UV radiation-induced biosynthesis, stability and antioxidant activity of mycosporine-like amino acids (MAAs) in a unicellular cyanobacterium Gloeocapsa sp. CU2556
Introduction
Cyanobacteria are one of the most dominant photoautotrophic prokaryotes and are major biomass producers in both aquatic and terrestrial ecosystems. The obligate requirement of solar light to perform some energy-dependent processes exposes them simultaneously to harmful doses of short wavelength ultraviolet (UV) radiation in their natural habitats. The increase in solar UV radiation on the Earth’s surface due to ozone depletion [1] is now recognized as a major environmental factor deleterious to all sun-exposed organisms [2], [3], [4], [5]. The high-energetic UV (mainly UV-B: 280–315 nm) radiation has great potential for cell damage either through direct effects on biologically relevant molecules (e.g., DNA and proteins) or indirect effects via the production of reactive oxygen species (ROS) [6], [7], [8], [9]. UV-B radiation is a highly active component of the solar radiation that brings about chemical modifications in DNA by the formation of pyrimidine dimers and strand breaks leading to mutagenesis and loss of normal cellular metabolic functions [8]. Several key physiological processes such as morphology, cell division and differentiation, growth, survival, motility and orientation, buoyancy, N2 fixation, CO2 uptake, photosynthesis, pigmentation and enzyme activity have been reported to be affected by UV radiation [10], [11], [12]. Consequently, several organisms including cyanobacteria that are exposed to solar radiation in their brightly lit habitats have evolved a number of mitigation strategies such as migration and mat formation, active repair mechanisms, production of quenching agents and synthesis/accumulation of UV radiation-absorbing/screening secondary compounds to counteract the negative effects of UV radiation [9], [12], [13], [14], [15], [16].
Biosynthesis of several UV-absorbing compounds in diverse organisms has received much attention for their putative role in UV photoprotection. In the past few years, some MAAs isolated from cyanobacteria and other taxonomic groups [13], [17] have become one of the most promising natural substances in the field of biotechnology and biomedical research [18] due to their strong UV absorption maxima (between 307 and 362 nm), high molar extinction coefficients (ε = 28,100–50,000 M−1 cm−1) and potential antioxidant properties. MAAs are colorless, water-soluble compounds composed of a cyclohexenone or cyclohexenimine chromophore conjugated with the nitrogen substituent of an amino acid or its imino alcohol [19]. The occurrence and/or UV-induced biosynthesis of MAAs is very limited or was not known in the unicellular cyanobacterium, Gloeocapsa sp. There are few reports on the stability of MAAs under different environmental conditions [20], [21], [22]. Moreover, the antioxidant activities of different MAAs in terms of scavenging of free radicals have been scarcely investigated which limits the specific use of MAAs. In the present study, we have investigated the presence of MAAs and its inducibility by UV stress, stability under different physicochemical stressors and total antioxidant potential in a unicellular cyanobacterium Gloeocapsa sp. isolated from stone monuments in the vicinity of Bangkok, facing high irradiation all year round especially during hot summers in their natural habitat.
Section snippets
Organism and growth conditions
The experimental organism Gloeocapsa sp. was isolated from the autotrophic biofilm covering stone monuments (13°44′47′′N 100°29′37′′E) located in the Phra Nakhon district, Bangkok, Thailand. Based on morphological characteristics this strain was identified with the help of standard taxonomic keys and monographs [23]. The morphology of the cyanobacterium was observed by using a light microscope (Seek model SK-100, USA). It is an autotrophically growing non-heterocystous, unicellular
Statistical analysis
All results are presented as mean values of three replicates. All data were analyzed by one-way analysis of variance (SPSS 15.0, Chicago, IL, USA). Once a significant difference was detected post hoc multiple comparisons were made by using the Tukey test.
Analysis of UV-absorbing compounds
A number of MAAs has been reported from diverse taxonomic groups of cyanobacteria and other organisms [13], [15], [17]. In the present study, the occurrence of UV-absorbing/screening compounds was analyzed in the cyanobacterium Gloeocapsa sp. The UV–Vis absorption spectra of a crude methanolic extract as well as partially purified MAA (in aqueous solution) of Gloeocapsa sp. exhibited the presence of a UV-absorbing compound with a UV absorption maximum (UVλmax) at 310 ± 2 nm, within the
Acknowledgments
Rajesh P. Rastogi is thankful to the Graduate School, Chulalongkorn University and Faculty of Science for Post-Doctoral Research Grant. Aran Incharoensakdi thanks the Ratchadaphiseksomphot Endowment Fund of Chulalongkorn University for the Research Grant (RES560530052-FW). We are also thankful to Prof. Dr. R.P. Sinha, Banaras Hindu University, India, for providing UV-cut-off filters. The authors declare that they have no conflict of interest.
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2020, Science of the Total EnvironmentCitation Excerpt :The absorbance of shinorine and M-307 decreased after one hour of exposure to UVR, being both more stable to UVA and UVB irradiation. UVC significantly decreased absorbances on both MAAs (Rastogi and Incharoensakdi, 2014b). Porphyra-334 is highly stable to UVA radiation, and its quantitative analysis on photolyzed solutions showed no formation of intermediate products such as radicals upon radiation, an indicative of good photostability (Conde et al., 2000).