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The current study explored the role of ionic copper (CuCl

Numerous advances have been made to mitigate the repercussion of toxic metals on photosynthetic prokaryotes, i.e. cyanobacteria. One strategy is exogenous supplementation of plant growth regulators (PGRs). This perhaps intensifies the resistance ability of cyanobacteria to encounter the environmental stresses by augmenting their physiological functioning and decontaminant efficiency. Thus, the aim of the present study was to determine if the PGRs auxin (indole-3 acetic acid, IAA; 300 nM) and cytokinin (kinetin, KN; 10 nM) are able to mitigate the toxic environmentally relevant doses of chromium (CrVI; 100 and 150 µM) in cyanobacteria (Nostoc muscorum ATCC 27,683 and Anabaena sp. PCC 7120). The response was assessed by estimating oxidative stress [in vitro and in vivo (superoxide radical; O2·−, hydrogen peroxide H2O2 and malondialdehyde equivalents contents; MDA)], feasible alteration in whole cell oxygen evolution, ascorbate–glutathione cycle and their metabolites. Intracellular Cr accumulation raised the production of ROS by down-regulating the photosynthetic rate due to decreased pigment contents (Chl a; chlorophyll a, Car; carotenoids and PC; phycocyanin) and reduced growth despite the accelerated activity of the ascorbate–glutathione (AsA-GSH) cycle enzymes (viz. APX; ascorbate peroxidase, GR; glutathione reductase and DHAR; dehydroascorbate reductase). Concurrently, exogenous PGRs (auxin and cytokinin) moderated the negative effects of Cr on the growth, by lowering the cellular Cr accumulation and ROS level and by modulating the pigment system, photosynthesis and a further rise in the efficiency of AsA-GSH cycle, thereby acting as positive regulators of the metal detoxification machinery. Thus, exogenous application of PGRs (auxin and cytokinin) could improve the growth in cyanobacteria in Cr-polluted environments, hence maintaining productivity in paddy cultivation as well as having potential to be used as bio-remediator in metal-contaminated aquatic systems.

Background Cyanobacteria are well known for their inherent ability to serve as atmospheric nitrogen fixers and as bio-fertilizers; however, increased contaminants in aquatic ecosystem significantly decline the growth and function of these microbes in paddy fields. Plant growth regulators play beneficial role in combating the negative effects induced by heavy metals in photoautotroph. Current study evaluates the potential role of indole acetic acid (IAA; 290 nm) and kinetin (KN; 10 nm) on growth, nitrogen metabolism and biochemical constituents of two paddy field cyanobacteria Nostoc muscorum ATCC 27893 and Anabaena sp. PCC 7120 exposed to two concentrations of chromium (CrVI; 100 μM and 150 μM). Results Both the tested doses of CrVI declined the growth, ratio of chlorophyll a to carotenoids (Chl a/Car), contents of phycobiliproteins; phycocyanin (PC), allophycocyanin (APC), and phycoerythrin (PE), protein and carbohydrate associated with decrease in the inorganic nitrogen (nitrate; NO3— and nitrite; NO2—) uptake rate that results in the decrease in nitrate and ammonia assimilating enzymes; nitrate reductase (NR), nitrite reductase (NiR), glutamine synthetase (GS), glutamate synthase (GOGAT) except glutamate dehydrogenase (GDH). However, exogenous supplementation of IAA and KN exhibited alleviating effects on growth, nitrogen metabolism and exopolysaccharide (EPS) (first protective barrier against metal toxicity) contents in both the cyanobacteria, which probably occurred as a result of a substantial decrease in the Cr uptake that lowers the damaging effects. Conclusion Overall result of the present study signifies affirmative role of the phytohormone in minimizing the toxic effects induced by chromium by stimulating the growth of cyanobacteria thereby enhancing its ability as bio-fertilizer that improved fertility and productivity of soil even in metal contaminated condition.

Exposure time, metal bio-accumulation, and upregulation of ascorbate-glutathione (AsA-GSH) cycle are the key factor that provide tolerance against heavy metal stress. Thus, the current study is an endeavor to prove our hypothesis that regulation of arsenate (AsV: 50, 100, and 150 mM) and arsenite (AsIII: 50, 100, and 150 μM) toxicity is time dependent (48-96 h) due to modulation in bio-accumulation pattern, AsA-GSH cycle, and non-enzymatic antioxidants in two paddy field cyanobacteria Nostoc muscorum ATCC27893 and Anabaena sp. PCC7120. After 48 h, reduction in growth associated with increased sensitivity index, As bio-accumulation, and oxidative stress was observed which further intensified after 96 h but the degree of damage was lesser than 48 h. It denotes a significant recovery in growth after 96 h which is correlated with decreased As bio-accumulation and oxidative stress due to increased efficiency of AsA-GSH cycle and non-enzymatic antioxidants. Both the species of As caused significant rise in oxidative biomarkers as evident by in -vitro analysis of O2·-, H2O2, and MDA equivalent contents despite appreciable rise in the activity antioxidative enzymes APX, DHAR, and GR. The study concludes that among both forms of arsenic, AsIII induced more toxic effect on growth by over-accumulating the ROS as evident by weak induction of AsA-GSH cycle to overcome the stress as compared to AsV. Further, with increasing the time exposure, apparent recovery was noticed with the lower doses of AsV, i.e., 50 and 100 mM and AsIII, i.e., 50 and 100 μM; however, the toxicity further aggravated with higher dose of both AsV and AsIII. Study proposes the deleterious impact of AsV and AsIII on cyanobacteria N. muscorum and Anabaena sp. but the toxicity was overcome by time-dependent recovery.

Arsenic poisoning in aquatic ecosystem is a global concern that obstructs the productivity of agricultural lands (paddy fields) by targeting the growth of cyanobacteria. The cyanobacteria also tolerate and accumulate elevated concentration of arsenic (As) inside the cell and excrete out from cells in less toxic forms after the successive time interval. Thus to validate this, the study was carried out at two different time intervals, i.e., 48 h and 96 h. Two redox forms of As arsenate (AsV) and arsenite (AsIII) at different concentrations (50, 100, and 150 mM AsV; 50, 100, and 150 μM AsIII) caused substantial reduction in growth, pigments (Chl a/Car and phycobiliproteins: phycocyanin, allophycocyanin, and phycoerythrin), inorganic nitrogen ( nitrate (NO3−) and nitrite (NO2−)) uptake, activity of enzymes (NR, NiR, GS, and GOGAT) of nitrogen metabolism, biochemical constituents (protein, carbohydrate, and exopolysaccharide (EPS) contents of Nostoc muscorum, and Anabaena sp. PCC7120. The tested doses of AsV and AsIII after 48 h of exposure exhibited adverse impact on these parameters, but after 96 h with lower doses of AsV (50 mM and 100 mM) and AsIII (50 μM and 100 μM), significant recovery was recorded. Contrary to this, at higher dose of AsV (150 mM) and AsIII (150 μM), the adverse impact was further aggravated with increasing time exposure. Contrary to the activity of NR, NiR, GS, and GOGAT, GDH activity (alternative NH3+ assimilating enzyme) was found to increase, and after 96 h, the activity showed declining trend but still higher than the control. The biochemical constituent EPS (first protective barrier) under scanning electron microscope showed more accumulation of dry adsorbent in the case of AsIII stress hence displayed more toxic nature of AsIII than AsV. The study concludes that with increasing time exposure, the recovery in growth and related parameters mainly at lower doses of AsV and AsIII points toward adaptability of cyanobacteria which was more pronounced in Nostoc muscorum.

The current study was undertaken to elucidate the impact of the herbicide pretilachlor (3 µg ml−1 and 6 µg ml−1) on cyanobacteria, Nostoc muscorum ATCC 27893 and Anabaena sp. PCC 7120 under three levels of photoacclimatization (suboptimum, 25 µmol photon m−2 s−1; optimum, 75 µmol photon m−2 s−1; and supra-optimum, 225 µmol photon m−2 s−1) by analyzing certain physiological (biomass accumulation, photosynthesis, Chl a fluorescence and respiration) and biochemical parameters (photosynthetic pigments‒ chlorophyll a, carotenoids and phycocyanin; reactive oxygen species‒ O2•¯, H2O2, lipid peroxidation; antioxidant system‒ superoxide dismutase, peroxidise, catalase and glutathione-S-transferase). The light conditioning played the most prominent role in deciding the extent of herbicide toxicity on both the tested cyanobacteria as the maximum toxicity was observed in suboptimum light acclimatized cyanobacterial cells corroborated by the least growth in the same cells. The impact of pretilachlor treatment on photosystem II photochemistry viz. φP0, Ѱ0, φE0, PIABS, ABS/RC, TR0/RC, ET0/RC and DI0/RC was also altered by light acclimatization. The percent rise in oxidative stress markers (SOR and H2O2) and consequent lipid peroxidation (MDA equivalents) were also highest in suboptimum light acclimatized cells exposed to pretilachlor which could not be prospered with compatible antioxidant performance. Conversely, supra-optimum light acclimatized cells of both the cyanobacteria was found to accelerate the activities of all the studied enzymes and thus able to counterbalance the pretilachlor toxicity and supported the healthier growth.

In recent days, the rapid development of industries and uncontrolled anthropogenic activities increased the concentration of various heavy metals in all types of environments, that is, aquatic as well as terrestrial/soil. Therefore, organisms thriving in such environments regularly face high metal concentrations which in turn adversely affect their growth and development. Among these organisms, cyanobacteria are very precious due to their tremendous ability to perform oxygenic photosynthesis. Apart from this, a number of cyanobacteria are well known to fix atmospheric N2 and play important role in geochemical cycling of N2. In this chapter, we have provided information about the impact of heavy metals on the important physiological processes affecting biomass accumulation. We have also tried providing the strategies adapted at cellular (secretion of some chelates) as well as molecular (OMICS approach) levels by cyanobacteria to overcome the stress generated by heavy metals. However, there is still a dearth of knowledge related to mechanisms that help in establishing this tolerance/adaptation, most of the studies are being performed in plant system, while cyanobacterial system is still not much interrogated at the molecular level so that they can be used in several metal remediation programs. The future studies must utilize multidisciplinary approach including OMICS to establish the role of these cyanobacteria under heavy metal stress tolerance.