Biophotovoltaic (BPV) devices hold promise as a decentralized and environmentally friendly alternative for generating electricity, harnessing the photosynthetic capabilities of algae. We have identified a new green algal strain, Parachlorella kessleri MACC-38, that produces approximately ten times higher electricity output than already characterized Chlamydomonas and Chlorella strains. This breakthrough marks a significant advancement in BPV technology.
The remarkable electricity generation of MACC-38 stems primarily from its photosynthetic activity, ensuring sustainability. Additionally, the strain maintains its physiological integrity throughout the process, further enhancing its potential as a viable BPV technology component.
Chlamydomonas produces H2 by its hydrogenase, which is O2-sensitive and compete with CO2-fixation for electrons. We developed an anaerobic, carbon-limited protocol that keeps the Calvin-Benson cycle inactive, and the evolved O2 is scavenged by an absorbent. The PGR5-deficient mutant has excellent performance in this system. However, as the pgr5 mutant is sensitive to fluctuating light conditions, it was important to determine how it reacts to changes in light intensity during H2 production. We have developed an automated system to monitor H2 production under simulated daily light conditions and found that the pgr5 mutant outperforms the wild type strain by 100%. Photosynthetic subunits and hydrogenase activity were preserved after 5 days of H2 production in simulated daily light conditions. Hence, the pgr5 mutant is a promising candidate for H2 production under the adverse light conditions that algae may encounter in bioindustry settings.
In this study, we characterized CrPHT4-7, a chloroplast envelope Pi transporter in Chlamydomonas reinhardtii. It is similar to AtPHT4;4, an ascorbate transporter in Arabidopsis, but it does not transport ascorbate. Crpht4-7 mutants had impaired growth, ATP level, and non-photochemical quenching in high light, while overexpressing lines had enhanced biomass accumulation. We demonstrated that CrPHT4-7 transported Pi and was essential for Pi homeostasis and photosynthesis in Chlamydomonas reinhardtii. To our knowledge, this is the first detailed characterization of a phosphate transporter in green algae.
Tóth SZ (2023) Int J Mol Sci 24: 2537
Ascorbate (Asc) is a multifunctional metabolite essential for various cellular processes in plants and animals. The best-known property of Asc is to scavenge reactive oxygen species, in a highly regulated manner. In this review, the intricate relationship between Asc and photosynthesis in plants and algae is summarized in the following major points: (i) regulation of Asc biosynthesis by light, (ii) interaction between photosynthetic and mitochondrial electron transport in relation to Asc biosynthesis, (iii) Asc acting as an alternative electron donor of photosystem II, (iv) Asc inactivating the oxygen-evolving complex, (v) the role of Asc in non-photochemical quenching, and (vi) the role of Asc in ROS management in the chloroplast. The review also discusses differences in the regulation of Asc biosynthesis and the effects of Asc on photosynthesis in algae and vascular plants.
The lifetime of the oxygen-evolving complex subunit PSBO depends on light intensity and carbon availability in Chlamydomonas
Vidal-Meireles et al., (2023) Plant Cell Physiol 46: 422-439.
PSBO is essential for the assembly of the oxygen-evolving complex but information on its lifetime was lacking in green algae. Using nitrate-inducible PSBO amiRNA lines in Chlamydomonas reinhardtii, we have demonstrated that the lifetime of PSBO strongly depends on the light intensity and carbon availability, and thus, on the metabolic status of the cells. We also confirmed that PSBO is required for photosystem II stability in C. reinhardtii and demonstrate that its specific loss also entails substantial changes in cell morphology and cell cycle.
Széles et al., (2022) Cells 11: 285
To facilitate studying the relationship between morphology and photochemistry in C. reinhardtii on a single cell level, we established microfluidics in combination with chlorophyll a fluorescence induction measurements. We developed two types of microfluidic platforms for single-cell investigations: (i) The traps of the “Tulip” device are suitable for capturing and immobilizing single cells, enabling the assessment of their photosynthesis for several hours without binding to a solid support surface. (ii) The traps of the “Pot” device were designed for capturing single cells and allowing the growth of the daughter cells within the traps. Our microfluidic devices represent versatile platforms for the simultaneous morphological and photosynthetic investigations of C. reinhardtii on a single-cell level.
Nagy et al., (2021) Biores. Technol. 333: 125217
Earlier, we established a photoautotrophic H2 production system based on anaerobic induction, where the Calvin-Benson cycle is inactive and O2 scavenged by an absorbent. In this paper, we employed thin layer cultures, resulting in a three-fold increase in H2 production relative to bulk CC-124 cultures. Productivity was maintained when increasing the light intensity to 1000 µmol photons m-2s−1 and the cell density to 150 µg chlorophyll/ml. Remarkably, the L159I-N230Y photosystem II mutant and the pgrl1 photosystem I cyclic electron transport mutant produced 50% more H2 than CC-124, while the pgr5 mutant generated 250% more (1.2 ml H2/ml culture in six days). The photosynthetic apparatus of the pgr5 mutant and its in vitro HydA activity remained remarkably stable.
Podmaniczki et al., (2021) Physiol Plantarum 171: 232-245
In this study, we acquired knowledge on the role of ascorbate in dark‐induced leaf senescence using Arabidopsis thaliana as a model organism. One of the earliest effects of prolonged darkness is the inactivation of oxygen‐evolving complexes (OEC). We found that ascorbate inactivates the OEC in prolonged darkness by over‐reducing the Mn‐complex that is probably enabled by a dark‐induced dissociation of the extrinsic OEC subunits. Our study is an example that ascorbate may negatively affect certain cellular processes and thus its concentration and localization need to be highly controlled.
Ascorbate deficiency does not limit non-photochemical quenching in Chlamydomonas reinhardtii
Vidal-Meireles et al., (2020) Plant Physiol 182: 597–611
In this paper we demonstrated that Chlorophycean violaxanthin de-epoxidase found in C. reinhardtii does not require ascorbate as a reductant. The rapidly induced, energy-dependent non-photochemical quenching component was not limited by ascorbate deficiency either. On the other hand, a reactive oxygen species-induced photoinhibitory quenching component was greatly enhanced upon ascorbate deficiency. These results demonstrate that ascorbate has distinct roles in non-photochemical quenching in C. reinhardtii as compared to vascular plants.
Paradigm shift in algal H2 production: Bypassing competitive processes
Tóth SZ, Yacoby I (2019) Trends Biotechnol 37: 1159-1163
Green algae can produce hydrogen photosynthetically using their efficient but oxygen-sensitive hydrogenases. In this paper we overview recently developed hydrogen production strategies that aim at bypassing the competing Calvin-Benson cycle to provide a promising route for scaling up algal hydrogen production.
The mechanism of photosystem II inactivation during sulphur deprivation-induced H2 production in Chlamydomonas reinhardtii
Nagy et al., (2018) Plant J 94: 548-561
In the green alga Chlamydomonas reinhardtii, sulphur limitation causes a large number of physiological changes, including the inactivation of photosystem II (PSII), leading to the establishment of hypoxia, essential for the increase in hydrogenase expression and activity. In this paper we show that sulphur limitation imposes oxidative stress early on, which leads to strong ascorbate accumulation. In the mM range, ascorbate may inactivate the oxygen‐evolving complex and provide electrons to PSII, albeit at a low rate. In the absence of a functional donor side and sufficient electron transport, PSII reaction centers are inactivated and degraded.
Water-splitting-based, sustainable, and efficient H2 production in green algae as achieved by substrate limitation of the Calvin-Benson-Bassham cycle
Nagy et al., (2018) Biotechnol Biofuels 11: 69
We reported on the establishment of a novel H2 production method by green algae that is based on a short anaerobic induction, keeping the Calvin–Benson cycle inactive by substrate limitation and preserving hydrogenase activity by applying a simple catalyst to remove the evolved O2. Cultures remained photosynthetically active for several days, with the electrons feeding the hydrogenases mostly derived from water. The amount of H2 produced is remarkable and the process is photoautotrophic.
Our protocol demonstrated that it is possible to sustainably use algal cells as whole-cell catalysts for H2 production, which enables industrial application of algal biohydrogen production.
See also: Nagy V and Tóth SZ (2017) European Patent EPA 17155168.2).
Regulation of ascorbate biosynthesis in green algae has evolved to enable rapid stress-induced response via the VTC2 gene encoding GDP-L-galactose phosphorylase
Vidal-Meireles et al. (2017) New Phytol 214: 668-681
Ascorbate plays essential roles in plants; however, little was known about its biosynthesis in algae. In this study we demonstrated that in green algae ascorbate biosynthesis occurs via the Smirnoff-Wheeler pathway, just like in higher plants. In addition, we showed that: ascorbate biosynthesis is rapidly induced by reactive oxygen species; in contrast to plants, there is no circadian regulation of ascorbate biosynthesis; photosynthesis is not required per se for ascorbate biosynthesis; and Chlamydomonas VTC2 lacks negative feedback regulation by ascorbate in the physiological concentration range.