Models of Protocells Undergoing Asymmetrical Division

The conditions that allow for the sustained growth of a protocell population are investigated in the case of asymmetrical division. The results are compared to those of previous studies concerning models of symmetrical division, where synchronization (between duplication of the genetic material and fission of the lipid container) was found under a variety of different assumptions about the kinetic equations and about the place where molecular replication takes place. Such synchronization allows a sustained proliferation of the protocell population. In the asymmetrical case, there can be no true synchronization, since the time to duplication may depend upon the initial size, but we introduce a notion of homogeneous growth that actually allows for the sustained reproduction of a population of protocells. We first analyze Surface Reaction Models, defined in the text, and we show that in many cases they undergo homogeneous growth under the same kinetic laws that lead to synchronization in the symmetrical case. This is the case also for Internal Reaction Models (IRMs), which, however, require a deeper understanding of what homogeneous growth actually means, as discussed belo

Photosynthesis regulation in response to fluctuating light in the secondary endosymbiont alga Nannochloropsis gaditana

In nature, photosynthetic organisms are exposed to highly dynamic environmental conditions where the excitation energy and electron flow in the photosynthetic apparatus need to be continuously modulated. Fluctuations in incident light are particularly challenging since they drive oversaturation of photosynthesis, with consequent oxidative stress and photoinhibition. Plants and algae have evolved several mechanisms to modulate their photosynthetic machinery to cope with light dynamics, such as thermal dissipation of excited chlorophyll states (Non-Photochemical Quenching, NPQ) and regulation of electron transport. The regulatory mechanisms involved in the response to light dynamics have adapted during evolution and exploring biodiversity is a valuable strategy for expanding our understanding of their biological roles. In this work, we investigated the response to fluctuating light in Nannochloropsis gaditana, a eukaryotic microalga of the phylum Heterokonta originating from a secondary endosymbiotic event. N. gaditana is negatively affected by light fluctuations, leading to large reductions in growth and photosynthetic electron transport. Exposure to light fluctuations specifically damages photosystem I, likely because of ineffective regulation of electron transport in this species. The role of Non-Photochemical Quenching, also assessed using a mutant strain specifically depleted of this response, was instead found to be minor, especially in responding to the fastest light fluctuations

Generation of random mutants to improve light-use efficiency of Nannochloropsis gaditana cultures for biofuel production

Background The productivity of an algal culture depends on how efficiently it converts sunlight into biomass and lipids. Wild-type algae in their natural environment evolved to compete for light energy and maximize individual cell growth; however, in a photobioreactor, global productivity should be maximized. Improving light use efficiency is one of the primary aims of algae biotechnological research, and genetic engineering can play a major role in attaining this goal. Results In this work, we generated a collection of Nannochloropsis gaditana mutant strains and screened them for alterations in the photosynthetic apparatus. The selected mutant strains exhibited diverse phenotypes, some of which are potentially beneficial under the specific artificial conditions of a photobioreactor. Particular attention was given to strains showing reduced cellular pigment contents, and further characterization revealed that some of the selected strains exhibited improved photosynthetic activity; in at least one case, this trait corresponded to improved biomass productivity in lab-scale cultures. Conclusions This work demonstrates that genetic modification of N. gaditana has the potential to generate strains with improved biomass productivity when cultivated under the artificial conditions of a photobioreactor

Antenna complexes protect Photosystem I from Photoinhibition

Background Photosystems are composed of two moieties, a reaction center and a peripheral antenna system. In photosynthetic eukaryotes the latter system is composed of proteins belonging to Lhc family. An increasing set of evidences demonstrated how these polypeptides play a relevant physiological function in both light harvesting and photoprotection. Despite the sequence similarity between antenna proteins associated with the two Photosystems, present knowledge on their physiological role is mostly limited to complexes associated to Photosystem II. Results In this work we analyzed the physiological role of Photosystem I antenna system in Arabidopsis thaliana both in vivo and in vitro. Plants depleted in individual antenna polypeptides showed a reduced capacity for photoprotection and an increased production of reactive oxygen species upon high light exposure. In vitro experiments on isolated complexes confirmed that depletion of antenna proteins reduced the resistance of isolated Photosystem I particles to high light and that the antenna is effective in photoprotection only upon the interaction with the core complex. Conclusions We show that antenna proteins play a dual role in Arabidopsis thaliana Photosystem I photoprotection: first, a Photosystem I with an intact antenna system is more resistant to high light because of a reduced production of reactive oxygen species and, second, antenna chlorophyll-proteins are the first target of high light damages. When photoprotection mechanisms become insufficient, the antenna chlorophyll proteins act as fuses: LHCI chlorophylls are degraded while the reaction center photochemical activity is maintained. Differences with respect to photoprotection strategy in Photosystem II, where the reaction center is the first target of photoinhibition, are discussed

Stem photosynthetic efficiency across woody angiosperms and gymnosperms with contrasting drought tolerance

Key messageStem photosynthesis seems to play an adaptive role for woody plants that prosper in hot and dry ecosystems.Stem photosynthesis is thought to be involved in tree resistance/resilience to water shortage. Recent studies have focused on the coordination between stem photosynthesis and hydraulics, but the generality of association of stem photosynthetic efficiency with species-specific adaptation to drought is still unclear. We quantified bark and wood chlorophyll a fluorescence (in terms of F-v/F-m) in current-year, 1-year and 2-year-old stems of several woody species harvested in diverse habitats. We ranked species in terms of relative drought tolerance on the basis of their vulnerability to xylem embolism (P-50), and compared stem photosynthetic efficiency of drought-tolerant vs drought-sensitive species. F-v/F-m values decreased with increasing stem age, and were generally higher for Angiosperms than Gymnosperms. F-v/F-m both at the bark and wood level was higher for drought-tolerant Angiosperms compared to drought-sensitive ones. Our results highlight the potential adaptive role of stem photosynthesis in drought-tolerant species, thriving under arid conditions likely leading to prolonged stomatal closure and halt of leaf-level carbon gain

Reactive oxygen species and transcript analysis upon excess light treatment in wild-type Arabidopsis thaliana vs a photosensitive mutant lacking zeaxanthin and lutein

<p>Abstract</p> <p>Background</p> <p>Reactive oxygen species (ROS) are unavoidable by-products of oxygenic photosynthesis, causing progressive oxidative damage and ultimately cell death. Despite their destructive activity they are also signalling molecules, priming the acclimatory response to stress stimuli.</p> <p>Results</p> <p>To investigate this role further, we exposed wild type <it>Arabidopsis thaliana </it>plants and the double mutant <it>npq1lut2 </it>to excess light. The mutant does not produce the xanthophylls lutein and zeaxanthin, whose key roles include ROS scavenging and prevention of ROS synthesis. Biochemical analysis revealed that singlet oxygen (¹O₂) accumulated to higher levels in the mutant while other ROS were unaffected, allowing to define the transcriptomic signature of the acclimatory response mediated by ¹O₂which is enhanced by the lack of these xanthophylls species. The group of genes differentially regulated in <it>npq1lut2 </it>is enriched in sequences encoding chloroplast proteins involved in cell protection against the damaging effect of ROS. Among the early fine-tuned components, are proteins involved in tetrapyrrole biosynthesis, chlorophyll catabolism, protein import, folding and turnover, synthesis and membrane insertion of photosynthetic subunits. Up to now, the <it>flu </it>mutant was the only biological system adopted to define the regulation of gene expression by ¹O₂. In this work, we propose the use of mutants accumulating ¹O₂by mechanisms different from those activated in <it>flu </it>to better identify ROS signalling.</p> <p>Conclusions</p> <p>We propose that the lack of zeaxanthin and lutein leads to ¹O₂accumulation and this represents a signalling pathway in the early stages of stress acclimation, beside the response to ADP/ATP ratio and to the redox state of both plastoquinone pool. Chloroplasts respond to ¹O₂accumulation by undergoing a significant change in composition and function towards a fast acclimatory response. The physiological implications of this signalling specificity are discussed.</p

Light-Harvesting Complex Stress-Related Proteins Catalyze Excess Energy Dissipation in Both Photosystems of Physcomitrella patens

Two LHC-like proteins, Photosystem II Subunit S (PSBS) and Light-Harvesting Complex Stress-Related (LHCSR), are essential for triggering excess energy dissipation in chloroplasts of vascular plants and green algae, respectively. The mechanism of quenching was studied in Physcomitrella patens, an early divergent streptophyta (including green algae and land plants) in which both proteins are active. PSBS was localized in grana together with photosystem II (PSII), but LHCSR was located mainly in stroma-exposed membranes together with photosystem I (PSI), and its distribution did not change upon high-light treatment. The quenched conformation can be preserved by rapidly freezing the high-light-treated tissues in liquid nitrogen. When using green fluorescent protein as an internal standard, 77K fluorescence emission spectra on isolated chloroplasts allowed for independent assessment of PSI and PSII fluorescence yield. Results showed that both photosystems underwent quenching upon high-light treatment in the wild type in contrast to mutants depleted of LHCSR, which lacked PSI quenching. Due to the contribution of LHCII, P. patens had a PSI antenna size twice as large with respect to higher plants. Thus, LHCII, which is highly abundant in stroma membranes, appears to be the target of quenching by LHCSR

Cultivation of Gongolaria barbata (Fucales, Phaeophyceae) with a seaweed‐derived biostimulant in order to improve photophysiological fitness and promote fertility to advance the restoration of marine macroalgal forests

As a result of several anthropogenic factors, Cystoseira sensu lato forests have declined or become regionally extinct in many coastal regions of the Mediterranean. Given the low natural recovery of lost populations, research efforts have been encouraged to develop sustainable and efficient restoration of macroalgal forests on a large scale. By promoting growth and fertility of collected thallus branches under controlled laboratory conditions, the availability of seedlings for restoration could be ensured without jeopardizing natural populations. Here we investigated the effect of a commercial algal biostimulant (AlgatronCifo®) on the photophysiology, growth and fertility of Gongolaria barbata (Stackhouse) Kuntze (Fucales, Phaeophyceae). In a factorial laboratory experiment, two different temperatures (10 oC and 14 °C) and two culture media [i.e. seawater (SW) and Algatron (AT)] were tested. The photosynthetic performance of G. barbata doubled after three weeks of culture with AT, while it decreased by 25% when cultivated in SW. The highest photosynthetic performance and growth were achieved at 14oC with AT, where fertile receptacles also developed, followed by seedling settlements. The thalli cultured in AT had similar or better photosynthetic performance than the initial control thalli. AT-cultured thalli had a greater ability to quench energy via photochemical pathways (qP) than those from the SW, which on the contrary, had higher levels of non-photochemical responses (qN, NPQmax). This limited photosynthetic performance was probably linked to the higher P-limitation experienced under that treatment. The algal biostimulant enhanced the physiological performance and induced fertility of G. barbata, demonstrating its valorization potential and setting a new path for improved restoration applications