Revised assignment of room-temperature chlorophyll fluorescence emission bands in single living cells of Chlamydomonas reinhardtii

Enhancing Urban Wastewater Treatment through Isolated Chlorella Strain-Based Phytoremediation in Centrate Stream: An Analysis of Algae Morpho-Physiology and Nutrients Removal Efficiency

The release of inadequately treated urban wastewater is the main cause of environmental pollution of aquatic ecosystems. Among efficient and environmentally friendly technologies to improve the remediation process, those based on microalgae represent an attractive alternative due to the potential of microalgae to remove nitrogen (N) and phosphorus (P) from wastewaters. In this work, microalgae were isolated from the centrate stream of an urban wastewater treatment plant and a native Chlorella-like species was selected for studies on nutrient removal from centrate streams. Comparative experiments were set up using 100% centrate and BG11 synthetic medium, modified with the same N and P as the effluent. Since microalgal growth in 100% effluent was inhibited, cultivation of microalgae was performed by mixing tap-freshwater with centrate at increasing percentages (50%, 60%, 70%, and 80%). While algal biomass and nutrient removal was little affected by the differently diluted effluent, morpho-physiological parameters (F_V/F_M ratio, carotenoids, chloroplast ultrastructure) showed that cell stress increased with increasing amounts of centrate. However, the production of an algal biomass enriched in carotenoids and P, together with N and P abatement in the effluent, supports promising microalgae applications that combine centrate remediation with the production of compounds of biotechnological interest; for example, for organic agriculture.

Plastoquinone pool redox state and control of state transitions in Chlamydomonas reinhardtii in darkness and under illumination

Movement of LHCII between two photosystems has been assumed to be similarly controlled by the redox state of the plastoquinone pool (PQ-pool) in plants and green algae. Here we show that the redox state of the PQ-pool of Chlamydomonas reinhardtii can be determined with HPLC and use this method to compare the light state in C. reinhardtii with the PQ-pool redox state in a number of conditions. The PQ-pool was at least moderately reduced under illumination with all tested types of visible light and oxidation was achieved only with aerobic dark treatment or with far-red light. Although dark incubations and white light forms with spectral distribution favoring one photosystem affected the redox state of PQ-pool differently, they induced similar Stt7-dependent state transitions. Thus, under illumination the dynamics of the PQ-pool and its connection with light state appears more complicated in C. reinhardtii than in plants. We suggest this to stem from the larger number of LHC-units and from less different absorption profiles of the photosystems in C. reinhardtii than in plants. The data demonstrate that the two different control mechanisms required to fulfill the dual function of state transitions in C. reinhardtii in photoprotection and in balancing light utilization are activated via different means.

Removal of Nitrogen and Phosphorus from Thickening Effluent of an Urban Wastewater Treatment Plant by an Isolated Green Microalga

Microalgae are photosynthetic microorganisms and are considered excellent candidates for a wide range of biotechnological applications, including the removal of nutrients from urban wastewaters, which they can recover and convert into biomass. Microalgae-based systems can be integrated into conventional urban wastewater treatment plants (WW-TP) to improve the water depuration process. However, microalgal strain selection represents a crucial step for effective phytoremediation. In this work, a microalga isolated from the effluent derived from the thickening stage of waste sludge of an urban WW-TP was selected and tested to highlight its potential for nutrient removal. Ammonium and phosphate abatements by microalgae were evaluated using both the effluent and a synthetic medium in a comparative approach. Parallelly, the isolate was characterized in terms of growth capability, morphology, photosynthetic pigment content and photosystem II maximum quantum yield. The isolated microalga showed surprisingly high biomass yield and removal efficiency of both ammonium and phosphate ions from the effluent but not from the synthetic medium. This suggests its clear preference to grow in the effluent, linked to the overall characteristics of this matrix. Moreover, biomass from microalgae cultivated in wastewater was enriched in photosynthetic pigments, polyphosphates, proteins and starch, but not lipids, suggesting its possible use as a biofertilizer.

Enhanced photosynthetic linear electron flow in mixotrophic green microalga Ettlia oleoabundans UTEX 1185

Basic understanding of the photosynthetic physiology of the oleaginous green microalga Ettlia oleoabundans is still very limited, including the modulation of the photosynthetic membrane upon metabolism conversion from autotrophy to mixotrophy. It was previously reported that, upon glucose supply in the culture medium, E. oleoabundans preserves photosystem II (PSII) from degradation by virtue of a higher packing of thylakoid complexes. In this work, it was investigated whether in the mixotrophic exponential growth phase the PSII activity is merely preserved or even enhanced. Modulated fluorescence parameters were then recorded under short-term treatments with increasing irradiance values of white light. It was found that the mixotrophic microalga down-regulated the chlororespiratory electron recycling from photosystem I (PSI), but enhanced the linear electron flow from PSII to PSI. Ability to keep PSII more open than in autotrophic growth conditions indicated that the respiration of the glucose taken up from the medium fed the carbon fixing reactions with CO₂. The overall electron poise was indeed well regulated, with a lesser need for thermal dissipation of excess absorbed energy. It is proposed that the significant, though small, increase in PSII maximum quantum yield in mixotrophic cells just reflects an improved light energy use and an increased photochemical capacity as compared to the autotrophic cells.

Spectroanalysis in native gels (SING): rapid spectral analysis of pigmented thylakoid membrane complexes separated by CN-PAGE

Photosynthetic organisms rapidly adjust the capture, transfer and utilization of light energy to optimize the efficiency of photosynthesis and avoid photodamage. These adjustments involve fine-tuning of expression levels and mutual interactions among electron/proton transfer components and their associated light-harvesting antenna. Detailed studies of these interactions and their dynamics have been hindered by the low throughput and resolution of currently available research tools, which involve laborious isolation, separation and characterization steps. To address these issues, we developed an approach that measured multiple spectroscopic properties of thylakoid preparations directly in native polyacrylamide gel electrophoresis gels, enabling unprecedented resolution of photosynthetic complexes, both in terms of the spectroscopic and functional details, as well as the ability to distinguish separate complexes and thus test their functional connections. As a demonstration, we explore the thylakoid membrane components of Chlamydomonas reinhardtii acclimated to high and low light, using a combination of room temperature absorption and 77K fluorescence emission to generate a multi-dimensional molecular and spectroscopic map of the photosynthetic apparatus. We show that low-light-acclimated cells accumulate a photosystem I-containing megacomplex that is absent in high-light-acclimated cells and contains distinct LhcII proteins that can be distinguished based on their spectral signatures.

Frequently asked questions about chlorophyll fluorescence, the sequel

Using chlorophyll (Chl) a fluorescence many aspects of the photosynthetic apparatus can be studied, both in vitro and, noninvasively, in vivo. Complementary techniques can help to interpret changes in the Chl a fluorescence kinetics. Kalaji et al. (Photosynth Res 122:121-158, 2014a) addressed several questions about instruments, methods and applications based on Chl a fluorescence. Here, additional Chl a fluorescence-related topics are discussed again in a question and answer format. Examples are the effect of connectivity on photochemical quenching, the correction of F V /F M values for PSI fluorescence, the energy partitioning concept, the interpretation of the complementary area, probing the donor side of PSII, the assignment of bands of 77 K fluorescence emission spectra to fluorescence emitters, the relationship between prompt and delayed fluorescence, potential problems when sampling tree canopies, the use of fluorescence parameters in QTL studies, the use of Chl a fluorescence in biosensor applications and the application of neural network approaches for the analysis of fluorescence measurements. The answers draw on knowledge from different Chl a fluorescence analysis domains, yielding in several cases new insights.

Iron deficiency cause changes in photochemistry, thylakoid organization, and accumulation of photosystem II proteins in Chlamydomonas reinhardtii

A trace element, iron (Fe) plays a pivotal role in photosynthesis process which in turn mediates the plant growth and productivity. Here, we have focused majorly on the photochemistry of photosystem (PS) II, abundance of proteins, and organization of supercomplexes of thylakoids from Fe-depleted cells in Chlamydomonas reinhardtii. Confocal pictures show that the cell's size has been reduced and formed rosette-shaped palmelloids; however, there is no cell death. Further, the PSII photochemistry was reduced remarkably. Further, the photosynthetic efficiency analyzer data revealed that both donor and acceptor side of PSII were equally damaged. Additionally, the room-temperature emission spectra showed the fluorescence emission maxima increased due to impaired energy transfer from PSII to PSI. Furthermore, the protein data reveal that most of the proteins of reaction center and light-harvesting antenna were reduced in Fe-depleted cells. Additionally, the supercomplexes of PSI and PSII were destabilized from thylakoids under Fe-deficient condition showing that Fe is an important element in photosynthesis mechanism.

Probing molecular events associated with early development of thylakoid membranes by comparative proteomics and low temperature fluorescence

A comparison of protein profiles between prolamellar bodies from dark-grown etioplasts and thylakoid membranes from de-etioplasts illuminated respectively for 1, 5 and 9h revealed 155 differentially expressed CBB-stained spots. Clear results showed that the nonphototransformable Pchlide627-632 was the dominant pigment form in the PLBs of rice etioplasts during plant development in dark and transformed slowly to chlorophyllide in rice etioplasts when exposed to light. The light-induced accumulation of ACC oxidase, which catalyzes the final step of ethylene synthesis using ACC as substrate, would facilitate chlorophyll synthesis by inducing PORa/b expression via ethylene signaling. It could be also suggested that cyclic electron transport might play an important role in generation of ATP for carbon fixation and photoprotection of photosystems from excessive light in prothylakoid. Furthermore, the overproduction of ClpC1, which targets proteins to the ClpPR core complex for degradation, was observed only in Stage 1, during which period PLBs disrupted and converted into prothylakoids, suggesting that ClpC1 was of particular importance for disassembly of PLBs of etioplasts when exposed to light. This study revealed the possible biochemical and physiological processes lead to the formation of functional thylakoid membranes.

BIOLOGICAL SIGNIFICANCE

In this study, we monitored the light-induced transformation of prolamellar bodies into thylakoid membranes, which is correlated to the biogenesis of photosynthetic apparatus involving a complex cascade of biochemical and structural events. Three stages of thylakoid development classified according to the thylakoid development status (Adam et al., 2011) were studied for biogenesis of photosynthetic apparatus: Stage 1, prothylakoids emerge from the disrupted PLBs; Stage 2, prothylakoids converted into primary thylakoids which were dispersed in the stroma; Stage 3, the continuous grana and stroma thylakoids are formed. The development stage-dependent changes in the proteomic profile of the thylakoids were analyzed by two-dimensional electrophoresis (2-DE). This information was complemented with the steady-state 77K chlorophyll fluorescence of thylakoids at the corresponding development stage. Together, these analyses allowed us to further understand the molecular processes connected to the formation of functional thylakoid membranes.

Frequently asked questions about in vivo chlorophyll fluorescence: practical issues

The aim of this educational review is to provide practical information on the hardware, methodology, and the hands on application of chlorophyll (Chl) a fluorescence technology. We present the paper in a question and answer format like frequently asked questions. Although nearly all information on the application of Chl a fluorescence can be found in the literature, it is not always easily accessible. This paper is primarily aimed at scientists who have some experience with the application of Chl a fluorescence but are still in the process of discovering what it all means and how it can be used. Topics discussed are (among other things) the kind of information that can be obtained using different fluorescence techniques, the interpretation of Chl a fluorescence signals, specific applications of these techniques, and practical advice on different subjects, such as on the length of dark adaptation before measurement of the Chl a fluorescence transient. The paper also provides the physiological background for some of the applied procedures. It also serves as a source of reference for experienced scientists.

Growth and lipid synthesis promotion in mixotrophic Neochloris oleoabundans (Chlorophyta) cultivated with glucose

In the recent years, the studies concerning the cultivation of Neochloris oleoabundans for biofuel purposes have increased, in relation to its capability to accumulate lipids when grown under nutrient starvation. Unfortunately, this cultivation mode does not allow to reach high biomass densities, which are required to improve the feasibility of the process. Increasing knowledge of the microalgal physiology is necessary to obtain new useful information for the improvement of culture performance in the perspective of large-scale cultivation. In this work, the mixotrophic cultivation of N. oleoabundans in a brackish medium added with different glucose concentrations has been tested under shaking, with the aim of stimulating growth alongside lipid accumulation inside cells. Cell morphology, glucose consumption, photosynthetic pigment content and photosynthetic efficiency were also investigated. Among all tested glucose concentrations (0-30 g L(-1)), it was observed that 2.5 g L(-1) was the optimal concentration, allowing to obtain the best compromise between glucose supplement, biomass production and lipid accumulation. Growth was highly enhanced in mixotrophic cultures, linked to the release of cells from sporocysts. A unique feature characterising mixotrophy in N. oleoabundans was the promotion of the maximum quantum yield of Photosystem II. Moreover, when mixotrophic cells entered the stationary phase, high lipid accumulation was induced. This study shows that the addition of glucose to N. oleoabundans remarkably increases the production of biomass enriched in lipids and represents an advancement for the cultivation of this microalga for applied purposes.

Comparison of photosynthesis recovery dynamics in floating leaves of Trapa natans after inhibition by manganese or molybdenum: effects on Photosystem II

The aquatic plant Trapa natans L. is highly resistant to Mn and moderately resistant to Mo, mainly thanks to its ability to sequestrate the metals by chelation in the vacuole. Excess of Mn and Mo causes somewhat aspecific toxicity symptoms in plants, but the main target of their toxicity seems to be the photosynthetic process. In this work, we aimed at understanding how the effect on photosynthesis caused by Mn (130 μM, full recovery) or Mo (50 μM, partial recovery) in T. natans is linked to changes occurring in the photosynthetic apparatus, with emphasis on Photosystem II (PSII), during a 10 day treatment with these metals. The time-course of net photosynthesis, photosynthetic pigment content, amount of PSII and its peripheral antenna LHCII, and room-temperature fluorescence emission ratios F694/F680 and F700/(F685 + F695) showed that the early inhibiting effect of Mo and Mn (one day exposure) was essentially non-specific with respect to the metal, though more marked in Mo- than in Mn-treated plants. During the subsequent recovery phase, Mo still impaired PSII assembly and, consequently, photosynthesis could not reach the control values. Conversely, in Mn-treated plants the amount of PSII was fully re-established, as was photosynthesis, but the metal induced the accumulation of LHCII. The extent of inhibition and the effectiveness of photosynthesis recovery are proposed to reflect the different ability of T. natans to sequestrate safely excess Mn or Mo in vacuoles.

Chlorophyll triplet quenching and photoprotection in the higher plant monomeric antenna protein Lhcb5

In oxygenic photosynthetic organisms, chlorophyll triplets are harmful excited states readily reacting with molecular oxygen to yield the reactive oxygen species (ROS) singlet oxygen. Carotenoids have a photoprotective role in photosynthetic membranes by preventing photoxidative damage through quenching of chlorophyll singlets and triplets. In this work we used mutation analysis to investigate the architecture of chlorophyll triplet quenching sites within Lhcb5, a monomeric antenna protein of Photosystem II. The carotenoid and chlorophyll triplet formation as well as the production of ROS molecules were studied in a family of recombinant Lhcb5 proteins either with WT sequence, mutated into individual chlorophyll binding residues or refolded in vitro to bind different xanthophyll complements. We observed a site-specific effect in the efficiency of chlorophyll-carotenoid triplet-triplet energy transfer. Thus chlorophyll (Chl) 602 and 603 appear to be particularly important for triplet-triplet energy transfer to the xanthophyll bound into site L2. Surprisingly, mutation on Chl 612, the chlorophyll with the lower energy associated and in close contact with lutein in site L1, had no effect on quenching chlorophyll triplet excited states. Finally, we present evidence for an indirect role of neoxanthin in chlorophyll triplet quenching and show that quenching of both singlet and triplet states is necessary for minimizing singlet oxygen formation.

Morphophysiological analyses of Neochloris oleoabundans (Chlorophyta) grown mixotrophically in a carbon-rich waste product

Neochloris oleoabundans is considered one of the most promising oil-rich microalgae because of its ability to store lipids under nitrogen starvation. However, high biomass densities, required for applications on medium to large scale, are not reached in this condition of growth. As previous studies on other microalgae have shown that mixotrophy allows to obtain higher biomass in comparison to autotrophic cultures, we performed morphophysiological analyses in order to test the mixotrophic growth capability of N. oleoabundans. A carbon-rich manure derived from the apple vinegar production (AWP) was added to the medium. Cells were also cultivated under nutrient starvation (tap water), to observe the expected lipids accumulation, and combining AWP to water, to test the potential of this waste in a low-cost culture system. The results highlighted that AWP in the medium allowed to obtain the highest final cell density. Moreover, starch granules were stored inside chloroplast at the beginning of the experiment. The presence of AWP did not induce variations on light harvesting complex II (LHCII)-photosystem II (PSII) assembly, even if an interesting promotion of pigment synthesis in cells was observed. On the other hand, in starved cells, chloroplast degeneration, pigment content decrease, altered LHCII-PSII assembly and accumulation of high amount of lipid globules were observed, irrespective of the presence of AWP. The results suggest that mixotrophy promotes growth in N. oleoabundans and open up the possibility of using waste products from agri-food industries for this purpose. After growth, cells could be transferred under nutrient starvation to induce lipid accumulation.

Low photosynthetic activity is linked to changes in the organization of photosystem II in the fruit of Arum italicum

The low photosynthetic activity of fleshy green fruits is currently attributed to their special anatomy rather than to a down-regulation of photosystem II (PSII). However, it is unclear whether the organization of PSII, which is highly conserved in leaves, is also shared by non-foliar structures, such as fleshy fruits. To obtain new information on this aspect, the photosynthetic activity and the organization of PSII were investigated in the berry of Arum italicum Miller during maturation (ivory to green) and early ripening (green to yellow). The berry developed an "internal CO(2) recycling" photosynthesis; gross photosynthesis at the green stage was 25% of the leaf lamina. SDS-PAGE, BN-PAGE and 77 K spectrofluorimetry showed that the thylakoid membrane accumulated a very high amount of free LHCII trimers and only few PSII and PSI complexes. The pattern of PSII forms was similar to that of the lamina (monomers, dimers, LHCII-PSII supercomplexes), but increase in CP43-less PSII cores and low F695/F680 fluorescence ratio at room temperature indicated that PSII was less stable than in the leaf lamina. Beside effective PSII photoprotection, we propose that LHCII serves as a temporary storage of chlorophylls to provide a visual signal that fruit is not mature for seed dispersal. We conclude that the low photosynthetic activity of A. italicum berry depends on the scantiness of reaction centres and the reduced functionality of PSII.