Photosynthetic Carbon Reduction Cycle Research Articles

Overexpression of glucose-6-phosphate isomerase in Synechocystis sp. PCC 6803 with disrupted glycogen synthesis pathway improves exopolysaccharides synthesis

The exopolysaccharides (EPS) acting as an extracellular protective molecule in cyanobacteria have unique characteristics suitable for various applications. The synthesis of EPS which has a complex structure is regulated by the expression of multiple gene clusters. In order to increase the EPS production, it is essential to enhance the synthesis of fundamental sugar units. Thus, the glgC encoding glucose-1-phosphate adenylyltransferase involved in glycogen synthesis was disrupted to redirect most of the carbon sources including glucose into EPS synthesis pathway. On the other hand, the gpi encoding glucose-6-phosphate isomerase was overexpressed to enhance the sugar metabolism from photosynthetic carbon reduction cycle, which led to the accumulation of various sugar monomers. The glgC knockout strain containing gpi overexpression (▲gpi-∆glgC) produced the highest content of EPS accompanied by the increases in its composition of glucose, galactose, xylose, mannose, rhamnose, fucose and uronic acid. Furthermore, ▲gpi-∆glgC strain with the total EPS content of about 251 μg/108 cells was more tolerant to high light intensity as well as UV irradiance.

Plant aquaporins: A frontward to make crop plants drought resistant.

Drought stress alters gene expression and causes cellular damage in crop plants. Drought inhibits photosynthesis by reducing the content and the activity of the photosynthetic carbon reduction cycle, ultimately decreasing the crop yield. The role of aquaporins (AQP) in improving the growth and adaptation of crop plants under drought stress is of importance. AQP form channels and control water transport in and out of the cells and are associated with drought tolerance mechanisms. The current review addresses: (1) the evolution of AQPs in plants, (2) the classification of plant AQPs, (3) the role of AQPs in drought alleviation in crop plants, and (4) the phytohormone crosstalk with AQPs in crops exposed to drought stress.

Remembering Melvin Calvin (1911-1997), a highly versatile scientist of the 20th century.

Melvin Calvin (1911-1997) was the recipient of the 1961 Nobel Prize in Chemistry for the discovery of the canonical photosynthetic carbon reduction cycle. We present here a very brief glimpse of this extraordinary American scientist, who in his time was a preeminent force in physical and organic chemistry. Besides natural photosynthesis, Calvin's prolific career included artificial photosynthesis, colors of organic substances, the origin of life, cancer, moon rocks, molecular basis of learning, and plant lipids & algal hydrocarbons as potential renewable sources of transport fuels.

The importance of the photosynthetic Gibbs effect in the elucidation of the Calvin-Benson-Bassham cycle.

The photosynthetic carbon reduction cycle, or Calvin–Benson–Bassham (CBB) cycle, is now contained in every standard biochemistry textbook. Although the cycle was already proposed in 1954, it is still the subject of intense research, and even the structure of the cycle, i.e. the exact series of reactions, is still under debate. The controversy about the cycle's structure was fuelled by the findings of Gibbs and Kandler in 1956 and 1957, when they observed that radioactive 14CO2 was dynamically incorporated in hexoses in a very atypical and asymmetrical way, a phenomenon later termed the ‘photosynthetic Gibbs effect’. Now, it is widely accepted that the photosynthetic Gibbs effect is not in contradiction to the reaction scheme proposed by CBB, but the arguments given have been largely qualitative and hand-waving. To fully appreciate the controversy and to understand the difficulties in interpreting the Gibbs effect, it is illustrative to illuminate the history of the discovery of the CBB cycle. We here give an account of central scientific advances and discoveries, which were essential prerequisites for the elucidation of the cycle. Placing the historic discoveries in the context of the modern textbook pathway scheme illustrates the complexity of the cycle and demonstrates why especially dynamic labelling experiments are far from easy to interpret. We conclude by arguing that it requires sound theoretical approaches to resolve conflicting interpretations and to provide consistent quantitative explanations.

Overexpression of the rubisco activase gene improves growth and low temperature and weak light tolerance in Cucumis sativus.

Rubisco activase (RCA) is an important enzyme that can catalyze the carboxylation and oxygenation activities of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), which is involved in the photosynthetic carbon reduction cycle. Here, we studied the effects of changes in RCA activity on photosynthesis, growth and development, as well as the low temperature and weak light tolerance of RCA overexpressing transgenic cucumber (Cucumis sativus) plants. CsRCA overexpression increased the plant height, leaf area and dry matter, and decreased the root/top ratio in transgenic cucumber plants compared with the wild-type (WT) plants. Low temperature and low light stress led to decreases in the CsRCA expression and protein levels, the photosynthetic rate (Pn) and the stomatal conductance (Gs), but an increase in the intercellular CO2 (Ci) concentration in cucumber leaves. The actual photochemical efficiency and maximal photochemical efficiency of photosystem II in cucumber seedlings also declined, but the initial fluorescence increased during low temperature and weak light stress. Transgenic plants showed a lower decrease in the CsRCA expression level and actual and maximal photochemical efficiencies, as well as increases in the Ci and initial fluorescence relative to the WT plants. Low temperature and low light stress resulted in a significant increase in the malondialdehyde (MDA) content; however, this increase was reduced in transgenic plants compared with that in WT plants. Thus, the overexpression of CsRCA may promote the growth and low temperature and low light tolerance of cucumber plants in solar greenhouses.

Expression of cyanobacterial FBP/SBPase in soybean prevents yield depression under future climate conditions.

Predictions suggest that current crop production needs to double by 2050 to meet global food and energy demands. Based on theory and experimental studies, overexpression of the photosynthetic enzyme sedoheptulose-1,7-bisphosphatase (SBPase) is expected to enhance C3 crop photosynthesis and yields. Here we test how expression of the cyanobacterial, bifunctional fructose-1,6/sedoheptulose-1,7-bisphosphatase (FBP/SBPase) affects carbon assimilation and seed yield (SY) in a major crop (soybean, Glycine max). For three growing seasons, wild-type (WT) and FBP/SBPase-expressing (FS) plants were grown in the field under ambient (400 μmol mol−1) and elevated (600 μmol mol−1) CO2 concentrations [CO2] and under ambient and elevated temperatures (+2.7 °C during daytime, +3.4 °C at night) at the SoyFACE research site. Across treatments, FS plants had significantly higher carbon assimilation (4–14%), Vc,max (5–8%), and Jmax (4–8%). Under ambient [CO2], elevated temperature led to significant reductions of SY of both genotypes by 19–31%. However, under elevated [CO2] and elevated temperature, FS plants maintained SY levels, while the WT showed significant reductions between 11% and 22% compared with plants under elevated [CO2] alone. These results show that the manipulation of the photosynthetic carbon reduction cycle can mitigate the effects of future high CO2 and high temperature environments on soybean yield.

Application of the ecosystem model and Markov Chain Monte Carlo for parameter estimation and productivity prediction

It is increasingly being recognized that global ecological research requires novel methods and strategies in which to combine process‐based ecological models and data in cohesive, systematic ways. In process‐based model applications, inherent spatial and temporal heterogeneities found within terrestrial ecosystems may lead to the uncertainties of model predictions. To reduce simulation uncertainties due to inaccurate model parameters, the Markov Chain Monte Carlo (MCMC) method was applied in this study to improve the estimations of four key parameters used in the process‐based ecosystem model of TRIPLEX‐FLUX. These four key parameters include a maximum photosynthetic carboxylation rate of 25°C (Vmax), an electron transport (Jmax) light‐saturated rate within the photosynthetic carbon reduction cycle of leaves, a coefficient of stomatal conductance (m), and a reference respiration rate of 10°C (R10). Seven forest flux tower sites located across North America were used to investigate and facilitate understanding of the daily variation in model parameters for three deciduous forests, three evergreen temperate forests, and one evergreen boreal forest. Eddy covariance CO2 exchange measurements were assimilated to optimize the parameters in the year 2006. After parameter optimization and adjustment took place, net ecosystem production prediction significantly improved (by approximately 25%) compared to the CO2 flux measurements taken at the seven forest ecosystem sites. Results suggest that greater seasonal variability occurs in broadleaf forests in respect to the selected parameters than in needleleaf forests. This study also demonstrated that the model‐data fusion approach by incorporating MCMC method is able to better estimate parameters and improve simulation accuracy for different ecosystems located across North America.

Decreased TK activity alters growth, yield and tolerance to low temperature and low light intensity in transgenic cucumber plants.

Four CsTK antisense transgenic cucumber plants were obtained. Decreased TK activity decreased the photosynthetic rate, seed germination rate, growth yield, and the tolerance to low temperature and weak light stress. Transketolase (TK, EC 2.2.1.1) is a key enzyme in the photosynthetic carbon reduction cycle (Calvin cycle). A cDNA fragment (526 bp) encoding transketolase was cloned from cucumber plants (Cucumis sativa L. cv 'Jinyou 3') by RT-PCR. The antisense expression [(PBI-CsTK(-)] vector containing the CsTK gene fragment was constructed. The resulting plasmid was introduced into the cucumber inbred lines '08-1' using the agrobacterium-mediated method, and four antisense transgenic cucumber plants were obtained. Decreased CsTK expression either unaltered or slightly increased the mRNA abundance and activities of the other main enzymes in the Calvin cycle, however, it decreased the TK activity and net photosynthetic rate (Pn) in antisense transgenic cucumber leaves. Antisense plants showed decreases in the growth, ratio of female flowers and yield compared with the wild-type (WT) plants. The decrease in Pn, stomatal conductance (Gs), transpiration rate (Tr), photochemical efficiency (Fv/Fm) and actual photochemical efficiency of PSII (ΦPSII) and the increase in electrolyte leakage (EL) were greater in antisense transgenic plants than in WT plants under low temperature (5 °C) and low light intensity (100 μmol m(-2) s(-1)).

Cloning and expression analysis of transketolase gene in Cucumis sativus L.

Transketolase (TK, EC 2.2.1.1) is a key enzyme in the photosynthetic carbon reduction cycle (Calvin cycle). A full-length cDNA encoding transketolase (TK, designated as CsTK) was isolated from cucumber leaves (Cucumis sativa L. cv 'Jinyou 3') by RT-PCR and RACE. The cDNA contained 2368 nucleotides with a complete open reading frame (ORF) of 2229 nucleotides, which was predicted to encode a peptide of 742 amino acids. Expression analysis by real-time PCR and western blot revealed that TK mRNA was abundant in cucumber leaves and detectable in stems, fruits and roots. TK activity and the gene expression at the mRNA and protein levels was higher in mid-position leaves (4th apical leaves) than in upper position leaves (1st) and base position leaves (12th). The diurnal variation of CsTK expression and TK activity in the optimal functional leaf (4th leaf) was a single-peak curve, and the peak appeared at 14:00 on a sunny day. Low temperature (5°C) and low light (100μmolm(-2)s(-1)) induced CsTK expression, but the expression level decreased after 24h of chilling stress. Over-expression of CsTK increased the TK activity, mRNA abundance and activities of other main enzymes in Calvin cycle, and net photosynthetic rate (Pn) in transgenic cucumber leaves. Transgenic plants showed a higher ratio of female flower and yield relative to the wild type (WT) plants. The decreases in Pn and carboxylation efficiency (CE) were less in transgenic plants than that in WT during low temperature and low light intensity.

Will C3 crops enhanced with the C4 CO2-concentrating mechanism live up to their full potential (yield)?

Sustainably feeding the world's growing population in future is a great challenge and can be achieved only by increasing yield per unit land surface. Efficiency of light interception and biomass partitioning into harvestable parts (harvest index) has been improved substantially via plant breeding in modern crops. The conversion efficiency of intercepted light into biomass still holds promise for yield increase. This conversion efficiency is to a great extent constrained by the metabolic capacity of photosynthesis, defined by the characteristics of its components. Genetic manipulations are increasingly applied to lift these constraints, by improving CO2 or substrate availability for the photosynthetic carbon reduction cycle. Although these manipulations can lead to improved potential growth rates, this increase might be offset by a decrease in performance under stress conditions. In this review, we assess possible positive or negative effects of the introduction of a CO2-concentrating mechanism in C3 crop species on crop potential productivity and yield robustness.

Expression of the Chlamydomonas reinhardtii Sedoheptulose‐1,7‐bisphosphatase in Dunaliella bardawil leads to enhanced photosynthesis and increased glycerol production

Bioengineering of photoautotrophic microalgae into CO(2) scrubbers and producers of value-added metabolites is an appealing approach in low-carbon economy. A strategy for microalgal bioengineering is to enhance the photosynthetic carbon assimilation through genetically modifying the photosynthetic pathways. The halotolerant microalgae Dunaliella possess a unique osmoregulatory mechanism, which accumulates intracellular glycerol in response to extracellular hyperosmotic stresses. In our study, the Calvin cycle enzyme sedoheptulose 1,7-bisphosphatase from Chlamydomonas reinhardtii (CrSBPase) was transformed into Dunaliella bardawil, and the transformant CrSBP showed improved photosynthetic performance along with increased total organic carbon content and the osmoticum glycerol production. The results demonstrate that the potential of photosynthetic microalgae as CO(2) removers could be enhanced through modifying the photosynthetic carbon reduction cycle, with glycerol as the carbon sink.

Changes in photosynthesis and activities of enzymes involved in carbon metabolism during exposure to low light in cucumber (Cucumis sativus) seedlings

Two cucumber genotypes, S404 and S1 with low light-sensitivity and low light-tolerance, respectively were used to investigate the oxygen consumption rate of photosystem I, the oxygen evolution rate of photosystem II, cab transcript levels, and activities of enzymes involved in photosynthetic carbon reduction cycle. The results show that short term (24 h) / long term (five and 10 days) low light stress had significant effect on PSII activities while PSI’s effect was not significant. Under the low light stress, S1 cab gene transcript levels were quickly recovered while S404 cab gene transcript levels were slowly recovered. The total dry mass and leaf area of S1 was lower than S404. Low light treatment decreased C 3 photosynthetic carbon cycle enzyme activities involved in ribulose-1,5-bisphosphate carboxylase oxygenase (rubisco) carboxylation and fructose-1,6-bisphosphatase (FBPase), and increased C 4 photosynthetic carbon cycle enzyme activities involved in nicotinamide adenine dinucleotide phosphate malate dehydrogenase (NADP-MDH). The NADP-MDH activity in S1 leaves increased significantly compared to S404. These observations suggest that S1 photosynthetic capacity is higher than S404 under low light conditions. Photosynthetic C 4 -microcycle possibly would have played a role in low light stress. Therefore, the transcript levels of cab and the involvement of NADP-MDH in low light-resistance need further research. Keywords: Low light, oxygen consumption rate, oxygen evolution rate, cab gene, NADP-MDH

Algal evolution in relation to atmospheric CO2: carboxylases, carbon-concentrating mechanisms and carbon oxidation cycles.

Oxygenic photosynthesis evolved at least 2.4 Ga; all oxygenic organisms use the ribulose bisphosphate carboxylase-oxygenase (Rubisco)-photosynthetic carbon reduction cycle (PCRC) rather than one of the five other known pathways of autotrophic CO(2) assimilation. The high CO(2) and (initially) O(2)-free conditions permitted the use of a Rubisco with a high maximum specific reaction rate. As CO(2) decreased and O(2) increased, Rubisco oxygenase activity increased and 2-phosphoglycolate was produced, with the evolution of pathways recycling this inhibitory product to sugar phosphates. Changed atmospheric composition also selected for Rubiscos with higher CO(2) affinity and CO(2)/O(2) selectivity correlated with decreased CO(2)-saturated catalytic capacity and/or for CO(2)-concentrating mechanisms (CCMs). These changes increase the energy, nitrogen, phosphorus, iron, zinc and manganese cost of producing and operating Rubisco-PCRC, while biosphere oxygenation decreased the availability of nitrogen, phosphorus and iron. The majority of algae today have CCMs; the timing of their origins is unclear. If CCMs evolved in a low-CO(2) episode followed by one or more lengthy high-CO(2) episodes, CCM retention could involve a combination of environmental factors known to favour CCM retention in extant organisms that also occur in a warmer high-CO(2) ocean. More investigations, including studies of genetic adaptation, are needed.

Molecular cloning and expression analysis of the gene encoding sedoheptulose-1, 7-bisphosphatase from Cucumis sativus

Sedoheptulose-1, 7-bisphosphatase (SBPase; EC 3.1.3.37) is a key enzyme in the photosynthetic carbon reduction cycle (Calvin cycle). In the present study, a full-length cDNA encoding SBPase (designated as CsSBP, GenBank accession no. FJ911553) was isolated from cucumber ( Cucumis sativus L. cv. ‘Jinyou 3’) grown in solar-greenhouse by RT-PCR and RACE. The cDNA contained 1452 nucleotides with a complete open reading frame (ORF) of 1167 nucleotides, which was deduced for encoding a peptide of 388 amino acids whose molecular mass was inferred to be 42 kDa with its isoelectric point at 6.17. Sequence comparison analysis showed that the deduced amino acid of SBPase from cucumber leaves had high homology to that from other species. Expression analysis by real-time PCR and Western blot revealed that CsSBP mRNA and protein was most abundant in leaves, detectable in stems and fruits, but invisible in roots. A similar difference was found in SBPase activity among leaves, stems, fruits and roots. CsSBP expression at the mRNA and protein level was the highest in mid-position leaves (4th–8th apical expanded leaves) of plants, medium in base position leaves (12th), and the lowest in upper position leaves (1st). However, SBPase activity decreased successively as leaf position declined. The net photosynthetic rate (Pn) of mid-position leaves was also found to be the highest, but that of base-position leaves was lower than upper-position leaves. The diurnal variations of gene expression and SBPase activity in optimal functional leaf (6th leaf) was a single-peak pattern curve, which was similar to that of Pn, and the peak appeared at 12:00. These data indicate that CsSBP expresses only in green tissues of cucumber plants and there are close positive correlations between SBPase activity and gene expression at the mRNA and protein levels, and between SBPase activity and Pn in leaves of cucumber plants.

Cyclic electron flow around PSI functions in the photoinhibited rice leaves

We investigated the regulation mechanism of cyclic electron flow around photosystem I (CEF-PSI) in the rice leaves which suffered from photosystem II (PSII) photoinhibition. High-light (HL) treatment [2000 µmol photon m–2 s–1 at 0% carbon dioxide (CO2), 2% oxygen (O2) and 25°C] of rice leaves decreased both the maximum quantum efficiency of PSII (Fv/Fm) and the light-dependent O2-evolution rate [V(O2)]. High-light treatment did not affect the relative electron flux in PSI [Φ(PSI) × PFD]. In non-treated leaves, increasing in the photon flux density (PFD) enhanced V(O2), Φ(PSI) × PFD and the ratio of oxidized P700 to total P700 [(P700+)/(P700)total]. Φ(PSI) × PFD continued to increase even after the saturation of V(O2) against PFD. These results suggested that the electrons not used for the major electron sink, photosynthetic carbon reduction-cycle, did turnover in PSI, that is, CEF-PSI functioned at a higher PFD. High-light treatments did not affect the activity of CEF-PSI and increased (P700+)/(P700)total in the lower PFDs, compared to non-treated leaves. The activity of CEF-PSI depends on the amount of oxidized PQ. Photoinhibition of PSII suppressed electron influx from PSII to photosynthetic linear electron transport. The enhanced (P700+)/(P700)total suggested the increase in the ratio of oxidized plastoquinone (PQ) to total PQ, which supported the activity of CEF-PSI in the photoinhibited leaves.

Enhancing Photosynthesis

The industrialization of cereal production in India about 50 years ago was associated with dramatic increases in yields worldwide. As a consequence, there was a reduction in famine. The worldwide impact was called the “Green Revolution,” for which Norman Borlaug was awarded the Nobel Peace Prize in 1970. The introduction of dwarfing genes into cereals enabled the use of greater amounts of fertilizer, which resulted in substantial yield increases. The dwarf stature also led to a larger fraction of crop biomass being allocated to grain, the harvested product. A concerted effort by plant breeders and agronomists has driven continuous improvement in cereal yields. World population has grown from 3 billion in 1960 to nearly 7 billion in 2010, increasing demand for food, animal feed, and bioenergy. With world population projected to pass 9 billion by 2050, a further increase in yield of at least 28% is necessary. Crops will also have to be adjusted for climate change, and production may be limited by the supply of irrigation water and loss of arable land. Reducing the gap between average farm yield and potential yield (that obtained with best management practice in the absence of water, nutrient, pest, and disease limitations) will enable yield increases to continue for some time. However, there is a growing realization that to sustain increases in potential yield, biomass production needs to be increased. Biomass is produced by photosynthesis. Once a crop has been adapted to fully exploit the interception of sunlight and allocate the maximum amount of biomass to grain, further increases in yield will require increasing biomass by enhancing photosynthesis. The year 2011 marks the 50th anniversary for the Nobel Prize in Chemistry awarded to Melvin Calvin for the elucidation of the photosynthetic carbon reduction cycle. A large body of knowledge has been accumulated about photosynthesis, which can be used to formulate strategies for enhancing photosynthesis, the concern of this Focus Issue. Fifteen Update articles in this issue target basic research covering diverse aspects of photosynthesis. These range from initial light capture, electron transport and photophosphorylation, carbon dioxide fixation, and metabolic pathways, to integration among organelles and transport from the subcellular to the whole-plant level. One aspect of enhancing photosynthesis will be the need to adjust other properties, such as leaf anatomy, to ensure that the biochemistry works efficiently. It is also important to consider the performance of photosynthesis at the level of the crop canopy. The Update articles identify many opportunities and targets where photosynthesis could be enhanced. Perhaps the clearest candidate for improvement is Rubisco, the enzyme used by all plants to ultimately fix carbon dioxide. This enzyme catalyzes both carboxylation and oxygenation reactions with the five-carbon sugar ribulose bisphosphate. The oxygenase reaction leads to the release of one-half a carbon dioxide and consumes ATP and NADPH, which could otherwise be used productively to fix more carbon. Photosynthesis can be increased by raising the carbon dioxide concentration surrounding Rubisco, either by CO2-concentrating mechanisms used by C4 plants, cyanobacteria, and algae, or by enriching the atmosphere, which is happening as a result of the human use of fossil fuels. There is abundant evidence demonstrating that photosynthetic rates can be increased in the short term in response to increasing atmospheric CO2 concentrations. Plant growth depends on how newly fixed carbon is invested. To translate enhanced photosynthesis into greater plant growth and crop yield, a great deal of research is required, but this should be attainable if sufficient resources are invested to support the research effort by the plant science research community. Recent increases in yield differ widely among crops; to a large extent, this reflects how much money has been invested into each crop. We thank the contributors for their insights and hope that this Focus Issue draws attention to the many possibilities that should be explored.

식물의 탄소대사공학 연구동향

Yield productivity of staple crops must be increased at least 50% by 2050, in order to feed the world population which is expected to reach 90 billions. Photosynthetic carbon assimilation and carbohydrate metabolism leading to the production of starch would be the final frontier to quest for new sources of technology enabling such a drastic increase of crop productivity. In this review, attempts to genetically engineer plant photosynthetic carbon reduction cycle and metabolic pathways to increase starch production are introduced.

Molecular Characterization of the Calvin Cycle Enzyme Phosphoribulokinase in the Stramenopile Alga Vaucheria litorea and the Plastid Hosting Mollusc Elysia chlorotica

Phosphoribulokinase (PRK), a nuclear-encoded plastid-localized enzyme unique to the photosynthetic carbon reduction (Calvin) cycle, was cloned and characterized from the stramenopile alga Vaucheria litorea. This alga is the source of plastids for the mollusc (sea slug) Elysia chlorotica which enable the animal to survive for months solely by photoautotrophic CO2 fixation. The 1633-bp V. litorea prk gene was cloned and the coding region, found to be interrupted by four introns, encodes a 405-amino acid protein. This protein contains the typical bipartite target sequence expected of nuclear-encoded proteins that are directed to complex (i.e. four membrane-bound) algal plastids. De novo synthesis of PRK and enzyme activity were detected in E. chlorotica in spite of having been starved of V. litorea for several months. Unlike the algal enzyme, PRK in the sea slug did not exhibit redox regulation. Two copies of partial PRK-encoding genes were isolated from both sea slug and aposymbiotic sea slug egg DNA using PCR. Each copy contains the nucleotide region spanning exon 1 and part of exon 2 of V. litorea prk, including the bipartite targeting peptide. However, the larger prk fragment also includes intron 1. The exon and intron sequences of prk in E. chlorotica and V. litorea are nearly identical. These data suggest that PRK is differentially regulated in V. litorea and E. chlorotica and at least a portion of the V. litorea nuclear PRK gene is present in sea slugs that have been starved for several months.

Contributions of anoxygenic and oxygenic phototrophy and chemolithotrophy to carbon and oxygen fluxes in aquatic environments

Estimates of aquatic primary productivity at local, regional or global scales commonly concentrate on oxygenic photolithotrophy. The analysis presented here briefly considers the occur- rence and metabolism of other autotrophs sensu lato, i.e. not just the organisms with an autotrophic inorganic carbon assimilation machinery. These other autotrophs include chemolithotrophs and anoxygenic photolithotrophs, of which some use the photosynthetic carbon reduction cycle as do oxy- genic photolithotrophs, while others use one of 4 other pathways. The category of other autotrophs also includes organisms that possess photochemical energy transduction machinery but lack autotrophic carbon assimilation; light stimulates the growth rates of these autotrophs and/or in- creases the fraction of the organic carbon substrate used that is converted into cell material when using light energy. Organisms lacking autotrophic inorganic carbon assimilation influence food webs by increasing the rate, or efficiency, of conversion of dissolved organic carbon ultimately derived from autotrophic inorganic carbon assimilation into particulate organic carbon. Chemolithotrophs and anoxygenic autotrophs today depend on the activities of oxygenic chemolithotrophs for one or more of their growth substrates, and thus contribute to global net primary productivity but not to global gross primary productivity. Global net aquatic primary productivity by oxygenic photo- lithotrophs is at least 50 Pg C yr -1 , while chemolithotrophs and anoxygenic photolithotrophs together contribute about 0.40 Pg C yr -1 . Before the occurrence of oxygenic photolithotrophs in the Archaean, chemolithotrophs and anoxygenic photolithotrophs had a net primary productivity of about 3.4 Pg C yr -1 , which is higher than the present values largely because anoxygenic photolithotrophs were able to inhabit the euphotic zone worldwide.

Contributions of anoxygenic and oxygenic phototrophy and chemolithotrophy to carbon and oxygen fluxes in aquatic environments

Estimates of aquatic primary productivity at local, regional or global scales commonly concentrate on oxygenic photolithotrophy. The analysis presented here briefly considers the occur- rence and metabolism of other autotrophs sensu lato, i.e. not just the organisms with an autotrophic inorganic carbon assimilation machinery. These other autotrophs include chemolithotrophs and anoxygenic photolithotrophs, of which some use the photosynthetic carbon reduction cycle as do oxy- genic photolithotrophs, while others use one of 4 other pathways. The category of other autotrophs also includes organisms that possess photochemical energy transduction machinery but lack autotrophic carbon assimilation; light stimulates the growth rates of these autotrophs and/or in- creases the fraction of the organic carbon substrate used that is converted into cell material when using light energy. Organisms lacking autotrophic inorganic carbon assimilation influence food webs by increasing the rate, or efficiency, of conversion of dissolved organic carbon ultimately derived from autotrophic inorganic carbon assimilation into particulate organic carbon. Chemolithotrophs and anoxygenic autotrophs today depend on the activities of oxygenic chemolithotrophs for one or more of their growth substrates, and thus contribute to global net primary productivity but not to global gross primary productivity. Global net aquatic primary productivity by oxygenic photo- lithotrophs is at least 50 Pg C yr -1 , while chemolithotrophs and anoxygenic photolithotrophs together contribute about 0.40 Pg C yr -1 . Before the occurrence of oxygenic photolithotrophs in the Archaean, chemolithotrophs and anoxygenic photolithotrophs had a net primary productivity of about 3.4 Pg C yr -1 , which is higher than the present values largely because anoxygenic photolithotrophs were able to inhabit the euphotic zone worldwide.