Development of light-shielding hydrogel for nitrifying bacteria to prevent photoinhibition under strong light irradiation

Microalgae-nitrifying bacteria consortia have gained attention because photooxygenation of algae can supply oxygen to bacteria which eliminates the need for costly mechanical aeration. However, nitrifying bacteria are known to suffer from photoinhibition. In this study, we developed “Light-shielding hydrogel”, in which bacteria were immobilized in hydrogel and light-shielding particles (carbon black) were incorporated, and evaluated its effectiveness to mitigate photoinhibition for bacteria under strong light irradiation. For comparison, “Hydrogel”, in which bacteria were immobilized in hydrogel without carbon black, and “Dispersion” which was simply suspended bacteria were prepared. At 1600 μmol photons m⁻² s^–1, the nitrification performance markedly decreased to 15.1 and 48.0% compared to the dark condition in the Dispersion and the Hydrogel, respectively. Meanwhile, it was successfully maintained for the Light-shielding hydrogel. Our results showed that the effectiveness of light-shielding hydrogel to mitigate photoinhibition on nitrifying bacteria even under strong light irradiation.     

Overall assessment of Monodus subterraneus cultivation and EPA production in outdoor helical and bubble column reactors

The outdoor production of Monodus subterraneus wasstudied in bubble column and helical reactors, mainly analysing the influenceofdilution rate, air flow rate and solar irradiance on growth rate andbiochemicalcomposition. Photoinhibition and photo-oxidation phenomena were also analysed.The cultures were stressed at high solar irradiance and dissolved oxygenconcentrations. A clear relationship between stress of the cultures and thefluorescence from PSII measurements was observed, the Fv/Fm ratio being lowerinthe helical reactor than in the bubble column. Growth rate and biomassproductivity were both a function of the average irradiance and the Fv/Fmratio;maximum values of 0.040 h^–1 and 0.54 gL^–1 d^–1 were measured. The influenceofphotoinhibition and average irradiance was modelled, the model also fitting theexperimental data reported by another author. The chlorophyll contenthyperbolically decreased, whereas the carotenoid content decreased linearlywiththe average irradiance. The higher the dilution rate the higher the protein andcarbohydrate content of the biomass, and the lower the lipid content. Theeicosapentaenoic acid (EPA) content ranged from 2.3 to 3.2% d.wt, the higherthe dilution rate, the lower EPA content, although the higher the EPAproportion. Maximum EPA productivity was only 9 mg L^–1d^–1, due to the stress to which the cultures wereexposed.     

A new paradigm for the action of reactive oxygen species in the photoinhibition of photosystem II

Inhibition of the activity of photosystem II (PSII) under strong light is referred to as photoinhibition. This phenomenon is due to the imbalance between the rate of photodamage to PSII and the rate of the repair of damaged PSII. Photodamage is initiated by the direct effects of light on the oxygen-evolving complex and, thus, photodamage to PSII is unavoidable. Studies of the effects of oxidative stress on photodamage and subsequent repair have revealed that reactive oxygen species (ROS) act primarily by inhibiting the repair of photodamaged PSII and not by damaging PSII directly. Thus, strong light has two distinct effects on PSII; it damages PSII directly and it inhibits the repair of PSII via production of ROS. Investigations of the ROS-induced inhibition of repair have demonstrated that ROS suppress the synthesis de novo of proteins and, in particular, of the D1 protein, that are required for the repair of PSII. Moreover, a primary target for inhibition by ROS appears to be the elongation step of translation. Inhibition of the repair of PSII by ROS is accelerated by the deceleration of the Calvin cycle that occurs when the availability of CO₂ is limited. In this review, we present a new paradigm for the action of ROS in photoinhibition.     

Photosynthetic regulation under fluctuating light in field-grown Cerasus cerasoides: A comparison of young and mature leaves

Photosystem I (PSI) is a potential target of photoinhibition under fluctuating light. However, photosynthetic regulation under fluctuating light in field-grown plants is little known. Furthermore, it is unclear how young leaves protect PSI against fluctuating light under natural field conditions. In the present study, we examined chlorophyll fluorescence, P700 redox state and the electrochromic shift signal in the young and mature leaves of field-grown Cerasus cerasoides (Rosaceae). Within the first seconds after any increase in light intensity, young leaves showed higher proton gradient (ΔpH) across the thylakoid membranes than the mature leaves, preventing over-reduction of PSI in the young leaves. As a result, PSI was more tolerant to fluctuating light in the young leaves than in the mature leaves. Interestingly, after transition from low to high light, the activity of cyclic electron flow (CEF) in young leaves increased first to a high level and then decreased to a stable value, while this rapid stimulation of CEF was not observed in the mature leaves. Furthermore, the over-reduction of PSI significantly stimulated CEF in the young leaves but not in the mature leaves. Taken together, within the first seconds after any increase in illumination, the stimulation of CEF favors the rapid lumen acidification and optimizes the PSI redox state in the young leaves, protecting PSI against photoinhibition under fluctuating light in field-grown plants.     

Spatial and temporal variation of photosynthesis in intertidal Mazzaella laminarioides (Bory)Fredericq (Rhodophyta, Gigartinales)

The red alga Mazzaella laminarioides is an economically important species with an extended latitudinal distribution along the Chilean coast. Its populations form mid-intertidal stands, several meters wide, and therefore are differentially exposed to environmental variables that result in temporal and spatial variability in productivity. We evaluated the effect of latitude and intertidal height on productivity by in situ measurement of photosynthetic performance. Daily and seasonal variations of O₂-evolution rate and maximal quantum yield (F _v/F _m) were determined in plants from the upper and lower intertidal zone at two localities 1500,km apart. Results suggest that plant responses were mainly affected by irradiation, temperature and desiccation. At local level, upper intertidal plants showed a reduced photosynthetic rate and quantum efficiency as compared to those displayed by plants from the lower intertidal, indicating their higher level of excitation energy acclimation. Stronger acclimation differences between upper and lower intertidal plants were observed in spring and summer. Differences in photosynthetic parameters between reproductive phases were recorded in autumn and winter, regardless of the position of the individuals in the intertidal zone. The effects of tidal elevation on seasonal patterns of photosynthesis were also influenced by latitude. Seasonal variation in photosynthetic efficiency was observed in plants from the northern population at both intertidal elevations, but only at the upper intertidal level in the southern population. This study shows that production variability in M. laminarioides results from differences in the intensity of environmental factors observed seasonally at local (intertidal) and latitudinal scales.     

Photosynthetic responses to overnight frost in Eucalyptus nitens and E. globulus

Significant expansion in the area of eucalypt plantations in Tasmania has led to their establishment at altitudes that are close to the upper limits of the planting distributions of Eucalyptus nitens and E. globulus, the main species planted. This has implications for plantation productivity. We investigated the processes that limit productivity in these environments through a study of freezing-induced depression of photosynthesis of E. nitens saplings in the field and plantlets of E. nitens and E. globulus clones in a controlled environment cabinet. In the field consecutive frosts of around –4.6°C had a cumulative effect, reducing maximum net photosynthesis ( A _max) by 17%, and then a further 9%, respectively, compared with saplings insulated from the frosts. Shading saplings pre-dawn had no effect on A _max measured after 1030 hours indicating that the reduction in A _max at this time was independent of photoinhibition. Recovery of A _max to pre-frost levels required at least two consecutive frost-free nights and was dependent on the severity of frost. Photosynthetic light response curves indicated that reduced A _max was associated also with decreased quantum yield and stomatal conductance. Similar intracellular carbon dioxide concentration between exposed and insulated saplings indicated that low stomatal conductance did not limit photosynthesis through carbon dioxide limitation. The timing of frost events was critical: E. nitens saplings took less time to recover from reduced A _max in the field when they were hardened. Unhardened plantlets of E. nitens and E. globulus clones had greater reduction of A _max and took longer to recover from frost events than hardened plantlets. E. globulus was more susceptible to frost-induced reduction of A _max than E. nitens. This is consistent with its planting range which is restricted to mild sites compared with that of E. nitens.     

Formation of radicals from singlet oxygen produced during photoinhibition of isolated light-harvesting proteins of photosystem II

Electron spin resonance spectroscopy and liquid chromatography have been used to detect radical formation and fragmentation of polypeptides during photoinhibition of purified major antenna proteins, free of protease contaminants. In the absence of oxygen and light, no radicals were observed and there was no damage to the proteins. Similarly illumination of the apoproteins did not induce any polypeptide fragmentation, suggesting that chlorophyll, light and atmospheric oxygen are all participating in antenna degradation. The use of TEMP and DMPO as spin traps showed that protein damage initiates with generation of ¹O₂, presumably from a triplet chlorophyll, acting as a Type II photosensitizer which attacks directly the amino acids causing a complete degradation of protein into small fragments, without the contribution of proteases. Through the use of scavengers, it was shown that superoxide and H₂O₂ were not involved initially in the reaction mechanism. A higher production of radicals was observed in trimers than in monomeric antenna, while radical production is strongly reduced when antennae were organized in the photosystem II (PSII) complex. Thus, monomerization of antennae as well as their incorporation into the PSII complex seem to represent physiologically protected forms. A comparison is made of the photoinhibition mechanisms of different photosynthetic systems.     

Core protein phosphorylation facilitates the repair of photodamaged photosystem II at high light

Phosphorylation of photosystem II (PSII) reaction center protein D1 has been hypothesised to function as a signal for the migration of photodamaged PSII core complex from grana membranes to stroma lamellae for concerted degradation and replacement of the photodamaged D1 protein. Here, by using the mutants with impaired capacity (stn8) or complete lack (stn7 stn8) in phosphorylation of PSII core proteins, the role of phosphorylation in PSII photodamage and repair was investigated. We show that the lack of PSII core protein phosphorylation disturbs the disassembly of PSII supercomplexes at high light, which is a prerequisite for efficient migration of damaged PSII complexes from grana to stroma lamellae for repair. This results in accumulation of photodamaged PSII complexes, which in turn results, upon prolonged exposure to high light (HL), in general oxidative damage of photosynthetic proteins in the thylakoid membrane.     

Phenotypic plasticity in response to light in the coffee tree

Phenotypic plasticity to light availability was examined at the leaf level in field-grown coffee trees (Coffea arabica). This species has been traditionally considered as shade-demanding, although it performs well without shade and even out-yields shaded coffee. Specifically, we focused our attention on the morpho-anatomical plasticity, the balance between light capture and excess light energy dissipation, as well as on physiological traits associated with carbon gain. A wide natural light gradient, i.e., a diurnal intercepted photon irradiance differing by a factor of 25 between the deepest shade leaves and the more exposed leaves in the canopy, was explored. Responses of most traits to light were non-linear, revealing the classic leaf sun vs. leaf shade dichotomy (e.g., compared with sun leaves, shade leaves had a lower stomatal density, a thinner palisade mesophyll, a higher specific leaf area, an improved light capture, a lower respiration rate, a lower light compensating point and a limited capacity for photoprotection). The light-saturated rates of net photosynthesis were higher in sunlit than in shade leaves, although sun leaves were not efficient enough to use the extra light supply. However, sun leaves showed well-developed photoprotection mechanisms in comparison to shade leaves, which proved sufficient for avoiding photoinhibition. Specifically, a higher non-photochemical quenching coefficient was found in parallel to increases in: (i) zeaxanthin pools, (ii) de-epoxidation state of the xanthophyll cycle, and (iii) activities of some antioxidant enzymes. Intracanopy plasticity depended on the suite of traits considered, and was high for some physiological traits associated with photoprotection and maintenance of a positive carbon balance under low light, but low for most morpho-anatomical features. Our data largely explain the successful cultivation of the coffee tree in both exposed and shade environments, although with a poor resource-use efficiency in high light.     

Involvement of digalactosyldiacylglycerol in cellular thermotolerance in <Emphasis Type="Italic">Synechocystis</Emphasis> sp. PCC 6803

The effects of digalactosyldiacylglycerol (DGDG) deficiency on photosynthesis at high temperatures were examined using a dgdA mutant of Synechocystis sp. PCC 6803 incapable of DGDG biosynthesis. The dgdA mutant cells showed significant growth retardation when the temperature was increased from 30 to 38°C, although wild-type cells grew normally. The degree of growth retardation was enhanced by increasing light intensity. In addition, dgdA mutant cells showed increased sensitivity to the photoinhibition of photosynthesis when illuminated at 38°C. Analysis of photosynthesis in intact cells suggested that the inhibition of repair processes and accelerated photodamage resulted in growth retardation in dgdA mutant cells at high temperatures.