Search

Within the ancient vascular plant lineage known as lycophytes, many Selaginella species contain only one giant chloroplast in the upper epidermal cells of the leaf. In deep-shade species, such as S. martensii, the chloroplast is cup-shaped and the thylakoid system differentiates into an upper lamellar region and a lower granal region (bizonoplast). In this report, we describe the ultrastructural changes occurring in the giant chloroplast hosted in the epidermal cells of S. martensii during the daily relocation of the organelle. The process occurs in up to ca. 40% of the microphylls without the plants being exposed to high-light flecks. The relocated chloroplast loses its cup shape: first, it flattens laterally toward the radial cell wall and then assumes a more globular shape. The loss of the conical cell shape, the side-by-side lateral positioning of vacuole and chloroplast, and the extensive rearrangement of the thylakoid system to only granal cooperate in limiting light absorption. While the cup-shaped chloroplast emphasizes the light-harvesting capacity in the morning, the relocated chloroplast is suggested to support the renewal of the thylakoid system during the afternoon, including the recovery of photosystem II (PSII) from photoinhibition. The giant chloroplast repositioning is part of a complex reversible reshaping of the whole epidermal cell., (© 2023. The Author(s).)

As major primary producers in marine environments, diatoms are considered a valuable feedstock of biologically active compounds for application in several biotechnological fields. Due to their metabolic plasticity, especially for light perception and use and in order to make microalgal production more environmentally sustainable, marine diatoms are considered good candidates for the large-scale cultivation. Among physical parameters, light plays a primary role. Even if sunlight is cost-effective, the employment of artificial light becomes a winning strategy if a high-value microalgal biomass is produced. Several researches on marine diatoms are designed to study the influence of different light regimens to increase biomass production enriched in biotechnologically high-value compounds (lipids, carotenoids, proteins, polysaccharides), or with emphasised photonic properties of the frustule.

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.

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.

The high cost of recombinant enzymes for the production of biofuel from ligno-cellulosic biomass is a crucial factor affecting the economic sustainability of the process. The use of plants as biofactories for the production of the suitable recombinant enzymes might be an alternative to microbial fermentation. In the case of enzyme accumulation in chloroplasts, it is fundamental to focus on the issue of full photosynthetic efficiency of transplastomic plants in the field where they might be exposed to abiotic stress such as high light intensity and high temperature. Xylanases (EC 3.2.1.8), a group of enzymes that hydrolyse linear polysaccharides of beta-1,4-xylan into xylose, find an application in the biofuel industry favouring biomass saccharification along with other cell-wall degrading enzymes. In the present study, we analysed how a high level of accumulation of a thermostable xylanase in tobacco chloroplasts does not impact on photosynthetic performance of transplastomic plants grown outdoors. The recombinant enzyme was found to be stable during plant development, ex planta and after long-term storage.

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.

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.

Room temperature (RT) microspectrofluorimetry in vivo of single cells has a great potential in photosynthesis studies. In order to get new information on RT chlorophyll fluorescence bands, we analyzed the spectra of Chlamydomonas reinhardtii mutants lacking fundamental proteins of the thylakoid membrane and spectra of photoinhibited WT cells. RT spectra of single living cells were characterized thorough derivative analyses and Gaussian deconvolution. The results obtained suggest that the dynamism in LHCII assembly could be sufficient to explain the variations in amplitudes of F680 (free LHCII), F694 (LHCII-PSII) and F702 (LHCII aggregates); F686 was assigned to the PSII core. Based on the revised assignments and on the variations observed, we discuss the meaning of the two fluorescence emission ratios F680/(F686 + F694) and F702/(F686 + F694), showing that these are sensitive parameters under moderate photoinhibition. In the most photoinhibited samples, the RT spectra tended to degenerate, showing characteristics of mutants that are partly depleted in PSII.

The response to molybdenum (Mo) was studied in the metal-tolerant hydrophyte Trapa natans L. Previously, it was shown that the plant accumulates Mn in the floating lamina by means of phenolic compounds and responded with acclimation responses of the chloroplast. Since the involvement of phenolics has been proposed also in Mo resistance, we tested the response of T. natans to increasing doses (5, 50, 150, 600 microM) of Mo using the photosynthetic apparatus as an indicator of cellular stress. Only 5 microM Mo did not cause evident modifications with respect to controls. Conversely, 50 to 600 microM Mo induced progressively marked alterations of the lamina morphology. The chloroplast ultrastructure showed disorganisation of the thylakoid system, and correspondingly, the photosynthetic pigment pattern was altered with a fall-down in photosynthesis. Microspectrofluorimetry indicated alterations of photosystem II, with differences among the three cell layers (first and second palisade and spongy tissues). While the highest dose caused plant death, 50 and 150 microM Mo-treated plants underwent partial recovery, and the plant survived up to the end of the vegetative season. However, reproduction was unsuccessful. Mo treatment did not induce increase in total phenolics, but only in anthocyanin. In contrast to Mn, detoxification of Mo by chelation inside vacuoles, possibly by anthocyanins, is suggested to be an insufficient mechanism to reduce Mo toxicity, which probably includes an impairment of nitrogen metabolism. However, the metal was accumulated in the lamina. On the whole, T. natans showed limited capabilities to survive Mo excess as compared with Mn.