Probing the events of photoinhibition by altering electron-transport activity and light-harvesting capacity in chloroplast thylakoids

Abstract. The effect of photoinhibition on the activity of photosystem II (PSII) in spinach chloroplasts was investigated. Direct light-induced absorbance change measurements at 320 nm (Δ A ₃₂₀) provided a measure of the PSII charge separation reaction and revealed that photoinhibition prevented the stable photoreduction of the primary quinone acceptor Q_A. Sensitivity to photoinhibition was substantially enhanced by treatment of thylakoids with NH₂OH which extracts manganese from the H₂O-splitting enzyme and prevents electron donation to the reaction centre. Incubation with 3-(3,4,-dichlorophenyl)-1,1-dimethylurea (DCMU) during light exposure did not affect the extent of photoinhibitory damage. The chlorophyll (Chl) b -less chlorina (2 mutant of barley displayed a significantly smaller light-harvesting antenna size of PSII (about 20% of that in wild type chloroplasts) and, simultaneously, a lower sensitivity to photoinhibition. These observations suggest that photoinhibition depends on the amount of light absorbed by PSII and that the process of photoinhibition is accelerated when electron donation to the reaction centre is prevented. It is postulated that the probability of photoinhibition is greater when excitation energy is trapped by P680⁺, the oxidized form of the PSII reaction centre. The results are discussed in terms of the D1/D2 heterodimer which contains the functional PSII components P680, pheophytin, Q_A and Q_B.     

Effects of salinity stress on photosystem II function in cyanobacterial Spirulina platensis cells

The changes in PSII photochemistry in Spirulina platensis cells exposed to salinity stress (0–0.8 M NaCl) for 12 h were studied. Salinity stress induced a decrease in oxygen evolution activity, which correlated with the decrease in the quantum yield of PSII electron transport ( Φ _PSII). Phycocyanin content decreased significantly while chlorophyll content remained unchanged in salt-stressed cells. Salinity stress induced an increase in non-photochemical quenching (q_N) and a decrease in photochemical quenching (q_P). Analyses of the polyphasic fluorescence transients (OJIP) showed that with the increase in salt concentration, the fluorescence yield at the phases J, I and P declined sharply and the transient almost levelled off at salt concentration of 0.8 M NaCl. The effects of DCMU on the polyphasic rise of fluorescence transients decreased significantly. Salinity stress resulted in a decrease in the efficiency of electron transfer from Q_A⁻ to Q_B. The slope at the origin of the relative variable fluorescence curves (dV/dt_o) and the relative variable fluorescence at phase J (V_J) increased in the absence of DCMU, but decreased in the presence of DCMU. The shape of the relative variable fluorescence transients in salt-stressed cells was comparable to that of the control cells incubated with DCMU. The results in this study suggest that salt stress inhibited the electron transport at both donor and acceptor sides of PSII, resulted in damage to phycobilisome and shifted the distribution of excitation energy in favour of PSI.     

Effects of manganese toxicity on photosynthesis of white birch (Betula platyphylla var. japonica) seedlings

The effects of manganese (Mn) toxicity on photosynthesis in white birch ( Betula platyphylla var. japonica ) leaves were examined by the measurement of gas exchange and chlorophyll fluorescence in hydroponically cultured plants. The net photosynthetic rate at saturating light and ambient CO₂ (C_a) of 35 Pa decreased with increasing leaf Mn concentrations. The carboxylation efficiency, derived from the difference in CO₂ assimilation rate at intercellular CO₂ pressures attained at C_a of 13 Pa and O Pa, decreased with greater leaf Mn accumulation. Net photosynthetic rate at saturating light and saturating CO₂ (5%) also declined with leaf Mn accumulation while the maximum quantum yield of O₂ evolution at saturating CO₂ was not affected. The maximum efficiency of PSII photochemistry (F_v/F_m) was little affected by Mn accumulation in white birch leaves over a wide range of leaf Mn concentrations (2–17 mg g₋₁ dry weight). When measured in the steady state of photosynthesis under ambient air at 430 μmol quanta m₋₂ s₋₁, the levels of photochemical quenching (qP) and the excitation capture efficiency of open PSII (F'^v/F'^m) declined with Mn accumulation in leaves. The present results suggest that excess Mn in leaves affects the activities of the CO² reduction cycle rather than the potential efficiency of photochemistry, leading to increases in Q_A reduction state and thermal energy dissipation, and a decrease in quantum yield of PSII in the steady state.     

Photosynthesis and photoinhibition in leaves of chlorophyll b-less barley in relation to absorbed light

The response of photosynthesis to absorbed light by intact leaves of wild-type ( Hordeum vulgare L. cv. Gunilla) and chlorophyll b -less barley ( H. vulgare L. cv. Dornaria, chlorina-f2²⁸⁰⁰) was measured in a light integrating sphere. Up to the section where the light response curve bends most sharply the responses of the b -less and wild-type barley were similar but not identical. Average quantum yield and convexity for the mutant light response curves were 0.89 and 0.90, respectively, times those of the wild-type barley. The maximum quantum yield for PSII photochemistry was also 10% lower as indicated by fluorescence induction kinetics (F_v/F_m). Just above the region where the light curve bends most sharply, photosynthesis decreased with time in the mutant but not in the wild-type barley. This decrease was associated with a decrease in F_v/F_m indicating photoinhibition of PSII. This photoinhibition occurred in the same region of the light response curve where zeaxanthin formation occurs. Zeaxanthin formation occurred in both the chlorophyll b -less and wild-type leaves. However, the epoxidation state was lower in the mutant than in the wild-type barley. The results indicate that chlorophyll b -less mutants will have reduced photosynthetic production as a result of an increased sensitivity to photoinhibition and possibly a lowered quantum yield and convexity in the absence of photoinhibition.     

Photoinhibition and the xanthophyll cycle are not enhanced in the salt-acclimated halophyte Artimisia anethifolia

The effects of high salinity (up to 400 m M NaCl) on photosystem II (PSII) photochemistry, photoinhibition and the xanthophyll cycle were investigated in the halophyte Artimisia anethifolia grown under outdoor conditions. In order to examine the changes in PSII photochemistry, photoinhibition, thermal dissipation associated with the xanthophyll cycle in salt-acclimated plants, the experiments were conducted at midday on a clear day (maximal irradiance 1500 μmol m⁻¹ s⁻¹) and on a cloudy day (maximal irradiance 700 μmol m⁻¹ s⁻¹), respectively. With increasing salt concentration, the accumulation of sodium and chloride in leaves increased considerably while the relative growth rate and CO₂ assimilation rate decreased significantly. Salinity induced no effects on PSII photochemistry, thermal energy dissipation, and the contents of the xanthophyll cycle pigments either on a clear day or on a cloudy day. However, when compared with those on a cloudy day, PSII photochemistry decreased and thermal energy dissipation increased significantly in both control and salt-acclimated plants on a clear day. The levels of zeaxanthin and antheraxanthin at the expense of violaxanthin were higher on a clear day than on a cloudy day. The results suggest that photoinhibition and the xanthophyll cycle were not induced by high salinity but by high light only in A. anethifolia plants. The results also suggest that A. anethifolia showed high resistance not only to high salinity, but also to photoinhibition even when it was treated with high salinity and exposed to full sunlight.     

Photosynthetic rates and partitioning of absorbed light energy in photoinhibited leaves

Parameters for the evaluation of the effects of photoinhibition on photosynthetic carbon gain were studied in Chenopodium album leaves. The light-response curve of photosynthetic rate was determined at 36 Pa CO₂ partial pressure and fitted by a non-rectangular hyperbola. Both the initial slope of the curve and the light-saturated rate decreased in photoinhibited leaves, although the decrease in the latter was small. The convexity of the curve was also smaller in photoinhibited leaves. The capacities of ribulose-1,5-bisphosphate carboxylation ( V _cmax) and electron transport ( J _max) were estimated from the CO₂-response curves. V _cmax and J _max decreased similarly with increasing photoinhibition. Energy partitioning in photosystem II (PSII) was estimated using chlorophyll fluorescence parameters. The fraction of energy that was consumed by photochemistry decreased with increasing photoinhibition. However, an increase in inactive PSII, decreasing energy partitioning to active PSII, relaxed the excitation pressure in PSII, and led to a reduction in the fraction of excess energy that was neither consumed by photochemistry nor dissipated as heat.     

Alleviation of photoinhibition by calcium supplement in salt-treated Rumex leaves

By analysis of gas exchange and chlorophyll fluorescence, the effects of NaCl treatment and supplemental CaCl₂ on photosynthesis, photosystem II (PSII) photochemistry and photoinhibition were investigated in Rumex leaves. Photosynthesis in Rumex leaves was strongly inhibited by 200 m M NaCl treatment. Such inhibition of photosynthesis was ameliorated by CaCl₂ supplement. Neither NaCl treatment nor CaCl₂ supplement had any significant effects on the PSII primary photochemical reaction in dark-adapted leaves. In light-adapted leaves, however, 200 m M NaCl treatment significantly decreased photochemical quenching (q_p), efficiency of excitation energy capture by open PSII reaction centers (F_V'/F_M') and quantum yield of PSII electron transport (Φ_PSII). These decreases in q_p, F_V'/F_M' and Φ_PSII were mitigated by CaCl₂ supplement with the maximum of its effect appearing at a concentration of 8 m M CaCl₂. A similar mitigating effect was shown in 200 m M NaCl-treated Rumex leaves when susceptibility of PSII to photoinhibition was determined under high irradiance. It is suggested that the mitigation of photoinhibition in NaCl-treated leaves is because of the amelioration of inhibition of photosynthesis.     

Changes in photosystem II function during senescence of wheat leaves

Analyses of chlorophyll fluorescence were undertaken to investigate the alterations in photosystem II (PSII) function during senescence of wheat ( Triticum aestivum L. cv. Shannong 229) leaves. Senescence resulted in a decrease in the apparent quantum yield of photosynthesis and the maximal CO₂ assimilation capacity. Analyses of fluorescence quenching under steady‐state photosynthesis showed that senescence also resulted in a significant decrease in the efficiency of excitation energy capture by open PSII reaction centers (F'_v/F'_m) but only a slight decrease in the maximum efficiency of PSII photochemistry (F'_v/F'_m). At the same time, a significant increase in non‐photochemical quenching (q_N) and a considerable decrease in photochemical quenching (q_P) were observed in senescing leaves. Rapid fluorescence induction kinetics indicated a decrease in the rate of Q_A reduction and an increase in the proportion of Q_B‐non‐reducing PSII reaction during senescence. The decrease in both F'_v/F'_m and q_P explained the decrease in the actual quantum yield of PSII electron transport ((φ_PSII). We suggest that the modifications in PSII function, which led to the down‐regulation of photosynthetic electron transport, would be in concert with the lower demand for ATP and NADPH in the Calvin cycle which is often inhibited in senescing leaves.     

Changes in the photosynthetic reaction centre II in the diatom Phaeodactylum tricornutum result in non-photochemical fluorescence quenching

Diatoms are an important group of primary producers in the aquatic environment. They are able to acclimate to fast changes in the light intensity by various mechanisms including a rise in non-photochemical fluorescence quenching (NPQ). The latter has been attributed to the xanthophyll cycle (XC) following activation of diadinoxanthin de-epoxidase by the acidification of the thylakoid lumen. To examine whether fluorescence quenching in the diatom Phaeodactylum tricornutum depends on the ΔpH generated by the photosynthetic electron transport, we arrested the latter by 3-(3',4'-dichlorophenyl)-1,1-dimethylurea (DCMU). This treatment hardly affected the NPQ or XC, even when methylviologen was present. Dissipation of the ΔpH by 2,4-dinitrophenol inhibited the XC but did not alter NPQ. Similar results, i.e. inhibition of the XC but normal fluorescence quenching, were observed when the experiments were performed at 3°C. Measurements of thermoluminescence showed that excess light treatment caused a marked decline in the signals obtained as a result of recombination of Q_B^- with the S₃ state of the Mn cluster; this was also observed in cells treated with DCMU (recombination of Q_A^- with S₂). Light treatment also diminished the Q_A^- re-oxidation signals. The data suggest that changes in PSII core centre itself due to exposure to excess light conditions play an important part in the acclimation of P. tricornutum to the changing light conditions.     

Photosystem II inhibition by moderate light under low temperature in intact leaves of chilling-sensitive and -tolerant plants

Photosystem II (PSII) activity was examsined in leaves of chilling-sensitive cucumber ( Cucumis sativus L.), tomato ( Lycopersicum esculentum L.), and maize ( Zea mays L.), and in chilling-tolerant barley ( Hordeum vulgare L.) illuminated with moderate white light (300 µmol m⁻² s⁻¹) at 4°C using chlorophyll a fluorescence measurements. PSII activity was inhibited in leaves of all the four plants as suggested by the decline in F _v/ F _m, 1/ F _o − 1/ F _m, and F _v/ F _o values. The changes in initial fluorescence level ( F _o), F _v/ F _m, 1/ F _o − /1/ F _m, and F _v/ F _o ratios indicate a stronger PSII inhibition in cucumber, maize and tomato plants. The kinetics of chlorophyll a fluorescence rise showed complex changes in the magnitudes and rise of O-J, J-I, and I-P phases caused by photoinhibition. The selective suppression of the J-I phase of fluorescence rise kinetics provides evidence for weakened electron donation from the oxidizing side, whereas the accumulation of reduced Q_A suggests damage to the acceptor side of PSII. These findings imply that the process of chilling-induced photoinhibition involves damage to more than one site in the PSII complexes. Furthermore, comparative analyses of the decline in F _v/ F _o and photooxidation of P700 explicitly show that the extent of photoinhibitory damage to PSII and photosystem I is similar in leaves of cucumber plants grown at a low irradiance level.     

Atrazine resistance entails a limited xanthophyll cycle activity,a lower PSII efficiency and an altered pattern of excess excitation dissipation

Atrazine-resistant (AR) weeds have a modified D1 protein structure, with a Ser₂₆₄→Gly mutation on the D1 protein, near the plastoquinone binding niche. The photosynthetic performance, the light response of the xanthophyll cycle and chlorophyll fluorescence quenching-related parameters were compared in attached leaves of susceptible (S) and AR biotypes of the C₃ dicot Chenopodium album L., Epilobium adenocaulon Hausskn., Erigeron canadensis L., Senecio vulgaris L. and Solanum nigrum L. and the C₄ dicot Amaranthus retroflexus L. grown under natural high-light conditions. No significant difference in CO₂ assimilation rate per leaf area unit was found between the S and AR biotypes of the investigated C₃ plants, whereas the AR biotype of A. retroflexus exhibited a relatively poor photosynthetic performance. The D1 protein mutant plants expressed a reduced activity of light-stimulated zeaxanthin formation. Neither the lower violaxanthin de-epoxidase activity nor the depletion of ascorbate seems to be the cause of the lower in vivo zeaxanthin formation in the AR plants. All the D1 mutant weeds had limited light-induced non-photochemical (NPQ) and photochemical (q_P) quenching capacities, and displayed a higher photosensitivity, as characterized by the ratio (1-q_P)/NPQ and a higher susceptibility to photoinhibition. Analysis of the chlorophyll fluorescence parameters showed that a lower proportion of excitation energy was allocated to PSII photochemistry, while a higher excess of excitation remained in the AR weeds relative to the S plants.     

Photoinhibition of photosynthesis in intact willow leaves in response to moderate changes in light and temperature

When willow leaves were transferred from 270 to 650 μmol m^-2 s^-1 photosynthetic photon flux density (PPFD), partial photoinhibition developed over the next hours. This was manifested as roughly parallel inhibitions of the ratio of variable over maximal chlorophyll fluorescence (F_v/F_M), and of the maximal quantum yield and the capacity of photosynthesis. This occurred even though photosynthesis was operating well below its capacity and only about one fourth of the reaction centres of photosystem (PS) II were in the closed state. When the air temperature was lowered from 25 to 15°C (18°C leaf temperature) photoinhibition was markedly accelerated. This temperature effect is suggested to be mediated largely by a decrease in the rate of energy dissipation through photosynthesis and indicated by a 50% increase in the number of closed PSII reaction centres. The pool size of the carotcnoid zeaxanthin and the extent of inhibition of the F_v/F_M ratio were positively correlated during the treatment. However, the relaxation following imposition of darkness was much faster for zeaxanthin than for the F_v/F_M ratio, ruling out the possibility of a direct causal relationship. The energy distribution between PSII and PSI was unaltered upon photoinhibition. However, the functioning of the PSII reaction centres was altered, as indicated by a rise in the minimal fluorescence, Fa.     

Relative contributions of photochemical and non-photochemical routes to excitation energy dissipation in rice and barley illuminated at a chilling temperature

The mechanistic basis for differential sensitivities to chilling-induced photoinhibition among two rice ( Oryza sativa L.) cultivars (an Indica and a Japonica type) and one barley cultivar ( Hordeum vulgare L. cv. Albori) was examined. When leaf segments were exposed to moderate illumination at 4°C, a sustained decrease in the photochemical efficiency of photosystem (PS) II measured as the ratio of variable to maximal fluorescence (F_v/F_m) was observed for several hours. An analysis of fluorescence quenching revealed a sudden drop in PSII-driven electron transport rate (ETR) and a rapid rise in the reduction state of the primary electron acceptor Q_A upon exposure to chilling in moderate light. There was no appreciable difference in the level of non-photochemical quenching (NPQ) nor in the xanthophyll cycle activity between Japonica rice and barley. However, barley was capable of sustaining a higher ETR, thereby keeping a lower reduction state of Q_A throughout the chilling for 6 h. The Indica rice was characterized by the lowest ability to develop the xanthophyll cycle-associated NPQ, particularly the fast relaxing NPQ component (qf), accompanied by the highest reduction state of Q_A and photoinhibitory quenching (qI). It is concluded that the lower susceptibility of barley to chilling-induced photoinhibition than Japonica rice is attributable to its higher potential to dissipate excess light energy via a photochemical mechanism, whereas Indica rice is more sensitive to photoinhibition at a chilling temperature than Japonica rice, due primarily to its lower capacity to develop an efficient NPQ pathway.     

Mechanism of copper-enhanced photoinhibition in thylakoid membranes

The effect of copper on photoinhibition of photosystem II (PSII) in vitro was studied in bean ( Phaseolus vulgaris L. cv. Dufrix) and pumpkin ( Cucurbita pepo L.) thylakoids. The thylakoids were illuminated at 200–2 000 μmol photons m⁻² s⁻¹ in the presence of 70–1 830 added Cu²⁺ ions per PSII. Three lines of evidence show that the irreversible damage of PSII caused by illumination of thylakoids in the presence of Cu²⁺ was mainly due to donor-side photoinhibition resulting from inhibition of the PSII donor side by Cu²⁺. First, addition of an artificial electron donor partially restored PSII activity of thylakoids that had been illuminated in the presence of Cu²⁺. Second, already moderate light was enough to cause rapid inhibition of PSII, and the inhibition could be saturated by light. Third, the extrinsic polypeptides of the oxygen-evolving complex were found to become oxidized by the combined effect of Cu²⁺ and light. The presence of oxygen was not necessary for the copper-induced enhancement of photoinhibition of PSII. When the illumination was prolonged, copper caused a gradual collapse of the thylakoid structure by increasing degradation of thylakoid proteins.     

Responses of Jatropha curcas seedlings to cold stress: photosynthesis-related proteins and chlorophyll fluorescence characteristics

Photosynthesis-related proteins and PSII functions of Jatropha curcas seedlings under cold stress were studied using proteomic and chlorophyll fluorescence approaches. The results of chlorophyll fluorescence measurement indicated that electron transport flux per reaction center (ET_o/RC) and performance index (PI_ABS) were relatively sensitive to low temperature, especially at early stage of cold stress. The increase in O–J phase and decrease in J–I phase of chlorophyll fluorescence transient indicated a protection mechanism of J.   curcas to photoinhibition at early stage of cold stress. Eight photosynthesis-related proteins significantly changed during cold stress were identified using liquid chromatography MS/MS. Results of correlation analyses between photosynthesis-related proteins and chlorophyll fluorescence parameters indicated that (1) ATP synthase and Rieske FeS protein were significantly correlated with electron transport of reaction center in PSII; (2) precursor for 33-kDa protein was positively correlated with fluorescence quenching of the O–J and J–I phases and PI_ABS during cold stress, which implies that it might be related to multiple process in PSII; (3) contrary correlations were found between F_J − F_o and two enzymes in the Calvin cycle, and the relations between these proteins and PSII function were unclear. The combined study using proteomic approaches and chlorophyll fluorescence measurements indicated that the early-stage (0–12 h) acclimation of PSII and the late-stage (after 24 h) H₂O₂ scavenging might be involved in the cold response mechanisms of J.   curcas seedlings.     

Tricolorin A, a potent natural uncoupler and inhibitor of photosystem II acceptor side of spinach chloroplasts

Tricolorin A, (11 S )-11-hydroxyhexadecanoic acid 11- O - α - l - rhamnopyranosyl-(1↠3)- O - α - l -{2- O -(2 S -methylbutanoyl)-4- O -(2 S -methylbutanoyl)}-rhamnopyranosil-(1↠2)- O - β - d -glucopyranosil-(1↠2)- β -fucopyranoside-(1,3'-lactone), the major phytogrowth inhibitor isolated from Ipomoea tricolor Cav. (Convolvulaceae) was found to be a potent uncoupler (U₅₀=0.33 μ M ) of photophosphorylation in spinach chloroplasts. Tricolorin A inhibited H⁺-uptake and adenosine 5'-triphosphate (ATP) synthesis, and stimulated basal and phosphorylating electron flows. Using a combination of two well-known fluorescent ΔpH probes, 9-aminoacridine and 9-amino-6-chloro-2-methoxyacridine, the uncoupling behavior of tricolorin A was also demonstrated for submitochondrial particles. Polarographic data showed that high concentrations (20 μ M ) of tricolorin A inhibited photosystem II (PSII) electron flow at the level of plastoquinone B (Q_B). Chlorophyll (Chl) a fluorescence analysis showed that tricolorin A induced accumulation of Q_A⁻ and strongly decreased the electron transport capacity, suggesting that the target of this molecule was located at the Q_B level. The macrocyclic lactone-type structure of this allelopathic agent proved to be an important structural requirement for uncoupling activity since its hydrolysis caused loss of the inhibitory potential.     

Light quality during growth of Tradescantia albiflora regulates photosystem stoichiometry, photosynthetic function and susceptibility to photoinhibition

Tradescantia albiflora (Kunth) was grown under two different light quality regimes of comparable light quantity: in red + far-red light absorbed mainly by photosystem I (PSI light) and yellow light absorbed mainly by photosystem II (PSII light). The composition, function and ultrastructure of chloroplasts, and photoinhibition of photosynthesis in the two types of leaves were compared. In contrast to regulation by light quantity (Chow et al. 1991. Physiol. Plant. 81: 175–182), light quality exerted an effect on the composition of pigment complexes, function and structure of chloroplasts in Tradescantia: PSII light-grown leaves had higher Chl a/b ratios, higher PSI concentrations, lower PSII/PSI reaction centre ratios and less extensive thylakoid stacking than PSI light-grown leaves. Light quality triggered modulations of chloroplast components, leading to a variation of photosynthetic characteristics. A larger proportion of primary quinone acceptor (Q_A) in PSI light-grown leaves was chemically reduced at any given irradiance. It was also observed that the quantum yield of PSII photochemistry was lower in PSI light-grown leaves. PSI light-grown leaves were more sensitive to photoinihibition and recovery was slower compared to PSII light-grown leaves, showing that the PSII reaction centre in PSI light-grown leaves was more easily impaired by photoinhibition. The increase in susceptibility of leaves to photoinhibition following blockage of chloroplast-encoded protein synthesis was greater in PSII light-grown leaves, showing that these leaves normally have a greater capacity for PSII repair. Inhibition of zeaxanthin formation by dithiothreitol slightly increased sensitivity to photoinhibition in both PSI and PSII light-grown leaves.     

Effect of darkness and CO2 starvation on NH4+ and NO3− assimilation in the unicellular alga Cyanidium caldarium

Cyanidium caldarium (Tilden) Geitler, a non-vacuolate unicellular alga, resuspended in medium flushed with air enriched with 5% CO₂, assimilated NH₄⁺ at high rates both in the light and in the dark. The assimilation of NO₃⁻, by contrast, was inhibited by 63% in the dark. In cell suspensions flushed with CO₂-free air, NH₄⁺ assimilation decreased with time both in the light and in the dark and ceased almost completely after 90 min. The addition of CO₂ completely restored the capacity of the alga to assimilate NH₄⁺. NO₃⁻ assimilation, by contrast, was 33% higher in the absence of CO₂ and was linear with time. It is suggested that NO₃⁻ and NH₄⁺ metabolism in C. caldarium are differently controlled in response to the light and carbon conditions of the cell.     

Bioenergetic changes in the microalgal photosynthetic apparatus by extremely high CO2 concentrations induce an intense biomass production

Unicellular green alga Chlorella minutissima , grown under extreme carbon dioxide concentrations (0.036–100%), natural temperature and light intensities (Mediterranean conditions), strongly increase the microalgal biomass through photochemical and non-photochemical changes in the photosynthetic apparatus. Especially, CO₂ concentrations up to 10% enhance the density of active reaction centers (RC/CS_o), decrease the antenna size per active reaction center (ABS/RC), decrease the dissipation energy (DI_o/RC) and enhance the quantum yield of primary photochemistry (F_v/F_m). Higher CO₂ concentrations (20–25%) combine the above-mentioned photochemical changes with enhanced non-photochemical quenching of surplus energy, which leads to an enhanced steady-state fraction of 'open' (oxidized) PSII reaction centers (q_p), and minimize the excitation pressure of PSII (1 − q_p) under very high light intensities (approximately 1700 μmol m⁻² s⁻¹ maximal value), avoiding the photoinhibition and leading to an enormous biomass production (approximately 2500%). In conclusion, these extreme CO₂ concentrations – about 1000 times higher than the ambient one – can be easily metabolized from the unicellular green alga to biomass and can be used, on a local scale at least, for the future development of microalgal photobioreactors for the mitigation of the factory-produced carbon dioxide.     

Methane-dependent nitrate production by a microbial consortium enriched from a cultivated humisol

Abstract A consortium was enriched from a humisol incubated with 3.6 kPa CH₄ and NH₄⁺. This consortium oxidized NH₄⁺ to NO₂⁻ and NO₃⁻ (NO₃⁻/NO₂⁻ ratio about 20) with smaller amounts of N₂O. This oxidation stopped in the stationary phase after depletion of CH₄. CH₃OH or CO₂ did not support oxidation. Growth and resting cell experiments suggested that nitrification was associated with methanotrophic activity and that chemoautotrophic nitrifiers were absent.