Photosynthetic Unit Research Articles

In-situ construction of biomineralized cadmium sulfide-Rhodopseudomonas palustris hybrid system: Mechanism of synergistic light utilization

Sulfide biomineralization is a microorganism-induced process for transforming the environmentally hazardous cadmium into useful resource utilization. This study successfully constructed cadmium sulfide nanoparticles-Rhodopseudomonas palustris (Bio-CdS NPs-R. palustris) hybrids. For the self-assembling hybrids, Bio-CdS NPs were treated as new artificial-antennas to enhance photosynthesis, especially under low light (LL). Bacterial physiological results of hybrids were significantly increased, particularly for cells under LL, with higher enhancement photon harvesting ability. The enhancement included the pigment contents, and the ratio of the peripheral light-harvesting complex Ⅱ (LH2) to light-harvesting Ⅰ (1.33 ± 0.01 under LL), leading to the improvements of light-harvesting, transfer, and antenna conversion efficiencies. Finally, the stimulated electron chain of hybrids improved bacterial metabolism with increased nicotinamide adenine dinucleotide (NADH, 174.5% under LL) and adenosine triphosphate (ATP, 41.1% under LL). Furthermore, the modified photosynthetic units were induced by the up-regulated expression of fixK, which was activated by reduced oxygen tension of the medium for hybrids. fixK up-regulated genes encoding pigments (crt, and bch) and complexes (puf, pucAB, and pucC), leading to improved light-harvesting and transfer, and transform ability. This study provides a comprehensive understanding of the solar energy utilization mechanism of in-situ semiconductor-phototrophic microbe hybrids, contributing to further theoretical insight into their practical application.

Effects of electrical stimulation on carbon sequestration of photosynthetic bacteria: Response of the photosynthetic unit to electrical stimulation

Bioelectrochemical systems have been used to enhance the carbon sequestration of photosynthetic bacteria (PSB), but it is unclear whether the electrical stimulation affects the structure of membrane proteins involved in photochemical activity. In this study, the rate of CO2 fixation in the photosynthetic bacteria R. palustris increased with electrical stimulation, and the enhancement partly remained even after the power off. The CO2 fixation rate in electrical-stimulated PSB was 10% higher than in non-stimulated PSB, and the activity of the CO2 fixation enzyme Rubisco was 16% higher. This resulted from the improved light utilization efficiency of the photosynthetic units in R. palustris after electrical stimulation, particularly due to the enhancement of the primary reaction mediated by the light capture protein RC-LH1. The molar extinction coefficient of the light capture protein RC-LH1 increased from 3301 cm−1 mM−1 to 3733 cm−1 mM−1, and the maximum rate of the primary reaction increased from 6.88 × 10−3 e−RC−1s−1 to 8.66 × 10−3 e−RC−1s−1. Circular dichroism spectroscopy and Fourier transform infrared spectroscopy indicated that the electrical stimulation increased the dipole moment of the α-helix in RC-LH1 to facilitate the quinone transport in the primary reaction. Nuclear magnetic resonance spectroscopy revealed that the electrical stimulation increased the formation of hydrogen bonds in bacteriochlorophyll a for electron transition. This study investigated the response of photosynthetic bacteria to electrical stimulation, providing a distinct outlook on the electrochemical regulation of photosynthesis for carbon capture.

Modelling dynamics of photosynthetic units and microalgae growth based on high throughput pulsed light screens

Light-dark cycles on the s timescale are typical of cell cycling through photobioreactors and high-rate ponds and can significantly impact microalgae cultivation efficiency. Here, a novel formulation of the Camacho-Rubio microalgae growth model was developed to simulate dynamic growth under pulsed light (s timescale). It simultaneously captures the dynamics of the photosynthetic units (PSUs: open, closed and photoinhibited) and their impact on biomass productivity over time. Theoretical knowledge of PSU mediated processes operating on characteristic timescales was used to identify intra- and inter-light cycle variable dynamics and guide the model formulation. For model calibration, dynamic profiles of microalgae growth under pulsed light in a High-Throughput Screening platform were employed, after Principal Component Analysis (PCA), to remove biological noise. Model parameters were estimated for Chlorella sp., and the model validated against additional data under both pulsed and continuous light. The average prediction error for OD750 (measure of biomass) was 8% under pulsed light and 15% under continuous light. For OD680/OD750 (measure of Chlorophyll cell−1) it was 8% under pulsed and continuous light, demonstrating the model’s ability to represent highly variable conditions with good accuracy.

Energy Transfer and Exciton Relaxation in B880-B800-RC Complex through Two-Dimensional Electronic Spectroscopy.

The light-harvesting (LH) and reaction center (RC) core complex of purple bacterium Roseiflexus castenholzii, B880-B800-RC, are different from those of the typical photosynthetic unit, (B850-B800)x-B880-RC. To investigate the excitation flowing dynamics in this unique complex, two-dimensional electronic spectroscopy is employed. The obtained time constants for the exciton relaxation in B880, exciton relaxation in B800, B800 → B880 energy transfer (EET), and B880 → closed RC EET are 43 fs, 177 fs, 1.9 ps, and 205 ps, respectively. These time constants result in an overall EET efficiency similar to that of the typical photosynthetic unit. Analysis of the oscillatory signals reveals that while several vibronic coherences are involved in the exciton relaxation process, only one prominent vibronic coherence, with a frequency of 27 cm-1 and coupled to the B880 electronic transition, may contribute to the B800 → B880 EET process.

Roadmap of electrons from donor side to the reaction center of photosynthetic purple bacteria with mutated cytochromes

In photosynthetic bacteria, the absorbed light drives the canonical cyclic electron transfer between the reaction center and the cytochrome bc1 complexes via the pools of mobile electron carriers. If kinetic or structural barriers hinder the participation of the bc1 complex in the cyclic flow of electrons, then the pools of mobile redox agents must supply the electrons for the multiple turnovers of the reaction center. These conditions were achieved by continuous high light excitation of intact cells of bacterial strains Rba. sphaeroides and Rvx. gelatinosus with depleted donor side cytochromes c2 (cycA) and tetraheme cytochrome subunit (pufC), respectively. The gradual oxidation by ferricyanide further reduced the availability of electron donors to pufC. Electron transfer through the reaction center was tracked by absorption change and by induction and relaxation of the fluorescence of the bacteriochlorophyll dimer. The rate constants of the electron transfer (~ 3 × 103 s‒1) from the mobile donors of Rvx. gelatinosus bound either to the RC (pufC) or to the tetraheme subunit (wild type) were similar. The electrons transferred through the reaction center dimer were supplied entirely by the donor pool; their number amounted to about 5 in wild type Rvx. gelatinosus and decreased to 1 in pufC oxidized by ferricyanide. Fluorescence yield was measured as a function of the oxidized fraction of the dimer and its complex shape reveals the contribution of two competing processes: the migration of the excitation energy among the photosynthetic units and the availability of electron donors to the oxidized dimer. The experimental results were simulated and rationalized by a simple kinetic model of the two-electron cycling of the acceptor side combined with aperiodic one-electron redox function of the donor side.

Peptide-bridged bis-porphyrin compounds: A photophysical and molecular dynamics study

Covalently linked peptide-porphyrin compounds are most suitable systems for fundamental studies aiming to the comprehension of the mechanisms driving photoinduced energy/electron transfer processes. Mimicking photosynthetic units, the porphyrin groups act as antenna moieties while the peptide chain is the active medium through which energy and/or electron funneling occur. In this contribution we studied the transfer of excitation between two identical tetraphenylporphyrin groups connected by short peptide chains of different length formed by non-coded conformationally constrained α-amino acids, i.e., Cα-methylvaline. The photophysical events following porphyrin photoexcitation were characterized from the microsecond to the picosecond time region by time-resolved spectroscopy techniques. Ultrafast transient absorption measurements revealed the presence of a transient species that we assign to a self-trapped exciton migrating through the peptide chain. The exciton species propagates the electronic coupling between the two porphyrin groups giving rise to a characteristic bisignate band measured by circular dichroism experiments. Molecular dynamics simulations strongly suggest that the long lifetime (hundreds of picoseconds) of the exciton species is determined by the rigidity of the Cα-methylvaline residues, that inhibited energy relaxation pathways coupled to torsional motions of the peptide chain.

Strong increase of photosynthetic pigments and leaf size in a pruned Ginkgo biloba tree.

A 50-year-old solitary, sun-exposed ginkgo tree had strongly been pruned in the fall of 2021. Very few buds for the formation of new leaves, twigs, and branches were left over. In spring 2022, these few remaining buds responded with the formation of a different leaf type. These leaves were 2.7 times larger and also thicker than in the years before. In addition, the mean content of total chlorophylls [Chl (a+b)] per leaf area unit of dark-green leaves was 1.45, those of green leaves two times higher as compared to the years before pruning and the two other ginkgo trees which had been investigated in parallel. A comparable increase was also found for the level of total carotenoids (x+c). The mean content for Chl (a+b) were 1,118 mg m-2 for dark-green and 898 mg m-2 for green leaves as compared to 435 to 770 mg m-2 in leaves of other trees. The higher values for Chl (a+b) and total carotenoid content showed up also on a fresh and dry mass basis. Thus, with the formation of a new, larger leaf type by changes in morphology (leaf size and thickness) and the increase of photosynthetic pigments, the pruned ginkgo tree was able to compensate for the much lower number of leaves and photosynthetic units.

The N:Chl-a ratio defines the optimal physiological state in marine microalgae

Several factors may influence the growth of microalgae and their biochemical composition. Thus, bioassays can contribute to understanding how the environment influences the chemical element:chlorophyll-a ratio and the metabolic processes in different microalgae. The objective of this study, under laboratory conditions, was to study the variation in the Carbon-to-Chlorophyll-a (C:Chl-a), Nitrogen-to-Chlorophyll-a (N:Chl-a), and Hydrogen-to-Chlorophyll-a (H:Chl-a) ratios as indicators of the physiological state of species representative of different functional groups of phytoplankton: Isochrysis galbana, Chaetoceros calcitrans, and Dunaliella tertiolecta. The results suggest that changes in the content of intracellular pigments can be directly attributed to maximizing phytoplankton growth through the metabolic requirements, where Nitrogen and Carbon uptake and assimilation are critical drivers related to their physiological needs rather than their photosynthesis processes per se. Our observations documented the behavior of phytoplanktonic microalgae, where I. galbana showed the lowest Chl-a values associated with metabolic processes to produce complex molecules for storage and as reserve compounds. In C. calcitrans, metabolic processes are mainly addressed to generate complex compounds of Carbon and Nitrogen inside the cell rather than for population increase. Finally, D. tertiolecta showed that these elements are used gradually but progressively, generating more photosynthetic units during the exponential growth phase. This work provides new evidence to understand metabolic mechanisms between different groups of phytoplankton, essential knowledge to produce macromolecules and bioactive metabolites with different applications.

Modeling Approach to Capture the Effect of High Frequency Flashing Light in Steady-State Microalgae Cultures

With the advent of efficient artificial light systems, the possibility of exploiting pulsed light (PL) in microalgal cultivation is gaining renewed attention. In this work, a model was developed to describeArthrospira maxima production in continuous cultivation systems, under both continuous and high-frequency pulsed light regimes. The model is based on the one originally developed by Camacho-Rubio, modified to link photosynthetic units activation to biomass growth, accounting also for maintenance. The modified model, calibrated in continuous light regime, was found to properly predict steady-state microalgal growth at different light intensities and residence times. The model was then validated on experimental data carried out under extreme PL conditions (high frequencies up to 3700 Hz and duty cycles as low as 0.01). Remarkably, it was able to account for the flashing light effect (FLE, i.e., increased biomass production with respect to continuous light supply) measured under such conditions and can therefore be a valuable tool to optimize the energetic efficiency of algal cultivation.

Emerging Trends in Nanomaterials for Photosynthetic Biohybrid Systems

Global warming and climate change are among the most immediate challenges confronting humans in the 21st century. Artificial photosynthesis represents a promising approach to mitigating the environmental crisis. Recently, people demonstrated that interfacing semiconductor, polymer, or metal-based nanomaterials with specific bacteria can generate built-in artificial photosynthetic systems, enabling solar-to-fuel conversion by forming a basic photosynthetic unit from a network of light-harvesting receptors, molecular water splitting and CO2, or proton reduction machinery. As a cutting-edge research direction, several strategies have been employed to create the artificial photosynthetic biohybrids. Notably, understanding of the molecular basis of these photosynthetic biohybrid systems is the key to improving the solar-to-chemical conversion efficiency. In the current review, we highlight the study of charge uptake channels in biohybrid artificial photosynthetic systems using various nanomaterials and microbes. We emphasize the importance of fully understanding the structures and operating mechanisms of these hybrid systems, as well as the criterion to select suitable microbes and photosensitized nanomaterials.

Optical cavity-mediated exciton dynamics in photosynthetic light harvesting 2 complexes

Strong light-matter interaction leads to the formation of hybrid polariton states and alters the photophysical dynamics of organic materials and biological systems without modifying their chemical structure. Here, we experimentally investigated a well-known photosynthetic protein, light harvesting 2 complexes (LH2) from purple bacteria under strong coupling with the light mode of a Fabry-Perot optical microcavity. Using femtosecond pump probe spectroscopy, we analyzed the polariton dynamics of the strongly coupled system and observed a significant prolongation of the excited state lifetime compared with the bare exciton, which can be explained in terms of the exciton reservoir model. Our findings indicate the potential of tuning the dynamic of the whole photosynthetic unit, which contains several light harvesting complexes and reaction centers, with the help of strong exciton-photon coupling, and opening the discussion about possible design strategies of artificial photosynthetic devices.

Comparative Study between the Photosynthetic Parameters of Two Avocado (Persea americana) Cultivars Reveals Natural Variation in Light Reactions in Response to Frost Stress

Avocado is a commercially important fruit tree which is sold worldwide. Originating in subtropical regions of the South America, this species is now grown worldwide and is sometimes exposed to cold temperatures. Specifically, frost stress harms the crop yield and its quality. While it is known in general that the photosynthetic apparatus changes in response to cold conditions, there is still not much information regarding the photosynthetic apparatus response to sporadic frost stress. In this study, we tracked the photosynthetic apparatus’ light reaction of ‘Hass’ and ‘Ettinger’ avocado cultivars to frost stress, with Ettinger being known to be more resilient to cold than Hass. We found that in avocado trees, the photosynthetic apparatus’ response to frost occurs at the level of photosystem II (PSII) itself, rather than a photoprotective response to a stress. The Hass apparatus incorrectly interprets the reduction in electron transport rate activity and by that increases its light harvesting complex size at the expense of its reaction centers which then increases the apparatus’ probability to generate reactive oxygen species. The results of this study open opportunities to further research the process which regulates the feedback mechanism that controls the photosynthetic unit’s size in Hass when compared to the Ettinger cultivar, and whether it is part of a feedback regulation from the carbon assimilation step or indirectly from a stomatal limitation which arises in these subtropical species. While corroborating past studies performed on avocados, this study suggests using advanced chlorophyll a fluorescence protocols when researching natural variation in crops.

Macro- and micro-scale adaptations allow distinct Stylophora pistillata-symbiodiniaceae holobionts to optimize performance across a broad light habitat.

In sessile organisms, phenotypic plasticity represents an important strategy for dealing with environmental variability. Here we test if phenotypic plasticity enables the common coral Stylophora pistillata to occupy a broad niche. We find clear differences in the photo-physiology of four putative species of photosynthetic dinoflagellate symbionts associated with the coral S. pistillata, namely, Cladocopium 'C35a', 'C79', 'C78a' and 'C8a'. Coral phenotypic responses were also tightly linked to symbiont identity. Corals with Cladocopium 'C8a' have more "open" macro-morphology compared to colonies associating with depth-restricted Cladocopium 'C35a' or 'C78a' in the same shallow water habitat. Corals with Cladocopium 'C8a' had 40 to 60% lower symbiont cell densities compared to other holobionts but were more efficient at acclimating over a range of light levels, with clear mechanisms to dissipate excess light energy. This holobiont contains host-based green fluorescent pigments, increased concentrations of symbiont-based mycosporine amino acids, and xanthophyll cycling in high light habitats. Photosynthetic efficiency was also adjusted over the light habitat. In contrast, limited micro-scale responses were observed between three depth-restricted symbionts: Cladocopium 'C79', 'C35a', and 'C78a'. To optimize light levels reaching the photosynthetic unit, these colonies rely on a more closed macro-morphology under high light levels, which reduces incident light levels by up to 43%, and higher symbiont densities . Our results show that distinct macro- and micro-scale adaptations lead to functional differences between four distinct S. pistillata holobionts, allowing them to co-exist by filling specific niches on a small, but environmentally diverse, spatial scale. Key index words: Light, Symbiodiniaceae, coral, pigments, Stylophora pistillata, ITS2, phenotypic plasticity, niche diversification.

Contribution of microbial photosynthesis to peatland carbon uptake along a latitudinal gradient

Abstract Phototrophic microbes, also known as micro‐algae, display a high abundance in many terrestrial surface soils. They contribute to atmospheric carbon dioxide fluxes through their photosynthesis, and thus regulate climate similar to plants. However, microbial photosynthesis remains overlooked in most terrestrial ecosystems. Here, we hypothesise that phototrophic microbes significantly contribute to peatland C uptake, unless environmental conditions limit their development and their photosynthetic activity. To test our hypothesis, we studied phototrophic microbial communities in five peatlands distributed along a latitudinal gradient in Europe. By means of metabarcoding, microscopy and cytometry analyses, as well as measures of photosynthesis, we investigated the diversity, absolute abundance and photosynthetic rates of the phototrophic microbial communities. We identified 351 photosynthetic prokaryotic and eukaryotic operational taxonomic units (OTUs) across the five peatlands. We found that water availability and plant composition were important determinants of the composition and the structure of phototrophic microbial communities. Despite environmental shifts in community structure and composition, we showed that microbial C fixation rates remained similar along the latitudinal gradient. Our results further revealed that phototrophic microbes accounted for approximately 10% of peatland C uptake. Synthesis. Our findings show that phototrophic microbes are extremely diverse and abundant in peatlands. While species turnover with environmental conditions, microbial photosynthesis similarly contributed to peatland C uptake at all latitudes. We estimate that phototrophic microbes take up around 75 MT CO2 per year in northern peatlands. This amount roughly equals the magnitude of projected peatland C loss due to climate warming and highlights the importance of phototrophic microbes for the peatland C cycle.

Unbranched Hybrid Conducting Redox Polymers for Intact Chloroplast-Based Photobioelectrocatalysis.

Photobioelectrocatalysis (PBEC) adopts the sophistication and sustainability of photosynthetic units to convert solar energy into electrical energy. However, the electrically insulating outer membranes of photosynthetic units hinder efficient extracellular electron transfer from photosynthetic redox centers to an electrode in photobioelectrocatalytic systems. Among the artificial redox-mediating approaches used to enhance electrochemical communication at this biohybrid interface, conducting redox polymers (CRPs) are characterized by high intrinsic electric conductivities for efficient charge transfer. A majority of these CRPs constitute peripheral redox pendants attached to a conducting backbone by a linker. The consequently branched CRPs necessitate maintaining synergistic interactions between the pendant, linker, and backbone for optimal mediator performance. Herein, an unbranched, metal-free CRP, polydihydroxy aniline (PDHA), which has its redox moiety embedded in the polymer mainchain, is used as an exogenous redox mediator and an immobilization matrix at the biohybrid interface. As a proof of concept, the relatively complex membrane system of spinach chloroplasts is used as the photobioelectrocatalyst of choice. A "mixed" deposition of chloroplasts and PDHA generated a 2.4-fold photocurrent density increment. An alternative "layered" PDHA-chloroplast deposition, which was used to control panchromatic light absorbance by the intensely colored PDHA competing with the photoactivity of chloroplasts, generated a 4.2-fold photocurrent density increment. The highest photocurrent density recorded with intact chloroplasts was achieved by the "layered" deposition when used in conjunction with the diffusible redox mediator 2,6-dichlorobenzoquinone (-48 ± 3 μA cm-2). Our study effectively expands the scope of germane CRPs in PBEC, emphasizing the significance of the rational selection of CRPs for electrically insulating photobioelectrocatalysts and of the holistic modulation of the CRP-mediated biohybrids for optimal performance.

An Unbranched, Hybrid Conductive-Redox Polymer for Interfacing Intact Chloroplasts and Electrode Surfaces during Photobioelectrocatalysis

Photobioelectrocatalysis (PBEC) adopts the sophistication of photosynthetic units, their abundant availability, easy processing and sustainability, to convert solar energy into electricity. Specifically, the use of chloroplasts as photobioelectrocatalysts is desirable due to their metabolic independence and photoprotection mechanisms. However, the protein-based outer membranes of chloroplasts impede efficient charge transfer onto electrode surfaces during PBEC. Typical bio-inspired redox polymers used to ameliorate this charge transfer are branched, and contain redox pendants peripherally attached to a backbone. Pendants extract, and propagate electrons via collision-based electron tunneling. Conversely, we investigate the ability of an unbranched hybrid conductive-redox polymer, polydihydroxy aniline (PDHA), which has its redox moiety embedded in a conductive backbone. The redox moiety of PDHA is a quinone derivative, with redox potentials higher than the quinone-based redox active sites of chloroplasts. The biohybrid chloroplast-PDHA electrode encapsulates chloroplasts in a conductive matrix of PDHA, improving charge transfer. Holistically, PDHA facilitates the immobilization of chloroplasts and mediate electron transfer between chloroplasts and electrode surfaces. Our results demonstrate that a 120 % photocurrent increment is obtained upon combining chloroplasts with PDHA. Sequentially layering chloroplasts and PDHA on electrode surfaces evinces a 260 % increment in photocurrent responses. We report the highest photocurrent recorded with chloroplasts during PBEC (-48±3 µA cm-2) by pairing chloroplast-PDHA electrodes with diffusible redox mediator, 2,6-dichlorobenzoquinone. The study shows that redox polymer designs for artificially mediating electron transfer from chloroplasts can extend to unbranched conductive polymers in PBEC systems. Figure 1

A new approach for calculating microalgae culture growth based on an inhibitory effect of the surrounding biomass

Ever since the potential of algae in biotechnology was recognized, models describing the growth of algae inside photobioreactors have been proposed. These models are the basis for the optimization of process conditions and reactor designs. Over the last few decades, models became more and more elaborate with the increase of computational capacity. Thus far, these models have been based on light attenuation due to the absorption and scattering effects of the biomass. This manuscript presents a new way of predicting the apparent growth inside photobioreactors using simple models for enzymatic kinetics to describe the reaction between photons and the photosynthetic unit. The proposed model utilizes an inhibition kinetic formula based on the surrounding biomass to describe the average growth rate of a culture, which is determined by the local light intensities inside the reactor. The result is a mixed-inhibition scheme with multiple inhibition sites. The parameters of the new kinetic equation are replaced by empirical regression functions to correlate their dependency on incident light intensity and reactor size. The calibrations of the parameters and the regression functions are based on the numerical solutions of the growth rate computed with a classical Type II model. As a final verification, we apply the new equation in predicting the growth behavior of three phototrophic organisms in reactors of three different sizes.

Simultaneous Presence of Bacteriochlorophyll and Xanthorhodopsin Genes in a Freshwater Bacterium.

Photoheterotrophic bacteria represent an important part of aquatic microbial communities. There exist two fundamentally different light-harvesting systems: bacteriochlorophyll-containing reaction centers or rhodopsins. Here, we report a photoheterotrophic Sphingomonas strain isolated from an oligotrophic lake, which contains complete sets of genes for both rhodopsin-based and bacteriochlorophyll-based phototrophy. Interestingly, the identified genes were not expressed when cultured in liquid organic media. Using reverse transcription quantitative PCR (RT-qPCR), RNA sequencing, and bacteriochlorophyll a quantification, we document that bacteriochlorophyll synthesis was repressed by high concentrations of glucose or galactose in the medium. Coactivation of photosynthesis genes together with genes for TonB-dependent transporters suggests the utilization of light energy for nutrient import. The photosynthetic units were formed by ring-shaped light-harvesting complex 1 and reaction centers with bacteriochlorophyll a and spirilloxanthin as the main light-harvesting pigments. The identified rhodopsin gene belonged to the xanthorhodopsin family, but it lacks salinixanthin antenna. In contrast to bacteriochlorophyll, the expression of xanthorhodopsin remained minimal under all experimental conditions tested. Since the gene was found in the same operon as a histidine kinase, we propose that it might serve as a light sensor. Our results document that photoheterotrophic Sphingomonas bacteria use the energy of light under carbon-limited conditions, while under carbon-replete conditions, they cover all their metabolic needs through oxidative phosphorylation.IMPORTANCE Phototrophic organisms are key components of many natural environments. There exist two main phototrophic groups: species that collect light energy using various kinds of (bacterio)chlorophylls and species that utilize rhodopsins. Here, we present a freshwater bacterium Sphingomonas sp. strain AAP5 which contains genes for both light-harvesting systems. We show that bacteriochlorophyll-based reaction centers are repressed by light and/or glucose. On the other hand, the rhodopsin gene was not expressed significantly under any of the experimental conditions. This may indicate that rhodopsin in Sphingomonas may have other functions not linked to bioenergetics.

Adjustments of the photosynthetic unit and compensation mechanisms of tolerance to high ammonia concentration in Chlorella sp. grown in food waste digestate

Anaerobically digested food waste (ADF) is a waste effluent produced from anaerobic digestion of food wastes. It is difficult to treat but also holds high potential for valorization. Microalgae cultivation is a promising path for its treatment and valorization. However, ADF is very turbid and has toxic concentration of ammonia nitrogen (NH3-N) (up to 5000 mg L−1 NH3-N). Here we looked at the physiological responses and acclimation patterns of Chlorella sp., Mur 269 to toxicity and turbidity in ADF, compared to bolds basal medium (BBM), under different irradiances. Indicators obtained via pulse amplitude modulation (PAM) fluorometry such as electron transport rate (ETR), fv′/fm′ and alpha (α), alongside oxygenesis as an indicator of photosynthesis, were used to study the state of the photosystem of the organism. Pigment (chlorophylls and carotenoids) ratios were studied as indicators of adjustments in the photosynthetic unit and growth rates and productivities were used to monitor cell growth. μmax and biomass productivities were 63 and 47% higher in ADF than in BBM. μmax and biomass productivity of Chlorella sp., Mur 269, remained close to their optimal values at 0.681 ± 0.03 d−1 and 165 ± 8 mg L−1 d−1 even at high irradiance intensities of 1500 μmol m−2 s−1, indicating applicability of this species in outdoor cultures at high irradiance. On the other hand, the results showed that in comparison with BBM, the efficiency of Chlorella sp., Mur 269 photosystem II at optimum irradiance, as reflected in Fv′/Fm′ values, was reduced by 16% in ADF. A critical look at the photosynthesis of the species by PAM fluorometry revealed that the adaptive methods of this microalgae to toxicity involves adjustment of the photosynthetic unit to maximize absorption of light and compensation mechanisms for reductions in PSII activity including switching to mixotrophic growth mode.

Far-red absorption and light-use efficiency trade-offs in chlorophyll f photosynthesis.

Plants and cyanobacteria use the chlorophylls embedded in their photosystems to absorb photons and perform charge separation, the first step of converting solar energy to chemical energy. While oxygenic photosynthesis is primarily based on chlorophyll a photochemistry, which is powered by red light, a few cyanobacterial species can harness less energetic photons when growing in far-red light. Acclimatization to far-red light involves the incorporation of a small number of molecules of red-shifted chlorophyll f in the photosystems, whereas the most abundant pigment remains chlorophyll a. Due to its different energetics, chlorophyll f is expected to alter the excited-state dynamics of the photosynthetic units and, ultimately, their performances. Here we combined time-resolved fluorescence measurements on intact cells and isolated complexes to show that chlorophyll f insertion slows down the overall energy trapping in both photosystems. While this marginally affects the efficiency of photosystem I, it substantially decreases that of photosystem II. Nevertheless, we show that despite the lower energy output, the insertion of red-shifted chlorophylls in the photosystems remains advantageous in environments that are enriched in far-red light and therefore represents a viable strategy for extending the photosynthetically active spectrum in other organisms, including plants. However, careful design of the new photosynthetic units will be required to preserve their efficiency.