Low Inorganic Carbon Research Articles

Key soil properties and their relationships with crop yields as affected by soil–landscape rehabilitation

AbstractTillage and water erosion induce spatially dependent changes in soil properties that influence productivity. Soil–landscape rehabilitation (returning translocated topsoil to landscape positions of soil loss by erosion) is one method to improve the productivity of severely eroded land. The objective of this study was to investigate relationships among key soil chemical, biological, and physical factors and crop growth and grain yield in eroded and rehabilitated landform positions. Soil–landscape rehabilitation was performed by moving 15–20 cm of topsoil from the lower slope to the upper slope positions of replicate plots; adjacent plots were left in their eroded condition. Crop response was monitored for 6 years. Rehabilitation resulted in large changes in the upper slope, especially in the most eroded landscape positions, where rehabilitated plots had lower inorganic carbon (IC) and higher organic carbon, available macronutrients, water infiltration rates, fungal and bacterial populations, and other measures of soil quality compared with control plots. Differences in surface soil IC that result from extensive erosion exposing calcareous subsoils, explained 70% of the yield variability in the upper slope. In the lower slope, the soil removal treatment was the main predictor of crop yield. Areas of soil removal had relatively high early‐season water content and low fungal and bacterial populations. Improving soil properties in areas of high soil loss by erosion increased both grain yield and yield stability across climatic conditions.

Identification of the cause of the difference among TOC quantitative methods according to the water sample characteristics

Total organic carbon (TOC) analysis with accurate determination of particulate organic carbon (POC) content in suspended solids (SS) containing water is critical for evaluating the environmental impact of particulate organic pollutants in water and calculating the carbon cycle mass balance. TOC analysis is divided into the non-purgeable organic carbon (NPOC) and differential (known as TC-TIC) methods; although the selection of method is greatly affected by the sample matrix characteristics of SS, no studies have investigated this. This study quantitatively evaluates the effect of SS containing inorganic carbon (IC) and purgeable organic carbon (PuOC), as well as that of sample pretreatment, on the accuracy and precision of TOC measurement in both methods for various environmental water sample types (12 wastewater influents and effluents and 12 types of stream water). For influent and stream water with high SS, the TC-TIC method expressed 110–200 % higher TOC recovery than that for the NPOC method due to POC component losses in SS owing to its conversion into PuOC during sample pretreatment (using ultrasonic) and subsequent loss in the NPOC purging process. Correlation analysis confirmed that particulated organic matter (POM, mg/L) content in SS directly affected this difference (r > 0.74, p < 0.01, n = 24); for POC water samples (those containing >10 mg/L of POM) featuring purgeable dissolved organic matter, TC-TIC was appropriate in securing TOC measurement accuracy. In constrast, in effluent and stream water with low SS (i.e., < ∼5 mg/L) and high IC (> 70 %) contents, the TOC measurement ratios (TC-TIC/NPOC) of both methods were similar, between 0.96 and 1.08, suggesting that NPOC is appropriate for improving precision. Our results provide useful basic data to establish the most reliable TOC analysis method considering SS contents and its properties along with the matrix characteristics of the sample.

The Birkhead and Murta formations—organic geochemistry and organic petrography of Mesozoic fluvio-lacustrine source rocks in the Eromanga Basin, central Australia

Due to a historic focus on the underlying Cooper Basin and a growing demand for energy in Australia, a reassessment of the Eromanga Basin in central and eastern Australia is necessary, including a more detailed characterization of its Jurassic-Cretaceous petroleum system elements. The Jurassic Birkhead Formation and the Cretaceous Murta Formation are considered the most important source rocks of the Eromanga Basin. To study the petroleum generation potential of these two formations, a set of 55 rock cores from eight petroleum exploration wells was analysed. The sample set was subject to multiple-organic geochemical and petrographical analyses to evaluate organic richness, hydrocarbon potential (Rock–Eval S1 and S2), maceral composition, thermal maturity (e.g. vitrinite reflectance), and saturated and aromatic biomarker composition. The investigated fluvio-lacustrine siltstones and carbonaceous shales exhibit low to excellent total organic carbon (TOC) content, very low sulphur and low inorganic carbon content. Thermal maturity of both formations is at the onset of the oil window (immature to early mature). The Cretaceous Murta Formation shows good hydrocarbon generation potential and, in part, high production indices, while the Jurassic Birkhead Formation generally shows low kerogen to bitumen conversion and a good to very good hydrocarbon generation potential. Dispersed, rather coarse organic matter of terrigenous origin in fine-grained siliciclastic matrix with well-preserved plant remnants including “cutinite flames” is typical. Liptinite (mainly lamalginite) content is generally higher in the Murta Formation (including the occurrence of Botryococcus green algae), while vitrinite is clearly dominant in the Birkhead Formation. The Birkhead solvent extracts exhibit a more heterogeneous distribution of n-alkanes with distinct maxima in the long-chain range, whereas shorter-chain n-alkanes dominate the Murta extracts. Based on their quality and quantity of incorporated organic matter as well as thermal maturity, the petroleum generation potential of the Murta and Birkhead formations is discussed in detail.

Mitochondrial phosphoenolpyruvate carboxylase contributes to carbon fixation in the diatom Phaeodactylum tricornutum at low inorganic carbon concentrations.

Photosynthetic carbon fixation is often limited by CO2 availability, which led to the evolution of CO2 concentrating mechanisms (CCMs). Some diatoms possess CCMs that employ biochemical fixation of bicarbonate, similar to C4 plants, but whether biochemical CCMs are commonly found in diatoms is a subject of debate. In the diatom Phaeodactylum tricornutum, phosphoenolpyruvate carboxylase (PEPC) is present in two isoforms, PEPC1 in the plastids and PEPC2 in the mitochondria. We used real-time quantitative polymerase chain reaction, Western blots, and enzymatic assays to examine PEPC expression and PEPC activity, under low and high concentrations of dissolved inorganic carbon (DIC). We generated and analyzed individual knockout cell lines of PEPC1 and PEPC2, as well as a PEPC1/2 double-knockout strain. While we could not detect an altered phenotype in the PEPC1 knockout strains at ambient, low or high DIC concentrations, PEPC2 and the double-knockout strains grown under ambient air or lower DIC availability conditions showed reduced growth and photosynthetic affinity for DIC while behaving similarly to wild-type (WT) cells at high DIC concentrations. These mutants furthermore exhibited significantly lower 13 C/12 C ratios compared to the WT. Our data imply that in P. tricornutum at least parts of the CCM rely on biochemical bicarbonate fixation catalyzed by the mitochondrial PEPC2.

Intelligent image-activated sorting of Chlamydomonas reinhardtii by mitochondrial localization.

Organelle positioning in cells is associated with various metabolic functions and signaling in unicellular organisms. Specifically, the microalga Chlamydomonas reinhardtii repositions its mitochondria, depending on the levels of inorganic carbon. Mitochondria are typically randomly distributed in the Chlamydomonas cytoplasm, but relocate toward the cell periphery at low inorganic carbon levels. This mitochondrial relocation is linked with the carbon-concentrating mechanism, but its significance is not yet thoroughly understood. A genotypic understanding of this relocation would require a high-throughput method to isolate rare mutant cells not exhibiting this relocation. However, this task is technically challenging due to the complex intracellular morphological difference between mutant and wild-type cells, rendering conventional non-image-based high-event-rate methods unsuitable. Here, we report our demonstration of intelligent image-activated cell sorting by mitochondrial localization. Specifically, we applied an intelligent image-activated cell sorting system to sort for C. reinhardtii cells displaying no mitochondrial relocation. We trained a convolutional neural network (CNN) to distinguish the cell types based on the complex morphology of their mitochondria. The CNN was employed to perform image-activated sorting for the mutant cell type at 180 events per second, which is 1-2 orders of magnitude faster than automated microscopy with robotic pipetting, resulting in an enhancement of the concentration from 5% to 56.5% corresponding to an enrichment factor of 11.3. These results show the potential of image-activated cell sorting for connecting genotype-phenotype relations for rare-cell populations, which require a high throughput and could lead to a better understanding of metabolic functions in cells.

Process optimization of the polyhydroxybutyrate production in the cyanobacteria Synechocystis sp. and Synechococcus sp.

The effect of four parameters (acetate, NaCl, inorganic carbon and days in darkness) affecting the polyhydroxybutyrate (PHB) production were tested and optimized for Synechococcus sp. and Synechocystis sp. using a Box-Behnken design. The optimal conditions for Synechocystis sp. were found to be 1.2 g L-1 of acetate, 4 gC L-1 of NaHCO3, 18 g L-1 of NaCl and 0 days in darkness. For Synechococcus sp., equal acetate concentration and days in darkness, and lower inorganic carbon and NaCl concentrations than those for Synechocystis sp. were needed (0.05 g L-1 inorganic carbon and 9 g L-1 NaCl). Optimal conditions were scaled up to 3 L photobioreactors. Using Synechocystis sp., 5.6 %dcw of PHB was obtained whether adding or not acetate. In opposition, a maximum of 26.1 %dcw by using acetate was reached with Synechococcus sp. These results provide an easy method to optimize the cultivation conditions to enhance PHB production with cyanobacteria.

Foraminifera-derived carbon contribution to sedimentary inorganic carbon pool: A case study from three Norwegian fjords.

Norwegian fjords have been recently recognized as hot spots for carbon burial due to the large amounts of terrestrial organic matter delivered to fjord sediments, as well as the high sediment accumulation rates. Here, we present the first data on the contribution of benthic foraminiferal inorganic carbon to the sediments of three Norwegian fjords. Our study shows that calcareous foraminifera, which are among the most abundant calcifying organisms in the modern global oceans, can constitute between 15% and 33% of inorganic carbon accumulated in the sediments of the two studied southern Norwegian fjords (Raunefjorden and Hjeltefjorden). In a northern Norwegian fjord (Balsfjorden), the contribution of calcareous foraminifera to the inorganic carbon pool is smaller (<1%) than the one observed in southern fjords. We also found that the amount of foraminifera-derived carbon is primarily dependent on the species composition of the foraminifera community. Large calcareous foraminifera species, despite a lower number of individuals, constitute, on average, 13%-29% of the inorganic carbon in the two southern Norwegian fjords, while the contribution of small, highly abundant species does not exceed 4% of the inorganic carbon pools in the sediments. Calcareous foraminifera species that are indicative of dysoxic conditions have been found to have low inorganic carbon contents per specimen compared to other analysed similar-sized calcareous foraminifera species. This relationship most likely exists due to the thin test walls of these foraminifera species, which may facilitate gas exchange. The results of our case study suggest that the climate-driven formation of near-bottom low-oxygen zones may lead to the dominance of foraminifera associated with dysoxic conditions and, in consequence, to the decrease of foraminifera-derived inorganic carbon. However, to properly analyse the contribution of carbon from thin-walled foraminifera to the sedimentary carbon pool, further studies analysing a broader range of these species is needed.

Condensation of Rubisco into a proto-pyrenoid in higher plant chloroplasts

Photosynthetic CO2 fixation in plants is limited by the inefficiency of the CO2-assimilating enzyme Rubisco. In most eukaryotic algae, Rubisco aggregates within a microcompartment known as the pyrenoid, in association with a CO2-concentrating mechanism that improves photosynthetic operating efficiency under conditions of low inorganic carbon. Recent work has shown that the pyrenoid matrix is a phase-separated, liquid-like condensate. In the alga Chlamydomonas reinhardtii, condensation is mediated by two components: Rubisco and the linker protein EPYC1 (Essential Pyrenoid Component 1). Here, we show that expression of mature EPYC1 and a plant-algal hybrid Rubisco leads to spontaneous condensation of Rubisco into a single phase-separated compartment in Arabidopsis chloroplasts, with liquid-like properties similar to a pyrenoid matrix. This work represents a significant initial step towards enhancing photosynthesis in higher plants by introducing an algal CO2-concentrating mechanism, which is predicted to significantly increase the efficiency of photosynthetic CO2 uptake.

Carbon and nutrients transfer from primary producers to lake sediments – A stoichiometric approach

We aimed to demonstrate different input of organic and inorganic carbon, nitrogen and phosphorus from three main groups of primary producers (phytoplankton, charophytes and vascular submerged macrophytes) to respective lake sediments. Studies were carried out in one eutrophic and two mesotrophic lakes. Samples of sediments were taken from profundal and from littoral zones, the latter divided into such overgrown by charophytes and others covered by vascular submerged macrophytes. We applied a stoichiometric approach to illustrate variable functional carbon to nutrients relationships. Among profundal sediments, the lowest organic to inorganic carbon ratio was found in sediments from the eutrophic lake due to precipitation of calcium carbonate during algal blooms. Extremely low inorganic carbon input to profundal sediment of one of the mesotrophic lakes may be explained by low phytoplankton production but also by dissolution of once deposited calcium carbonates. Charophyte-dominated littoral sediments contained significantly more inorganic carbon than other littoral and profundal sediments. Comparison of stoichiometric ratios between plant standing crop and underlying littoral sediments showed significant enrichment of sediments in nitrogen manifested by reduction of organic carbon to total nitrogen ratio during plant decomposition taking place both in charophyte and in vascular plant stands. We also attempted to divide phosphorus pool in sediments into organic P and calcium-bound P present in charophyte stands and in profundal sediments of eutrophic lake. In the former, calcium-bound P was estimated at 17–19 % of the total P pool while in profundal sediments it amounted 42 % of the total P. This difference suggests that calcium carbonate settling during algal blooms in a eutrophic lake may be more effective in P trapping than calcite encrustations covering charophyte plants in littoral sites. In conclusions, we underline the need of considering often neglected inorganic fractions of carbon and phosphorus to get better insight into carbon and nutrient burial in lake sediments.

Identification and Characterization of a Transient Receptor Potential Ion Channel (TRP2) Involved in Acclimation to Low CO2 Conditions in Chlamydomonas reinhardtii

The CO2 concentrating mechanism (CCM) is a key feature of green algal cells, induced in response to limiting CO2 conditions. Calcium-dependent signaling has been shown to play a role in this acclimation process to low inorganic carbon in Chlamydomonas reinhardtii, but the molecular players have not been characterized. One type of Ca2+ channel, the transient receptor potential (TRP) family of ion channels is specific to the algal members of the green lineage and not found in land plants. TRP channels generally mediate the flux of Ca2+ ions in response to environmental perturbations, and one recent study has revealed the role of Ca2+ signaling in acclimation to limiting CO2 in the green alga, C. reinhardtii. In this study, the gene Trp2, encoding a Ca2+ ion channel was identified through bioinformatics analyses as having a role in acclimation of algal cells to limiting CO2. Transcript abundance levels for this gene are significantly upregulated when cells are cultured in limiting CO2 and mutant cells missing the TRP2 protein show an impaired growth phenotype. We show that a calcium binding protein CAS, is downregulated in the trp2 mutant along with other CCM genes under the control of CAS. The results suggest that the TRP2 ion channel is involved in the acclimation of C. reinhardtii cells to limiting CO2.

Flavodiiron proteins 1-to-4 function in versatile combinations in O2 photoreduction in cyanobacteria.

Flavodiiron proteins (FDPs) constitute a group of modular enzymes widespread in Bacteria, Archaea and Eukarya. Synechocystis sp. PCC 6803 has four FDPs (Flv1-4), which are essential for the photoprotection of photosynthesis. A direct comparison of light-induced O2 reduction (Mehler-like reaction) under high (3% CO2, HC) and low (air level CO2, LC) inorganic carbon conditions demonstrated that the Flv1/Flv3 heterodimer is solely responsible for an efficient steady-state O2 photoreduction under HC, with flv2 and flv4 expression strongly down-regulated. Conversely, under LC conditions, Flv1/Flv3 acts only as a transient electron sink, due to the competing withdrawal of electrons by the highly induced NDH-1 complex. Further, in vivo evidence is provided indicating that Flv2/Flv4 contributes to the Mehler-like reaction when naturally expressed under LC conditions, or, when artificially overexpressed under HC. The O2 photoreduction driven by Flv2/Flv4 occurs down-stream of PSI in a coordinated manner with Flv1/Flv3 and supports slow and steady-state O2 photoreduction.

N2O emission in partial nitritation-anammox process

Nitrous oxide (N2O) is one of the significant greenhouse gases, and partial nitritation-anammox (PNA) process emits higher N2O than traditional nitrogen removal processes. N2O production in PNA mainly occurs in three different pathways, i.e., the ammonia oxidizing bacteria (AOB) denitrification, the hydroxylamine (NH2OH) oxidation and heterotrophic denitrifiers denitrification. N2O emission data vary significantly because of the different operational conditions, bioreactor configurations, monitoring systems and quantitative methods. Under the common operational parameter scopes of PNA, N2O emission via NH2OH oxidation dominates at relatively low dissolved oxygen (DO), low inorganic carbon (IC), high pH or low NO2− concentration, while N2O emission via AOB denitrification dominates at relative higher DO, higher IC, lower pH or higher NO2− concentration. AOB are highly enriched while nitrite-oxidizing bacteria (NOB) are rarely found in partial nitritation process, and the order Nitrosomonadales of AOB is the dominant group and N2O producer. Anammox bacteria, AOB and certain amount of heterotrophic denitrifying bacteria are observed in the anammox process, the genus Denitratisoma and the heterotrophic denitrifying bacteria in the deep layer of anammox granules are the dominant N2O generation bacteria. In one-stage PNA reactors, anammox bacteria account for a large fraction of the biomass, AOB account for small portion, and NOB account for even less. The microbial community, diversity and N2O producers in one-stage PNA reactors are similar with those in two-stage PNA reactors. The dominant anammox bacteria, AOB and NOB in PNA are the species Candidatus Brocadia, the genera of Nitrotoga, Nitrospira and Nitrobacter, and the genus Nitrosomonas, respectively. The relations between N2O emission pathways and microbial communities need further study in the future.

Nitrogen removal performance of marine anammox bacteria treating nitrogen-rich saline wastewater under different inorganic carbon doses: High inorganic carbon tolerance and carbonate crystal formation

With different inorganic carbon (IC) doses, nitrogen removal performance of marine anammox bacteria (MAB) treating nitrogen-rich saline wastewater was investigated in a sequencing batch reactor. Ammonium removal efficiency (ARE) was above 99% at 108–3600 mg/L IC, which indicated MAB had a good tolerance to high IC dose. When IC was 108–1200 mg/L, ARE reached 90% within 2.5 h. MAB activity was greatly promoted by providing adequate IC. Besides, the maximal substrate conversion rate (3.4 kg/(m3 d)) was achieved at 180 mg/L IC. Both the modified Logistic and Boltzmann models were appropriate to describe nitrogen removal at low IC doses, while the modified Gompertz model was more accurate at high IC doses. Calcium carbonate crystal was formed on the surface of MAB granule at high IC doses, which resulted in a significant deterioration of nitrogen removal performance.

Carbon Isotopes as Tracers of Organic and Inorganic Carbon in Baltic Sea Sediments

AbstractDistributions of Δ14CTOC studied in bottom sediments collected during 2011–2016 in the Curonian Lagoon and in the open Baltic Sea indicated wide variations of Δ14CTOC values. Laboratory experiments on differential carbon utilization by Pseudomonas putida isolated from bottom sediments were carried out for better understanding of impacts of different sources on Δ14CTOC variations. Preferential glucose uptake (up to 80%) as a carbon source and a rather low (2–10%) inorganic carbon incorporation was found in media with diesel fuel. Pseudomonas putida a specific biomarker analyzed in biomass cultivated on the media with different carbon sources has been used to characterize microbial communities responsible for degradation of organic substances in bottom sediments. Large 14C depletions observed in sediments collected in the Gotland Deep of the Baltic Sea may indicate leakage from dumped chemical weapons.

Unusual speleothem formation in the Thabazimbi mine cave, Limpopo Province, South Africa, and its chronology

An unusual cave is located in the hills southeast of the town of Thabazimbi in the Limpopo Province. This cave, a V-shaped shallow chamber measuring 110 × 40 m, was discovered serendipitously when an adit was driven into the hill in connection with iron ore mining. It has no natural entry point and no externally derived sediment is found in it. It is situated within the upper part of the Malmani Subgroup and although it was formed as a solution cavity, requiring a phreatic environment, dripwater flow must subsequently have been low: no large speleothems (stalactites, stalagmites or flowstones) were ever formed in the cave. Instead, evaporation produced crystallictitic frostwork bushes consisting of aragonite (metastably grown due to enhanced Mg-content) which were in part redissolved followed by the secondary formation of calcite corals and encrustations. This sequence of events can be understood in the context of an enclosed cave where the CO2 content of the air is high. U/Th disequilibrium dating of two aragonite frostwork samples from the cave yielded two very different ages (155.1 ± 7.4 and 67.2 ± 3.4 ka), thus showing that the formation of this type of frostwork occurred (probably episodically) over a long period of time. The two dates broadly coincide with the waning stages of periods of low total inorganic carbon (TIC) as well as high clastic input in the Tshwaing crater sediment record, both indicating enhanced rainfall at that locality. The younger age coincides with a brief negative excursion in chlorine content and a positive potassium anomaly at Tshwaing, both also indicating high rainfall. However, both ages are also coeval with glacial maxima, thus underlining earlier observations from the Tshwaing record that in southern Africa, wet periods are not necessarily coeval with northern hemisphere interglacials.

Sustainable Nutrient Management in Aerobic Finger Millet (Eleusine coracana (L.) Gaertn) under Rainfed Agriculture

The poor resource growers of southern Chhattisgarh have deprived of modern technologies of cultivation including externally managed inputs to augment the finger millet production. The low, negligible or no profit associated rainfed (aerobic) agriculture is still the only option owing to their socio-economic condition and physiographic location. The field experiment was conducted during the rainy (Kharif) seasons of 2011 and 2012 at Bastar district of Chhattisgarh with local finger millet (Eleusine coracana L. Gaertn) locally known as ‘Mandiya’ for assessing the sustainability of finger millet productivity. The soil was sandy loam, low in organic carbon (0.43%), and available N (178 kg/ha), P (21.4 kg/ha), medium in K (179 kg/ha) available with almost neutral (pH 6.8) in reaction. Application of fertilizer and manure significantly increased plant height and flag leaf length; N:P:K @ 40:30:15 kg/ha + 7 t/ha or 50:40:20 N:P:K kg/ha + Rice husk ash 4t/ha gave significantly taller plants, flag leaf length, tillers/hill, filled grains/finger and finger length than other fertilizer treatments along with Rice husk ash (RHA). Application of NPK above the level of 30:20:10 kg NPK/ha + FYM or RHA did not significantly increase finger millet grain yield. Higher net returns were produced when lower levels of NPK was applied along with RHA.

Climatic implications of hydrologic changes in two lake catchments on the central Tibetan Plateau since the last glacial

The numerous and widespread lakes of the Tibetan Plateau (TP) constitute the largest group of alpine lakes on Earth. Some of the lakes are fed mainly by glacier meltwater and others by precipitation and groundwater. Past changes in the environments of these lakes differed because of differences in lake hydrological regimes and the complex pattern of climate change on the TP. Here we present records of scanning XRF, inorganic carbon (IC) concentration n-alkanoic acid average chain length (ACL) and percent aquatic inputs (Paq) in sediment cores from two non-glaciated lakes on the central TP (Dagze Co and Jiang Co), which span the past 19,000 years. We used these measures to investigate past changes in catchment hydrology, climate and environment. Variations in the concentration of Ti and other lithogenic elements at the two sites were influenced mainly by surface runoff, which is supported by the variation of IC, Ca/(Al, Ti, Fe) (reflecting authigenic carbonate precipitation), Rb/Sr (a chemical weathering proxy), and ACL and Paq. We attribute variations in surface runoff to changes in the precipitation/evaporation ratio, caused by the pattern of climate change on the central TP since the late Pleistocene. During the late Pleistocene, stronger runoff (indicated by higher Ti, higher Rb/Sr and Paq, lower IC, Ca/(Al, Ti, Fe) and ACL) likely resulted from lower temperatures. Lower runoff during the Holocene may reflect intensified evaporation caused by higher temperatures. Comparison with records from glaciated lakes in the region reveals opposite trends in catchment hydrology. Overall, our results suggest that since the late Pleistocene the central TP was influenced mainly by the Indian Summer Monsoon.

Impacts of genetically engineered alterations in carbon sink pathways on photosynthetic performance

Genetic engineering of photosynthetic organisms typically redirects native metabolism towards desirable products, which thereby represent new metabolic sinks. There is limited information on how these modifications impact the evolved mechanisms of photosynthetic energy metabolism and cellular growth. Two engineered strains of Synechocystis sp. PCC 6803 with altered carbon sink capacity were assayed for their photosynthetic and CO2 concentrating mechanism properties in conditions of high and low inorganic carbon (Ci) availability. In the ΔglgC mutant, glycogen cannot be synthesized and a carbon sink pathway has been effectively removed. The JU547 strain has been engineered by integration of the Pseudomonas syringae ethylene forming enzyme and provides a new sink. When cultured under high carbon conditions, ΔglgC displayed diminished photochemical efficiency, a more reduced NADPH pool, delayed initiation of the Calvin-Benson-Bassham cycle, and impairment of linear and cyclic electron flows. It also exhibited a large decrease in photochemical quenching indicative of the accumulation of QA−, normally associated with a reduced PQ pool, but appears instead to be the result of an undefined dissipative mechanism to spill excess energy. In the case of carbon sink integration, JU547 displayed slightly more oxidized PQ and NADPH pools and increased rates of cyclic electron flow and an enhanced demand for inorganic carbon as suggested by increase in the expression of the bicarbonate transporter, SbtA. Overall, the results highlight the importance of the native regulatory network of autotrophic metabolism in governing photosynthetic performance and provide cogent examples of both predicable and difficult to predict phenotypic consequences upon installation of new pathways in autotrophs.

The effects of severe carbon limitation on the green seaweed, Ulva conglobata (Chlorophyta)

Low inorganic carbon (Ci) concentrations in seawater are usually an important factor controlling photosynthesis and growth of seaweeds. The green seaweed, Ulva conglobata Kjellm, collected from a rock pool in a middle intertidal zone located at Nanao Island, Shantou, China, were cultured under low Ci level for several days, to examine the effect of severe carbon limitation on photosynthesis. The rather high pH compensation points obtained from the pH-drift experiments indicated that U. conglobata was capable of acquiring HCO3− from surrounding seawater as its Ci source for photosynthesis. However, thalli of U. conglobata cultured in Ci-starved seawater exhibited a decline of biomass, showing that the realistic photosynthetic carbon gain could not compensate for the respiratory carbon consumption in the thalli under severe Ci limitation during laboratory culture. Compared with ambient Ci conditions, the culture under severe Ci limitation significantly had an increased pigment content, but a lower maximum quantum yield and photosynthetic electron transport rate. Additionally, the maximum carbon-saturating photosynthesis rate and the apparent photosynthetic conductance of U. conglobata thalli increased in cultures with severe Ci limitation compared with ambient Ci in low N-grown thalli. The results suggest that under severe Ci limitation, U. conglobata thalli increased capacities of both light absorption processes and carbon fixation pathways.

Genetic diversity of inorganic carbon uptake systems causes variation in CO2 response of the cyanobacterium Microcystis.

Rising CO2 levels may act as an important selective factor on the CO2-concentrating mechanism (CCM) of cyanobacteria. We investigated genetic diversity in the CCM of Microcystis aeruginosa, a species producing harmful cyanobacterial blooms in many lakes worldwide. All 20 investigated Microcystis strains contained complete genes for two CO2 uptake systems, the ATP-dependent bicarbonate uptake system BCT1 and several carbonic anhydrases (CAs). However, 12 strains lacked either the high-flux bicarbonate transporter BicA or the high-affinity bicarbonate transporter SbtA. Both genes, bicA and sbtA, were located on the same operon, and the expression of this operon is most likely regulated by an additional LysR-type transcriptional regulator (CcmR2). Strains with only a small bicA fragment clustered together in the phylogenetic tree of sbtAB, and the bicA fragments were similar in strains isolated from different continents. This indicates that a common ancestor may first have lost most of its bicA gene and subsequently spread over the world. Growth experiments showed that strains with sbtA performed better at low inorganic carbon (Ci) conditions, whereas strains with bicA performed better at high Ci conditions. This offers an alternative explanation of previous competition experiments, as our results reveal that the competition at low CO2 levels was won by a specialist with only sbtA, whereas a generalist with both bicA and sbtA won at high CO2 levels. Hence, genetic and phenotypic variation in Ci uptake systems provide Microcystis with the potential for microevolutionary adaptation to changing CO2 conditions, with a selective advantage for bicA-containing strains in a high-CO2 world.