Molecular cloning and expression analysis of a cytosolic Hsp70 gene from Antarctic ice algae Chlamydomonas sp. ICE-L

Salinity Stress Acclimation Strategies in Chlamydomonas sp. Revealed by Physiological, Morphological and Transcriptomic Approaches

Climate changes may include variations in salinity concentrations at sea by changing ocean dynamics. These variations may be especially challenging for marine photosynthetic organisms, affecting their growth and distribution. Chlamydomonas spp. are ubiquitous and are often found in extreme salinity conditions. For this reason, they are considered good model species to study salinity adaptation strategies. In the current study, we used an integrated approach to study the Chlamydomonas sp. CCMP225 response to salinities of 20‱ and 70‱, by combining physiological, morphological, and transcriptomic analyses, and comparing differentially expressed genes in the exponential and stationary growth phases under the two salinity conditions. The results showed that the strain is able to grow under all tested salinity conditions and maintains a surprisingly high photosynthetic efficiency even under high salinities. However, at the highest salinity condition, the cells lose their flagella. The transcriptomic analysis highlighted the up- or down-regulation of specific gene categories, helping to identify key genes responding to salinity stress. Overall, the findings may be of interest to the marine biology, ecology, and biotechnology communities, to better understand species adaptation mechanisms under possible global change scenarios and the potential activation of enzymes involved in the synthesis of bioactive molecules.

Cellular response of Parachlorella kessleri to a solid surface culture environment

Attached culture allows high biomass productivity and is a promising biomass cultivating system because neither a huge facility area nor a large volume of culture medium are needed. This study investigates photosynthetic and transcriptomic behaviors in Parachlorella kessleri cells on a solid surface after their transfer from liquid culture to elucidate the physiological and gene-expression regulatory mechanisms that underlie their vigorous proliferation. The chlorophyll content shows a decrease at 12 h after the transfer; however, it has fully recovered at 24 h, suggesting temporary decreases in the amounts of light harvesting complexes. On PAM analysis, it is demonstrated that the effective quantum yield of PSII decreases at 0 h right after the transfer, followed by its recovery in the next 24 h. A similar changing pattern is observed for the photochemical quenching, with the PSII maximum quantum yield remaining at an almost unaltered level. Non-photochemical quenching was increased at both 0 h and 12 h after the transfer. These observations suggest that electron transfer downstream of PSII but not PSII itself is only temporarily damaged in solid-surface cells just after the transfer, with light energy in excess being dissipated as heat for PSII protection. It thus seems that the photosynthetic machinery acclimates to high-light and/or dehydration stresses through its temporal size-down and functional regulation that start right after the transfer. Meanwhile, transcriptomic analysis by RNA-Seq demonstrates temporary upregulation at 12 h after the transfer as to the expression levels of many genes for photosynthesis, amino acid synthesis, general stress response, and ribosomal subunit proteins. These findings suggest that cells transferred to a solid surface become stressed immediately after transfer but can recover their high photosynthetic activity through adaptation of photosynthetic machinery and metabolic flow as well as induction of general stress response mechanisms within 24 h.

Temperature stress in psychrophilic green microalgae: Minireview

Photosynthetic algae are the main primary producers in polar regions, form the basis of polar food webs, and are responsible for a significant portion of global carbon fixation. Many cold-water algae are psychrophiles that thrive in the cold but cannot grow at moderate temperatures (≥20°C). Polar regions are at risk of rapid warming caused by climate change, and the sensitivity of psychrophilic algae to rising temperatures makes them, and the ecosystems they inhabit, particularly vulnerable. Recent research on the Antarctic psychrophile Chlamydomonas priscuii, an emerging algal model, has revealed unique adaptations to life in the permanent cold. Additionally, genome sequencing of C. priscuii and its relative Chlamydomonas sp. ICE-L has given rise to a plethora of computational tools that can help elucidate the genetic basis of psychrophily. This minireview summarizes new advances in characterizing the heat stress responses in psychrophilic algae and examines their extraordinary sensitivity to temperature increases. Further research in this field will help determine the impact of climate change on psychrophiles from threatened polar environments.

Can heat shock protein 70 (HSP70) serve as biomarkers in Antarctica for future ocean acidification, warming and salinity stress?

The Antarctic Peninsula is one of the fastest-warming places on Earth. Elevated sea water temperatures cause glacier and sea ice melting. When icebergs melt into the ocean, it “freshens” the saltwater around them, reducing its salinity. The oceans absorb excess anthropogenic carbon dioxide (CO2) causing decline in ocean pH, a process known as ocean acidification. Many marine organisms are specifically affected by ocean warming, freshening and acidification. Due to the sensitivity of Antarctica to global warming, using biomarkers is the best way for scientists to predict more accurately future climate change and provide useful information or ecological risk assessments. The 70-kilodalton (kDa) heat shock protein (HSP70) chaperones have been used as biomarkers of stress in temperate and tropical environments. The induction of the HSP70 genes (Hsp70) that alter intracellular proteins in living organisms is a signal triggered by environmental temperature changes. Induction of Hsp70 has been observed both in eukaryotes and in prokaryotes as response to environmental stressors including increased and decreased temperature, salinity, pH and the combined effects of changes in temperature, acidification and salinity stress. Generally, HSP70s play critical roles in numerous complex processes of metabolism; their synthesis can usually be increased or decreased during stressful conditions. However, there is a question as to whether HSP70s may serve as excellent biomarkers in the Antarctic considering the long residence time of Antarctic organisms in a cold polar environment which appears to have greatly modified the response of heat responding transcriptional systems. This review provides insight into the vital roles of HSP70 that make them ideal candidates as biomarkers for identifying resistance and resilience in response to abiotic stressors associated with climate change, which are the effects of ocean warming, freshening and acidification in Antarctic organisms.

Functional filtering and random processes affect the assembly of microbial communities of snow algae blooms at Maritime Antarctic

The increasing temperatures at the West Antarctic Peninsula (Maritime Antarctic) could lead to a higher occurrence of snow algal blooms which are ubiquitous events that change the snow coloration, reducing albedo and in turn exacerbating melting. However, there is a limited understanding of snow algae blooms biodiversity, composition, and their functional profiles, especially in one of the world's areas most affected by climate change. In this study we used 16S rRNA and 18S rRNA metabarcoding, and shotgun metagenomics to assess the diversity, composition, and functional potential of the snow algae blooms bacterial and eukaryotic communities at three different sites of Maritime Antarctic, between different colors of the algae blooms and between seasonal and semi-permanent snowfields. We tested the hypothesis that the functional potential of snow algae blooms is conserved despite a changing taxonomic composition. Furthermore, we determined taxonomic co-occurrence patterns of bacteria and eukaryotes and assessed the potential for the exchange of metabolites among bacterial taxa. Here, we tested the prediction that there are co-occurring taxa within snow algae whose biotic interactions are marked by the exchange of metabolites. Our results show that the composition of snow algae blooms vary significantly among sites. For instance, a higher abundance of fungi and protists were detected in Fildes Peninsula compared with Doumer Island and O'Higgins. Likewise, the composition varied between snow colors and snow types. However, the functional potential varied only among sampling sites with a higher abundance of genes involved in tolerance to environmental stress at O'Higgins. Co-occurrence patterns of dominant bacterial genera such as Pedobacter, Polaromonas, Flavobacterium and Hymenobacter were recorded, contrasting the absence of co-occurring patterns displayed by Chlamydomonadales algae with other eukaryotes. Finally, genome-scale metabolic models revealed that bacteria within snow algae blooms likely compete for resources instead of forming cooperative communities.

A new era of radiation resistance bacteria in bioremediation and production of bioactive compounds with therapeutic potential and other aspects: An in-perspective review

Microorganisms that survive in extreme environmental conditions are known as 'extremophiles'. Recently, extremophiles draw an impression in biotechnology/pharmaceutical researches/industries because of their novel molecules, known as 'extremolytes'. The intriguing phenomenon of microbial radiation resistance probably arose independently throughout their evolution of selective pressures (e.g. UV, X-ray, Gamma radiation etc.). Radiation produces multiple types of damage/oxidation to nucleic acids, proteins and other crucial cellular components. Most of the literature on microbial radiation resistance is based on acute γ-irradiation experiments performed in the laboratory, typically involving pure cultures isolation and their application on bioremediation/therapeutic field. There is much less information other than bioremediation and therapeutic application of such promising microbes we called as 'new era'. Here we discus origin and diversity of radiation resistance bacteria as well as selective mechanisms by which microorganisms can sustain in radiation rich environment. Potential uses of these radiations resistant microbes in the field of bioremediation, bioactive compounds and therapeutic industry. Last but not the least, which is the new aspect of radiation resistance microbes. Our review suggest that resistance to chronic radiation is not limited to rare specialized strains from extreme environments, but can occur among common microbial taxa, perhaps due to overlap molecular mechanisms of resistance to radiation and other stressors. These stress tolerance potential make them potential for radionuclides remediation, their extremolytes can be useful as anti-oxidant and anti-proliferative agents. In current scenario they can be useful in various fields from natural dye synthesis to nanoparticles production and anti-cancer treatment.

Cryopreservation of clonal and polyclonal populations of Chlamydomonas reinhardtii

Long-term preservation of laboratory strains of Chlamydomonas reinhardtii has historically involved either liquid nitrogen cryopreservation, which is expensive and labor intensive, or storage on agar plates, which requires frequent transfer to new plates, and which may leave samples susceptible to contamination as well as genetic drift and/or selection. The emergence of C. reinhardtii as a model organism for genetic analysis and experimental evolution has produced an increasing demand for an efficient method to cryopreserve C. reinhardtii populations. The GeneArt™ Cryopreservation Kit for Algae provides the first method for algal storage at −80°C; however, little is known about how this method affects recovery of different clones, much less polyclonal populations. Here, we compare postfreeze viability of clonal and genetically mixed samples frozen at −80°C using GeneArt™ or cryopreserved using liquid nitrogen. We find that the GeneArt™ protocol yields similar percent recoveries for some but not all clonal cultures, when compared to archiving via liquid N2. We also find that relative frequency of different strains recovered from genetically mixed populations can be significantly altered by cryopreservation. Thus, while cryopreservation using GeneArt™ is an effective means for archiving certain clonal populations, it is not universally so. Strain-specific differences in freeze–thaw tolerance complicate the storage of different clones, and may also bias the recovery of different genotypes from polyclonal populations.

Revealing the Characteristics of the Antarctic Snow Alga Chlorominima collina gen. et sp. nov. Through Taxonomy, Physiology, and Transcriptomics

Snow algae play crucial roles in cold ecosystems, however, many aspects related to their biology, adaptations and especially their diversity are not well known. To improve the identification of snow algae from colored snow, in the present study we used a polyphasic approach to describe a new Antarctic genus, Chlorominima with the species type Chlorominima collina. This new taxon was isolated of colored snow collected from the Collins Glacier (King George Island) in the Maritime Antarctic region. Microscopy revealed biflagellated ellipsoidal cells with a rounded posterior end, a C-shaped parietal chloroplast without a pyrenoid, eyespot, and discrete papillae. Several of these characteristics are typical of the genus Chloromonas, but the new isolate differs from the described species of this genus by the unusual small size of the cells, the presence of several vacuoles, the position of the nucleus and the shape of the chloroplast. Molecular analyzes confirm that the isolated alga does not belong to Chloromonas and therefore forms an independent lineage, which is closely related to other unidentified Antarctic and Arctic strains, forming a polar subclade in the Stephanosphaerinia phylogroup within the Chlamydomonadales. Secondary structure comparisons of the ITS2 rDNA marker support the idea that new strain is a distinct taxon within of Caudivolvoxa. Physiological experiments revealed psychrophilic characteristics, which are typical of true snow algae. This status was confirmed by the partial transcriptome obtained at 2°C, in which various cold-responsive and cryoprotective genes were identified. This study explores the systematics, cold acclimatization strategies and their implications for the Antarctic snow flora.

The Parallel Molecular Adaptations to the Antarctic Cold Environment in Two Psychrophilic Green Algae

Psychrophilic green algae from independent phylogenetic lines thrive in the polar extreme environments, but the hypothesis that their psychrophilic characteristics appeared through parallel routes of molecular evolution remains untested. The recent surge of transcriptome data enables large-scale evolutionary analyses to investigate the genetic basis for the adaptations to the Antarctic extreme environment, and the identification of the selective forces that drive molecular evolution is the foundation to understand the strategies of cold adaptation. Here, we conducted transcriptome sequencing of two Antarctic psychrophilic green algae (Chlamydomonas sp. ICE-L and Tetrabaena socialis) and performed positive selection and convergent substitution analyses to investigate their molecular convergence and adaptive strategies against extreme cold conditions. Our results revealed considerable shared positively selected genes and significant evidence of molecular convergence in two Antarctic psychrophilic algae. Significant evidence of positive selection and convergent substitution were detected in genes associated with photosynthetic machinery, multiple antioxidant systems, and several crucial translation elements in Antarctic psychrophilic algae. Our study reveals that the psychrophilic algae possess more stable photosynthetic apparatus and multiple protective mechanisms and provides new clues of parallel adaptive evolution in Antarctic psychrophilic green algae.

UV-Protective Compounds in Marine Organisms from the Southern Ocean

Solar radiation represents a key abiotic factor in the evolution of life in the oceans. In general, marine, biota-particularly in euphotic and dysphotic zones-depends directly or indirectly on light, but ultraviolet radiation (UV-R) can damage vital molecular machineries. UV-R induces the formation of reactive oxygen species (ROS) and impairs intracellular structures and enzymatic reactions. It can also affect organismal physiologies and eventually alter trophic chains at the ecosystem level. In Antarctica, physical drivers, such as sunlight, sea-ice, seasonality and low temperature are particularly influencing as compared to other regions. The springtime ozone depletion over the Southern Ocean makes organisms be more vulnerable to UV-R. Nonetheless, Antarctic species seem to possess analogous UV photoprotection and repair mechanisms as those found in organisms from other latitudes. The lack of data on species-specific responses towards increased UV-B still limits the understanding about the ecological impact and the tolerance levels related to ozone depletion in this region. The photobiology of Antarctic biota is largely unknown, in spite of representing a highly promising reservoir in the discovery of novel cosmeceutical products. This review compiles the most relevant information on photoprotection and UV-repair processes described in organisms from the Southern Ocean, in the context of this unique marine polar environment.

Acclimation of Antarctic Chlamydomonas to the sea-ice environment: a transcriptomic analysis

The Antarctic green alga Chlamydomonas sp. ICE-L was isolated from sea ice. As a psychrophilic microalga, it can tolerate the environmental stress in the sea-ice brine, such as freezing temperature and high salinity. We performed a transcriptome analysis to identify freezing stress responding genes and explore the extreme environmental acclimation-related strategies. Here, we show that many genes in ICE-L transcriptome that encoding PUFA synthesis enzymes, molecular chaperon proteins, and cell membrane transport proteins have high similarity to the gens from Antarctic bacteria. These ICE-L genes are supposed to be acquired through horizontal gene transfer from its symbiotic microbes in the sea-ice brine. The presence of these genes in both sea-ice microalgae and bacteria indicated the biological processes they involved in are possibly contributing to ICE-L success in sea ice. In addition, the biological pathways were compared between ICE-L and its closely related sister species, Chlamydomonas reinhardtii and Volvox carteri. In ICE-L transcripome, many sequences homologous to the plant or bacteria proteins in the post-transcriptional, post-translational modification, and signal-transduction KEGG pathways, are absent in the nonpsychrophilic green algae. These complex structural components might imply enhanced stress adaptation capacity. At last, differential gene expression analysis at the transcriptome level of ICE-L indicated that genes that associated with post-translational modification, lipid metabolism, and nitrogen metabolism are responding to the freezing treatment. In conclusion, the transcriptome of Chlamydomonas sp. ICE-L is very useful for exploring the mutualistic interaction between microalgae and bacteria in sea ice; and discovering the specific genes and metabolism pathways responding to the freezing acclimation in psychrophilic microalgae.

Response of heat-shock protein (HSP) genes to temperature and salinity stress in the antarctic psychrotrophic bacterium Psychrobacter sp. G

Temperature and salinity fluctuations are two of the most important factors affecting the growth of polar bacteria. In an attempt to better understand the function of heat-shock proteins (HSPs) in the adaptive mechanisms of the Antarctic psychrotrophic bacterium Psychrobacter sp. G to such conditions, genes Hsp845, Hsp2538, Hsp2666, and Hsp2667 were cloned on the basis of the draft genome. The expression characteristics of these HSP genes under different stress conditions were analyzed by the qRT-PCR method. Expression of Hsp845 and Hsp2667 was inhibited significantly by low temperature (0 and 10 °C, respectively). There was no difference of expression when Hsp2538 and Hsp2666 were exposed to 0 °C but the expression of Hsp2666 was inhibited when exposed to 10 °C. Expression of Hsp2538 and Hsp2667 was not sensitive but expression of Hsp845 and Hsp2666 was increased at low salinity (0 and 15, respectively). Expression of the four HSP genes was enhanced at high salinity (90 and 120) and at high temperature independent of salinity. By contrast, low temperature had no significant effect independent of salinity.

Radiation-resistant extremophiles and their potential in biotechnology and therapeutics

Extremophiles are organisms able to thrive in extreme environmental conditions. Microorganisms with the ability to survive high doses of radiation are known as radioresistant or radiation-resistant extremophiles. Excessive or intense exposure to radiation (i.e., gamma rays, X-rays, and particularly UV radiation) can induce a variety of mutagenic and cytotoxic DNA lesions, which can lead to different forms of cancer. However, some populations of microorganisms thrive under different types of radiation due to defensive mechanisms provided by primary and secondary metabolic products, i.e., extremolytes and extremozymes. Extremolytes (including scytonemin, mycosporine-like amino acids, shinorine, porphyra-334, palythine, biopterin, and phlorotannin, among others) are able to absorb a wide spectrum of radiation while protecting the organism's DNA from being damaged. The possible commercial applications of extremolytes include anticancer drugs, antioxidants, cell-cycle-blocking agents, and sunscreens, among others. This article aims to review the strategies by which microorganisms thrive in extreme radiation environments and discuss their potential uses in biotechnology and the therapeutic industry. The major challenges that lie ahead are also discussed.

Cloning and expression analysis of two different LhcSR genes involved in stress adaptation in an Antarctic microalga, Chlamydomonas sp. ICE-L

Light-harvesting complexes (LHCs) play essential roles in light capture and photoprotection. Although the functional diversity of individual LHCs in many plants has been well described, knowledge regarding the extent of this family in the majority of green algal groups is still limited. In this study, two different LhcSR genes, LhcSR1 and LhcSR2 from Chlamydomonas sp. ICE-L, were cloned from the total cDNA and characterized in response to high light (HL), low light (LL), UV-B radiation and high salinity. The lower F (v)/F (m) as well as the associated induction of non-photochemical quenching (NPQ), observed under those conditions, indicated that Chlamydomonas sp. ICE-L was under stress. Under HL stress, the expression of LhcSR1 and LhcSR2 increased rapidly from 0.5 h HL and reached a maximum after 3 h. In LL, LhcSR2 expression was up-regulated during the first 0.5 h after which it decreased, while the expression of LhcSR1 decreased gradually from the beginning of the experiment. In addition, the transcript levels of LhcSR1 and LhcSR2 increased under UV-B radiation and high salinity. These results showed that both genes were inducible and up-regulated under stress conditions. A higher NPQ was accompanied by the up-regulated LhcSR genes, suggesting that LhcSR plays a role in thermal energy dissipation. Overall, the results presented here suggest that LhcSR1 and LhcSR2 play a primary role in photoprotection in Chlamydomonas sp. ICE-L under stress conditions and provide an important basis for investigation of the adaptation mechanism of LhcSR in Antarctic green algae.

Molecular cloning and expression analysis of glutathione reductase gene in Chlamydomonas sp. ICE-L from Antarctica

A cDNA (GenBank ID: GU395492) encoding cytosolic glutathione reductase (named ICE-LGR) in Antarctic microalgae Chlamydomonas sp. ICE-L was successfully cloned by RT-PCR and rapid amplification of cDNA ends technique (RACE). The expression patterns of ICE-LGR under different salinity stresses were determined by real-time PCR. ICE-LGR cDNA has 1913 bp nucleotides with an open reading frame (ORF) of 1458 bp, encoding 485 amino acid residues. The deduced amino acid sequence shows 79% homology with glutathione reductase (GR) of Chlamydomonas reinhardtii. Activity assessment and mRNA expression analysis results showed that activity and expression level of GR in ICE-L cells were up-regulated under either high or low salinity. Together, our results revealed that ICE-LGR might play an important role in Antarctic ice algae Chlamydomonas sp. ICE-L acclimatizing to polar high salinity environment as well as low salinity. These results provide us valuable information on further investigating the molecular mechanism of ICE-LGR.

Characterization and expression analysis of hsp70 gene from Ulva prolifera J. Agardh (Chlorophycophyta, Chlorophyceae)

In the Yellow Sea of China, large-scale green tides have broken out consecutively from 2007 to 2011. Ulva prolifera, the causative species of green tide, showed great ability to acclimate to adverse circumstance. To explore the mechanisms of rapid growth and stress resistance during the bloom, we characterized and analyzed hsp70 from U. prolifera. The results showed that hsp70 gene had 6 exons and 5 introns. The promoter-like region contained multiple cis-acting elements. The transcription of hsp70 was up-regulated by UV irradiation, heat treatment and salinities induction, but less influenced by desiccation. In vitro expression of HSP70 protein and western blot was also conducted, and the recombinant protein will be used in detecting the interaction between HSP70 and related functional proteins in the future. The study suggested that hsp70 could be used in prediction of stress tolerance in algae and monitoring environmental changes.