ concentrating mechanisms (CCMs) to enhance the delivery of CO₂ to RuBisCO, while minimizing reactions with the competitive inhibitor, molecular O₂. These distinct types of CCMs have been extensively studied using genetics, biochemistry, cell imaging, mass spectrometry, and metabolic flux analysis. Highlighted in this paper, the cyanobacterial CCM features a bacterial microcompartment (BMC) called ‘carboxysome’ in which RuBisCO is co-encapsulated with the enzyme carbonic anhydrase (CA) within a semi-permeable protein shell. The cyanobacterial CCM is capable of increasing CO₂ around RuBisCO, leading to one of the most efficient processes known for fixing ambient CO₂. The carboxysome life cycle is dynamic and creates a unique subcellular environment that promotes activity of the Calvin–Benson (CB) cycle. The carboxysome may function within a larger cellular metabolon, physical association of functionally coupled proteins, to enhance metabolite channelling and carbon flux. In light of CCMs, synthetic biology approaches have been used to improve enzyme complex for CO₂ fixations. Research on CCM-associated metabolons has also inspired biologists to engineer multi-step pathways by providing anchoring points for enzyme cascades to channel intermediate metabolites towards valuable products.     

CO2‐concentrating mechanisms in three southern hemisphere strains of Emiliania huxleyi

Rising global CO₂ is changing the carbonate chemistry of seawater, which is expected to influence the way phytoplankton acquire inorganic carbon. All phytoplankton rely on ribulose‐bisphosphate carboxylase oxygenase (RUBISCO) for assimilation of inorganic carbon in photosynthesis, but this enzyme is inefficient at present day CO₂ levels. Many algae have developed a range of energy demanding mechanisms, referred to as carbon concentrating mechanisms (CCMs), which increase the efficiency of carbon acquisition. We investigated CCM activity in three southern hemisphere strains of the coccolithophorid Emiliania huxleyi W. W. Hay & H. P. Mohler. Both calcifying and non‐calcifying strains showed strong CCM activity, with HCO₃^? as a preferred source of photosynthetic carbon in the non‐calcifying strain, but a higher preference for CO₂ in the calcifying strains. All three strains were characterized by the presence of pyrenoids, external carbonic anhydrase (CA) and high affinity for CO₂ in photosynthesis, indicative of active CCMs. We postulate that under higher CO₂ levels cocco‐lithophorids will be able to down‐regulate their CCMs, and re‐direct some of the metabolic energy to processes such as calcification. Due to the expected rise in CO₂ levels, photosynthesis in calcifying strains is expected to benefit most, due to their use of CO₂ for carbon uptake. The non‐calcifying strain, on the other hand, will experience only a 10% increase in HCO₃^?, thus making it less responsive to changes in carbonate chemistry of water.     

The roles of carbonic anhydrases in photosynthetic CO2 concentrating mechanisms

Cyanobacteria, algae, aquatic angiosperms and higher plants have all developed their own unique versions of photosynthetic CO₂ concentrating mechanisms (CCMs) to aid Rubisco in efficient CO₂ capture. An important aspect of all CCMs is the critical roles that the specialised location and function that various carbonic anhydrase enzymes play in the overall process, participating the interconversion of CO₂ and HCO₃ ⁻ species both inside and outside the cell. This review examines what we currently understand about the nature of the carbonic anhydrase enzymes, their localisation and roles in the various CCMs that have been studied in detail. This revised version was published online in August 2006 with corrections to the Cover Date.     

Impacts of increased atmospheric CO<Subscript>2</Subscript> concentration on photosynthesis and growth of micro- and macro-algae

Marine photosynthesis drives the oceanic biological CO₂ pump to absorb CO₂ from the atmosphere, which sinks more than one third of the industry-originated CO₂ into the ocean. The increasing atmospheric CO₂ and subsequent rise of pCO₂ in seawater, which alters the carbonate system and related chemical reactions and results in lower pH and higher HCO₃ ⁻ concentration, affect photosynthetic CO₂ fixation processes of phytoplanktonic and macroalgal species in direct and/or indirect ways. Although many unicellular and multicellular species can operate CO₂-concentrating mechanisms (CCMs) to utilize the large HCO₃ ⁻ pool in seawater, enriched CO₂ up to several times the present atmospheric level has been shown to enhance photosynthesis and growth of both phytoplanktonic and macro-species that have less capacity of CCMs. Even for species that operate active CCMs and those whose photosynthesis is not limited by CO₂ in seawater, increased CO₂ levels can down-regulate their CCMs and therefore enhance their growth under light-limiting conditions (at higher CO₂ levels, less light energy is required to drive CCM). Altered physiological performances under high-CO₂ conditions may cause genetic alteration in view of adaptation over long time scale. Marine algae may adapt to a high CO₂ oceanic environment so that the evolved communities in future are likely to be genetically different from the contemporary communities. However, most of the previous studies have been carried out under indoor conditions without considering the acidifying effects on seawater by increased CO₂ and other interacting environmental factors, and little has been documented so far to explain how physiology of marine primary producers performs in a high-CO₂ and low-pH ocean.     

Microalgal carbon-dioxide-concentrating mechanisms: Chlamydomonas inorganic carbon transporters

Aquatic photosynthetic micro-organisms have adapted to the variable and often-limiting availability of CO(2), and inorganic carbon (Ci) in general, by development of inducible CO(2)-concentrating mechanisms (CCMs) that allow them to optimize carbon acquisition. Both microalgal and cyanobacterial CCMs function to facilitate CO(2) assimilation when Ci is limiting via active Ci uptake systems to increase internal Ci accumulation and carbonic anhydrase activity to provide elevated internal CO(2) concentrations through the dehydration of accumulated bicarbonate. These CCMs have been studied over several decades, and details of the cyanobacterial CCM function have emerged over recent years. However, significant advances in understanding of the microalgal CCM have been more recent. With the aid of mutational approaches and the availability of multiple microalgal genome sequences, an integrated picture of the functional components of the microalgal CCMs is emerging, together with the molecular details regarding the function and regulation of the CCM. This review will focus on the recent advances in identifying and characterizing the Ci transport components of the microalgal CCM, especially in the model organism Chlamydomonas reinhardtii Dangeard.     

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

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

A speculative review of factors controlling the evolution of phytoplankton during Paleozoic time

Compilations of acritarch taxon diversity through geologic time show a severe decline at the end of the Devonian Period. While this diversity drop may have been due to intrinsic (biological) factors, it seems more likely that acritarch diversity has changed in response to extrinsic changes in seawater chemistry. Since acritarchs largely serve as a fossil proxy for phytoplankton, that is, oxygenic photoautotrophes, the evolution of their physiology is more directly linked with the chemistry of the environment than are the physiologies of other groups of marine eukaryotic organisms. Two major shifts occurred in Earth systems chemistry during the end of the Devonian: (1) a decline in atmospheric pCO₂ from 12 to perhaps 20 times that of today down to within near present day levels, and, (2) a shift in seawater chemistry from low Mg calcite to high Mg calcite + aragonite seas. Additionally, the rise in standing carbon biomass on land due to the evolution of trees during the Frasnian–Tournasian interval, caused a marked increase in carbon flux to estuarine and marine habitats. In combination, the marine realm experienced a drop in Ca/Mg, Cl/SO₄, K/Na, and CO₂(aq) with an increase in particulate (POM) and dissolved (DOM) organic matter. This last change would have brought with it an increased flux of P, which is mobilized in biological systems primarily as complexed with organic matter. Throughout their evolutionary history, cyanobacteria and the algae have shown a progressive acquisition of carbon concentration mechanisms (CCMs) that are required for inorganic carbon (C_i) uptake by their anabolic physiology. Paleozoic phytoplankton lacking these CCMs could have been growth-limited by C_i uptake. Given the possibility of a lack of such CCMs during the relatively high levels of CO₂ during the Lower Paleozoic, it seems possible that the acritarch decline was a combination of extinction due to inefficient C_i assimilation plus loss of cyst formation as a principal mechanism for survival following a large-scale shift to heterotrophic nutrition within multiple lineages.     

Evolutionary trends in RuBisCO kinetics and their co‐evolution with CO2 concentrating mechanisms

RuBisCO‐catalyzed CO₂ fixation is the main source of organic carbon in the biosphere. This enzyme is present in all domains of life in different forms (III, II, and I) and its origin goes back to 3500 Mya, when the atmosphere was anoxygenic. However, the RuBisCO active site also catalyzes oxygenation of ribulose 1,5‐bisphosphate, therefore, the development of oxygenic photosynthesis and the subsequent oxygen‐rich atmosphere promoted the appearance of CO₂ concentrating mechanisms (CCMs) and/or the evolution of a more CO₂‐specific RuBisCO enzyme. The wide variability in RuBisCO kinetic traits of extant organisms reveals a history of adaptation to the prevailing CO₂/O₂ concentrations and the thermal environment throughout evolution. Notable differences in the kinetic parameters are found among the different forms of RuBisCO, but the differences are also associated with the presence and type of CCMs within each form, indicative of co‐evolution of RuBisCO and CCMs. Trade‐offs between RuBisCO kinetic traits vary among the RuBisCO forms and also among phylogenetic groups within the same form. These results suggest that different biochemical and structural constraints have operated on each type of RuBisCO during evolution, probably reflecting different environmental selective pressures. In a similar way, variations in carbon isotopic fractionation of the enzyme point to significant differences in its relationship to the CO₂ specificity among different RuBisCO forms. A deeper knowledge of the natural variability of RuBisCO catalytic traits and the chemical mechanism of RuBisCO carboxylation and oxygenation reactions raises the possibility of finding unrevealed landscapes in RuBisCO evolution.     

Light-Modulated Responses of Growth and Photosynthetic Performance to Ocean Acidification in the Model Diatom Phaeodactylum tricornutum

Ocean acidification (OA) due to atmospheric CO₂ rise is expected to influence marine primary productivity. In order to investigate the interactive effects of OA and light changes on diatoms, we grew Phaeodactylum tricornutum, under ambient (390 ppmv; LC) and elevated CO₂ (1000 ppmv; HC) conditions for 80 generations, and measured its physiological performance under different light levels (60 µmol m⁻² s⁻¹, LL; 200 µmol m⁻² s⁻¹, ML; 460 µmol m⁻² s⁻¹, HL) for another 25 generations. The specific growth rate of the HC-grown cells was higher (about 12–18%) than that of the LC-grown ones, with the highest under the ML level. With increasing light levels, the effective photochemical yield of PSII (F_v′/F_m′) decreased, but was enhanced by the elevated CO₂, especially under the HL level. The cells acclimated to the HC condition showed a higher recovery rate of their photochemical yield of PSII compared to the LC-grown cells. For the HC-grown cells, dissolved inorganic carbon or CO₂ levels for half saturation of photosynthesis (K_1/2 DIC or K_1/2 CO₂) increased by 11, 55 and 32%, under the LL, ML and HL levels, reflecting a light dependent down-regulation of carbon concentrating mechanisms (CCMs). The linkage between higher level of the CCMs down-regulation and higher growth rate at ML under OA supports the theory that the saved energy from CCMs down-regulation adds on to enhance the growth of the diatom.     

Proteomics in translational cancer research: biomarker discovery for clinical applications

This special focus issue of Expert Review of Proteomics invites key opinion leaders to report their recent findings and views on the important topic of translating potential proteomic biomarkers to clinically useful, regulator-approved biomarkers: a challenging journey. The issue also highlights the difficulties associated with and the way forward in the discovery of proteomic cancer biomarkers for clinical applications, as well as presenting recent original research in the field.     

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

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

Feathers to Fur: the status of New Zealand ecological research in 2009

We outline the scope of this special issue of New Zealand Journal of Ecology, which reviews progress in New Zealand ecology to 2009, based on a symposium in 2007. Both the issue and symposium update a 1986 conference and 1989 special issue of NZ J Ecol called ?Moas, Mammals and Climate? which has been influential and widely cited. This issue revisits several themes featured in 1989, including the extent of recent and prehistoric extinctions in the New Zealand fauna; effects of introduced mammalian herbivores replacing now-extinct browsing birds such as moa; the impacts of introduced mammalian predators on native birds (hence the Feathers to Fur title); the role of islands as refuges and opportunities for restoration; and the status of bird?plant mutualisms like pollination and fruit dispersal. Several topics not discussed in 1989 are raised, including the unusual size and functional composition of New Zealand?s tree flora, and several taxonomic groups (invertebrates, fungi) and habitats (fresh waters) that received little attention in 1989. We summarise four symposium talks which are not included elsewhere in this issue. New Zealand leads the world in ways both unenviable (e.g. levels of impact of introduced species) and enviable (e.g. predator eradication, translocations, rare species management. The recent advances reviewed in this issues have relevance well beyond New Zealand.     

Differential effects of ocean acidification on carbon acquisition in two bloom‐forming dinoflagellate species

Dinoflagellates represent a cosmopolitan group of phytoplankton with the ability to form harmful algal blooms. Featuring a Ribulose‐1,5‐bisphosphate carboxylase/oxygenase (RubisCO) with very low CO₂ affinities, photosynthesis of this group may be particularly prone to carbon limitation and thus benefit from rising atmospheric CO₂ partial pressure (pCO₂) under ocean acidification (OA). Here, we investigated the consequences of OA on two bloom‐forming dinoflagellate species, the calcareous Scrippsiella trochoidea and the toxic Alexandrium tamarense. Using dilute batch incubations, we assessed growth characteristics over a range of pCO₂ (i.e. 180–1200 µatm). To understand the underlying physiology, several aspects of inorganic carbon acquisition were investigated by membrane‐inlet mass spectrometry. Our results show that both species kept growth rates constant over the tested pCO₂ range, but we observed a number of species‐specific responses. For instance, biomass production and cell size decreased in S. trochoidea, while A. tamarense was not responsive to OA in these measures. In terms of oxygen fluxes, rates of photosynthesis and respiration remained unaltered in S. trochoidea whereas respiration increased in A. tamarense under OA. Both species featured efficient carbon concentrating mechanisms (CCMs) with a CO₂‐dependent contribution of HCO₃^? uptake. In S. trochoidea, the CCM was further facilitated by exceptionally high and CO₂‐independent carbonic anhydrase activity. Comparing both species, a general trade‐off between maximum rates of photosynthesis and respective affinities is indicated. In conclusion, our results demonstrate effective CCMs in both species, yet very different strategies to adjust their carbon acquisition. This regulation in CCMs enables both species to maintain growth over a wide range of ecologically relevant pCO₂.     

Carbon acquisition by diatoms

Diatoms are responsible for up to 40% of primary productivity in the ocean, and complete genome sequences are available for two species. However, there are very significant gaps in our understanding of how diatoms take up and assimilate inorganic C. Diatom plastids originate from secondary endosymbiosis with a red alga and their Form ID Rubisco (ribulose-1,5-bisphosphate carboxylase-oxygenase) from horizontal gene transfer, which means that embryophyte paradigms can only give general guidance as to their C acquisition mechanisms. Although diatom Rubiscos have relatively high CO₂ affinity and CO₂/O₂ selectivity, the low diffusion coefficient for CO₂ in water has the potential to restrict the rate of photosynthesis. Diatoms growing in their natural aquatic habitats operate inorganic C concentrating mechanisms (CCMs), which provide a steady-state CO₂ concentration around Rubisco higher than that in the medium. How these CCMs work is still a matter of debate. However, it is known that both CO₂ and HCO₃⁻ are taken up, and an obvious but as yet unproven possibility is that active transport of these species across the plasmalemma and/or the four-membrane plastid envelope is the basis of the CCM. In one marine diatom there is evidence of C₄-like biochemistry which could act as, or be part of, a CCM. Alternative mechanisms which have not been eliminated include the production of CO₂ from HCO₃⁻ at low pH maintained by a H⁺ pump, in a compartment close to that containing Rubisco.     

Carbon concentrating mechanisms: in rescue of Rubisco inefficiency

Agricultural yields have kept pace with the rising demands in the recent past as a result of the breeding and improved farming practices, but these practices alone will not be able to meet the demands of the future. The focus is now on the enhancement of the photosynthetic machinery. In photosynthesis, the rate limiting step is the one catalyzed by RuBisCO- Ribulose-1,5-bisphosphate carboxylase/oxygenase (4.1.1.39), which, because of its loose specificity and low turnover rate, is the primary target of most research programs directed towards improved photosynthesis. The other avenues of photosynthetic machinery that are under investigation to enhance it include—improved stomatal regulation and membrane permeability, RuBisCO with high specificity for CO₂ and higher catalytic turnover; bypass of photorespiration and introduction of carbon concentrating mechanism (CCM) into the C₃ plants. Carbon concentrating mechanisms cause accumulation of carbon dioxide in vicinity of RuBisCO producing a high CO₂/O₂ ratio and hence an environment more suitable for carboxylation reactions than oxygenation reactions. This article includes the basic details of the major naturally occurring CCMs in various photosynthetic organisms to identify the knowledge gaps in each which could help study the prospects of its possible introduction into a non-native system as C₃ plants which are devoid of any CCM.     

Introduction: Re-thinking Ethnic and Racial Studies

The changing boundaries of the study of ethnicity and race have been the subject of much debate in recent years. New theoretical debates have come to the fore and empirical research has broached new questions. Taking its cue from the wide range of themes covered in this special issue, this article seeks to map out some of the key areas in which this transformation has become apparent and to highlight the implications for ethnic and race relations as a field of study. In doing so it engages with some of the key questions that run through the whole of this special issue, including the relationship between race, power and politics, identity and difference and the politics of multiculturalism. It concludes by touching on some issues that need further research and analysis.