Salt tolerance in the freshwater algae Chlamydomon as reinhardii: Effect of proline and taurine

Salt stress alters the spectrum of de novo mutation available to selection during experimental adaptation of Chlamydomonas reinhardtii

The genetic basis of adaptation is driven by both selection and the spectrum of available mutations. Given that the rate of mutation is not uniformly distributed across the genome and varies depending on the environment, understanding the signatures of selection across the genome is aided by first establishing what the expectations of genetic change are from mutation. To determine the interaction between salt stress, selection, and mutation across the genome, we compared mutations observed in a selection experiment for salt tolerance in Chlamydomonas reinhardtii to those observed in mutation accumulation (MA) experiments with and without salt exposure. MA lines evolved under salt stress had a single-nucleotide mutation rate of 1.1 × 10 - 9 $1.1 \times 10^{-9}$ , similar to that of MA lines under standard conditions ( 9.6 × 10 - 10 $9.6 \times 10^{-10}$ ). However, we found that salt stress led to an increased rate of indel mutations, but that many of these mutations were removed under selection. Finally, lines adapted to salt also showed excess clustering of mutations in the genome and the co-expression network, suggesting a role for positive selection in retaining mutations in particular compartments of the genome during the evolution of salt tolerance. Our study shows that characterizing mutation rates and spectra expected under stress helps disentangle the effects of environment and selection during adaptation.

Salinity Stress Responses and Adaptation Mechanisms in Eukaryotic Green Microalgae

High salinity is a challenging environmental stress for organisms to overcome. Unicellular photosynthetic microalgae are especially vulnerable as they have to grapple not only with ionic imbalance and osmotic stress but also with the generated reactive oxygen species (ROS) interfering with photosynthesis. This review attempts to compare and contrast mechanisms that algae, particularly the eukaryotic Chlamydomonas microalgae, exhibit in order to immediately respond to harsh conditions caused by high salinity. The review also collates adaptation mechanisms of freshwater algae strains under persistent high salt conditions. Understanding both short-term and long-term algal responses to high salinity is integral to further fundamental research in algal biology and biotechnology.

Sex increases the probability of evolutionary rescue in the presence of a competitor

The explanation for the continued existence of sex, despite its many costs, remains one of the major challenges of evolutionary biology. Previous experimental studies have demonstrated that sex increases the rate of adaptation in novel environments relative to asexual reproduction. Whereas these studies have investigated the impact of sex on adaptation to stressful abiotic environments, the potential for biotic interactions to influence this advantage of sex has been largely ignored. Species rarely exist in isolation in natural conditions, so the impact of sex on adaptation to a stressful abiotic environment may be altered by the interactions between coexisting species. To investigate the interplay of sex and competition on adaptation to deteriorating conditions, we allowed populations of the unicellular alga (Chlamydomonas reinhardtii) to evolve in an environment to which they were initially poorly adapted. We manipulated both their mode of reproduction and the presence of a competitor, and monitored population size and proportion of evolutionary rescue events for each mode of reproduction. The results indicate that sex may be the beneficial strategy in the presence of the competitor. Sexual populations had highest probability of evolutionary rescue irrespective of the presence of the competitor. The overall advantage of sex was also manifested through higher level of adaptedness of survived sexual populations relative to asexual populations. Since competitive interactions are commonplace in nature, one of the explanations for the maintenance of sex by natural selection may be the increased rate of adaptation of sexual populations both in the presence and absence of competitors.

Experimental macroevolution

The convergence of several disparate research programmes raises the possibility that the long-term evolutionary processes of innovation and radiation may become amenable to laboratory experimentation. Ancestors might be resurrected directly from naturally stored propagules or tissues, or indirectly from the expression of ancestral genes in contemporary genomes. New kinds of organisms might be evolved through artificial selection of major developmental genes. Adaptive radiation can be studied by mimicking major ecological transitions in the laboratory. All of these possibilities are subject to severe quantitative and qualitative limitations. In some cases, however, laboratory experiments may be capable of illuminating the processes responsible for the evolution of new kinds of organisms.

Introduction of taurine (2-aminoethanesulfonic acid) as a green bio-organic catalyst for the promotion of organic reactions under green conditions

Taurine (2-aminoethanesulfonic acid), a semi-essential amino acid that exists in the human body and numerous other living creatures, is used as a green bio-organic catalyst for the promotion of the Knoevenagel reaction between aldehydes and malononitrile.

Repeatability of adaptation in experimental populations of different sizes

The degree to which evolutionary trajectories and outcomes are repeatable across independent populations depends on the relative contribution of selection, chance and history. Population size has been shown theoretically and empirically to affect the amount of variation that arises among independent populations adapting to the same environment. Here, we measure the contribution of selection, chance and history in different-sized experimental populations of the unicellular alga Chlamydomonas reinhardtii adapting to a high salt environment to determine which component of evolution is affected by population size. We find that adaptation to salt is repeatable at the fitness level in medium (Ne = 5 × 10(4)) and large (Ne = 4 × 10(5)) populations because of the large contribution of selection. Adaptation is not repeatable in small (Ne = 5 × 10(3)) populations because of large constraints from history. The threshold between stochastic and deterministic evolution in this case is therefore between effective population sizes of 10(3) and 10(4). Our results indicate that diversity across populations is more likely to be maintained if they are small. Experimental outcomes in large populations are likely to be robust and can inform our predictions about outcomes in similar situations.

Experimental adaptation to marine conditions by a freshwater alga

The marine-freshwater boundary has been suggested as one of the most difficult to cross for organisms. Salt is a major ecological factor and provides an unequalled range of ecological opportunity because marine habitats are much more extensive than freshwater habitats, and because salt strongly affects the structure of microbial communities. We exposed experimental populations of the freshwater alga Chlamydomonas reinhardtii to steadily increasing concentrations of salt. About 98% of the lines went extinct. The ones that survived now thrive in growth medium with 36 g⋅L(-1) NaCl, and in seawater. Our results indicate that adaptation to marine conditions proceeded first through genetic assimilation of an inducible response to relatively low salt concentrations that was present in the ancestors, and subsequently by the evolution of an enhanced inducible response to high salt concentrations. These changes appear to have evolved through reversible and irreversible modifications, respectively. The evolution of marine from freshwater lineages is an example that clearly indicates the possibility of studying certain aspects of major ecological transitions in the laboratory.

Siting algae cultivation facilities for biofuel production in the United States: trade-offs between growth rate, site constructability, water availability, and infrastructure

Locating sites for new algae cultivation facilities is a complex task. The climate must support high growth rates, and cultivation ponds require appropriate land and water resources, as well as transportation and utility infrastructure. We employ our spatiotemporal Biomass Assessment Tool (BAT) to select promising locations based on the open-pond cultivation of Arthrospira sp. and strains of the order Sphaeropleales. A total of 64,000 sites across the southern United States were evaluated. We progressively applied screening criteria and tracked their impact on the number of potential sites, geographic location, and biomass productivity. Both strains demonstrated maximum productivity along the Gulf of Mexico coast, with the highest values on the Florida peninsula. In contrast, sites meeting all selection criteria for Arthrospira were located along the southern coast of Texas and for Sphaeropleales were located in Louisiana and southern Arkansas. Results were driven mainly by the lack of oil pipeline access in Florida and elevated groundwater salinity in southern Texas. The requirement for low-salinity freshwater (<400 mg L(-1)) constrained Sphaeropleales locations; siting flexibility is greater for salt-tolerant species like Arthrospira. Combined siting factors can result in significant departures from regions of maximum productivity but are within the expected range of site-specific process improvements.

Genetic correlation in relation to differences in dosage of a stressor

We exposed the strains of Chlamydomonas isolated from an outbred laboratory population to a range of concentrations of salt (NaCl) up to an extirpative level that the base population could not tolerate. The genetic variance of yield increased with stress over the first half of this range before collapsing to nearly zero. The genetic correlation decreased with environmental distance, whether measured as a difference in dosage or as an environmental variance. This result is consistent with previous studies and provides a basis for interpreting adaptation to a deteriorating environment and the process of evolutionary rescue.

Inhibition of plastocyanin to P(700)(+) electron transfer in Chlamydomonas reinhardtii by hyperosmotic stress

Oxygen electrode and fluorescence studies demonstrate that linear electron transport in the freshwater alga Chlamydomonas reinhardtii can be completely abolished by abrupt hyperosmotic shock. We show that the most likely primary site of inhibition of electron transfer by hyperosmotic shock is a blockage of electron transfer between plastocyanin (PC) or cytochrome c(6) and P(700). The effects on this reaction were reversible upon dilution of the osmolytes and the stability of plastocyanin or photosystem (PS) I was unaffected. Electron micrographs of osmotically shocked cells showed a significant decrease in the thylakoid lumen volume. Comparison of estimated lumenal width with the x-ray structures of plastocyanin and PS I suggest that lumenal space contracts during HOS so as to hinder the movement of docking to PS I of plastocyanin or cytochrome c(6).

Effect of compensatory organic osmolytes on resistance to freeze-drying of L929 cells and of their isolated chromatin

(1) Compensatory organic osmolytes are stabilizers of macromolecular structures. During acclimation to dehydration or high salinity, they accumulate in cells and effectively protect them against disruption that might otherwise result from increased inorganic ion concentrations. (2) Circular and electric dichroism, analysis of the kinetics of digestion by micrococcal nuclease, and UV spectra between 190 and 305 nm were used to investigate the resistance to dehydration upon freezing or freeze-drying that could confer such compounds to chromatin isolated from cultured L929 cells. Some work was also done on intact cells in vivo. (3) Sorbitol, sucrose, and trehalose appear to protect isolated chromatin very effectively; proline is less effective. (4) These compounds also effectively protect chromatin from the disrupting effects of NaCl. (5) Cells loaded and grown with sorbitol, sucrose, or proline can tolerate larger decreases in hydration than control cells. They cannot, however, tolerate complete dehydration.

Effect of high osmolarity acclimation on tolerance to hyperosmotic shocks in L929 cultured cells

Application of abrupt, large hyperosmotic shocks induces in L929 cultured cells changes similar to those previously described in other cell types, notably a hypercondensation of the nuclear chromatin. This paper shows that; 1) this phenomenon is concomitant with a complete disappearance of deoxyribonucleic acid, as visualized by immunogold labelling, from the nucleoplasmic spaces; 2) acclimation to high osmolarities (600 mOsm) by addition to the culture medium of NaCl, sorbitol or proline protects the cells from these effects, which appear to be largely attenuated-acclimated cells also survive much better to the osmotic shock than do control cells and; 3) the best protection seems to be provided by sorbitol and NaCl. Proline acclimation is less effective. These effects are discussed in terms of increased tolerance to NaCl load induced at the level of different macromolecules by so-called 'compensatory' organic compounds.

Increased tolerance to large hyperosmotic shocks and compensatory solutes in mammalian cultured cells

L929 and MDCK cells acclimated to sorbitol, proline or taurine media can tolerate a NaCl load that they would be unable to stand when cultured in normal conditions. The protecting effect is observed only at high concentrations, between 100 and 300 mM. In that range, the effect is increasing with increasing concentration. The presence of the compound is not necessary, in the extracellular medium, to induce tolerance. These results are discussed in the framework of the theory considering preferential interactions of proteins with solvent components to explain the protecting effect of such compounds.