Long-term survival of Chlamydomonas reinhardtii during conditional senescence

Photoacclimation strategies of Chlamydomonas reinhardtii in response to high-light stress in stationary phase

Under ideal conditions, Chlamydomonas reinhardtii can photoacclimate to excess light through various short- and long-term mechanisms. However, how microalgae handle excess light stress once they exit exponential growth, and especially in stationary phase, is less understood. Our study explored C. reinhardtii's photoprotection capacity and acclimation strategies during high-light stress once batch culture growth reached stationary phase. We monitored cultures of wildtype strain (CC125) over five days once they reached stationary phase under both low-light (LL) and high-light (HL) conditions. Under HL, many photosynthetic proteins were degraded but the stress-related light harvesting complex protein (LHCSR) was rapidly induced and contributed to the rapid activation of nonphotochemical quenching (NPQ). However, the LHCSR3-defective mutant (CC4614, npq4) lacked the rapid induction of quenching typical of post-exponential cultures, indicating that LHCSR3 is required for this response in stationary phase. Collectively, the main strategy for photoacclimation in stationary phase appears to be a dramatic reduction of photosystems while maintaining LHCII-LHCSR antenna complexes that prime the antenna for rapid activation of quenching upon light exposure. Part of this response to HL involves a resumption of cell growth after two days, that we hypothesized is due to the stimulation of growth-regulating pathways due to increased metabolite pools from the HL-induced protein turnover in the cell, something that remains to be tested. These findings demonstrate how C. reinhardtii manages high-light stress during stationary phases to maximize longevity.

From Division to Death: Metabolomic Analysis of Nicotiana tabacum BY-2 Cells Reveals the Complexity of Life in Batch Culture

Tobacco BY-2 cell culture is one of the most widely used models in plant biology. The main advantage of BY-2 suspension cultures is the synchronization of cell development and the appearance of polar elongation. In batch culture, BY-2 cells passed through the lag, proliferation, elongation, and stationary phases. During this process, the composition of the growth medium changed dramatically. Sucrose was rapidly eliminated; hexose first accumulated and then depleted. The medium's pH initially decreased and then rose with aging. As a result of the crosstalk between the internal and external stimuli, cells pass through complicated systemic rearrangements, which cause metabolomic alterations. The early stages were characterized by high levels of amino acids and sterols, which could be interpreted as the result of synthetic activity. The most intense rearrangements occurred between the proliferation and active elongation stages, including repression of amino acid accumulation and up-regulation of sugar metabolism. Later stages were distinguished by higher levels of secondary metabolites, which may be a non-specific response to deteriorating conditions. Senescence was followed by some increase in fatty acids and sterols as well as amino acids, and probably led to self-destructive processes. A correlation analysis revealed relationships between metabolites' covariation, their biochemical ratio, and the growth phase.

Features of the microalga Raphidocelis subcapitata: physiology and applications

The microalga Raphidocelis subcapitata was isolated from the Nitelva River (Norway) and subsequently deposited in the collection of the Norwegian Institute of Water Research as "Selenastrum capricornutum Printz". This freshwater microalga, also known as Pseudokirchneriella subcapitata, acquired much of its notoriety due to its high sensitivity to different chemical species, which makes it recommended by different international organizations for the assessment of ecotoxicity. However, outside this scope, R. subcapitata continues to be little explored. This review aims to shed light on a microalga that, despite its popularity, continues to be an "illustrious" unknown in many ways. Therefore, R. subcapitata taxonomy, phylogeny, shape, size/biovolume, cell ultra-structure, and reproduction are reviewed. The nutritional and cultural conditions, chronological aging, and maintenance and preservation of the alga are summarized and critically discussed. Applications of R. subcapitata, such as its use in aquatic toxicology (ecotoxicity assessment and elucidation of adverse toxic outcome pathways) are presented. Furthermore, the latest advances in the use of this alga in biotechnology, namely in the bioremediation of effluents and the production of value-added biomolecules and biofuels, are highlighted. To end, a perspective regarding the future exploitation of R. subcapitata potentialities, in a modern concept of biorefinery, is outlined. KEY POINTS: • An overview of alga phylogeny and physiology is critically reviewed. • Advances in alga nutrition, cultural conditions, and chronological aging are presented. • Its use in aquatic toxicology and biotechnology is highlighted.

Bioactive Compounds with Pesticide Activities Derived from Aged Cultures of Green Microalgae

The excessive use of synthetic pesticides has caused environmental problems and human health risks and increased the development of resistance in several organisms. Allelochemicals, secondary metabolites produced as part of the defense mechanisms in plants and microorganisms, are an attractive alternative to replace synthetic pesticides to remediate these problems. Microalgae are natural producers of a wide range of allelochemicals. Thus, they provide new opportunities to identify secondary metabolites with pesticide activities and an alternative approach to discover new modes of action and circumvent resistance. We screened 10 green microalgae strains belonging to the Chlorophyta phylum for their potential to inhibit the growth of photosynthetic and nonphotosynthetic organisms. Bioassays were established to assess microalgae extracts' effectiveness in controlling the growth of Chlorella sorokiniana, Arabidopsis thaliana, Amaranthus palmeri, and the model nematode Caenorhabditis elegans. All tested strains exhibited herbicidal, nematocidal, or algicidal activities. Importantly, methanol extracts of a Chlamydomonas strain effectively controlled the germination and growth of a glyphosate-resistant A. palmeri biotype. Likewise, some microalgae extracts effectively killed C. elegans L1 larvae. Comprehensive metabolic profiling using LC-MS of extracts with pesticide activities showed that the metabolite composition of Chlamydomonas, Chlorella, and Chloroidium extracts is diverse. Molecules such as fatty acids, isoquinoline alkaloids, aldehydes, and cinnamic acids were more abundant, suggesting their participation in the pesticide activities.

Life and death of Pseudokirchneriella subcapitata: physiological changes during chronological aging

The green alga Pseudokirchneriella subcapitata is widely used in ecotoxicity assays and has great biotechnological potential as feedstock. This work aims to characterize the physiology of this alga associated with the aging resulting from the incubation of cells for 21 days, in the OECD medium, with continuous agitation and light exposure, in a batch mode. After inoculation, cells grow exponentially during 3 days, and the culture presents a typical green color. In this phase, "young" algal cells present, predominantly, a lunate morphology with the chloroplast occupying a large part of the cell, maximum photosynthetic activity and pigments concentration, and produce starch as a reserve material. Between the 5^th and the 12^th days of incubation, cells are in the stationary phase. The culture becomes less green, and the cells stop dividing (≥ 99% have one nucleus) and start to age. "Old" algal cells present chloroplast shrinkage, an abrupt decline of chlorophylls content, and photosynthetic capacity (Fv/Fm and ɸ_PSII), accompanied by a degradation of starch and an increase of neutral lipids content. The onset of the death phase occurs after the 12^th day and is characterized by the loss of cell membrane integrity of some algae (cell death). The culture stays, progressively, yellow, and the majority of the population (~93%) is composed of live cells, chronologically "old," with a significant drop in photosynthetic activity (decay > 75% of Fv/Fm and ɸ_PSII) and starch content. The information here achieved can be helpful when exploring the potential of this alga in toxicity studies or in biotechnological applications. KEY POINTS: • Physiological changes of P. subcapitata with chronological aging are shown • "Young" algae exhibit a semilunar shape, high photosynthetic activity, and accumulated starch • "Old"-live algae show reduced photosynthetic capacity and accumulated lipids.

A life-history trade-off gene with antagonistic pleiotropic effects on reproduction and survival in limiting environments

Although life-history trade-offs are central to life-history evolution, their mechanistic basis is often unclear. Traditionally, trade-offs are understood in terms of competition for limited resources among traits within an organism, which could be mediated by signal transduction pathways at the level of cellular metabolism. Nevertheless, trade-offs are also thought to be produced as a consequence of the performance of one activity generating negative consequences for other traits, or the result of genes or pathways that simultaneously regulate two life-history traits in opposite directions (antagonistic pleiotropy), independent of resource allocation. Yet examples of genes with antagonistic effects on life-history traits are limited. This study provides direct evidence for a gene-RLS1, that is involved in increasing survival in nutrient-limiting environments at a cost to immediate reproduction in the single-celled photosynthetic alga, Chlamydomonas reinhardtii. Specifically, we show that RLS1 mutants are unable to properly suppress their reproduction in phosphate-deprived conditions. Although these mutants have an immediate reproductive advantage relative to the parental strain, their long-term survival is negatively affected. Our data suggest that RLS1 is a bona fide life-history trade-off gene that suppresses immediate reproduction and ensures survival by downregulating photosynthesis in limiting environments, as part of the general acclimation response to nutrient deprivation in photosynthetic organisms.