| Cyanobacteria and microalgae are progressively becoming attractive bionanofactories that produce renewable biochemicals and biofuels in industry due to their ability to remediate a diverse range of environmental pollutants and catalyze reactions for biotransformation of various organic substrates.We thus attempted to grow,characterize and successfully apply these bioremediation and biotransformation capabilities of cyanobacteria and microalgae for sustainability of environment.In the first part of my dissertation,siderophilic cyanobacterium Leptolyngbya strain JSC-1 inhabiting iron-rich hot springs is studied to provide insight into the mechanism of iron homeostasis and alleviation of oxidative stress.As known to all,iron intoxication induces severe oxidative stress by reactive oxygen species(ROS)produced in cyanobacteria,leading to membrane lipid peroxidation,altered morphology,impaired photosynthesis and other oxidative stress injuries.Given these stresses,mitigation of ROS is a prerequisite for all aerobic organisms.In this part,we investigated the morphophysiological and molecular mechanisms enabling this cyanobacterium to cope with iron-induced oxidative stress.Strain JSC-1 biomineralized extracellular iron in exopolymeric sheath(acting as first line of defense).It deposited intracellular iron into polyphosphate bodies(second line of defense)to minimize the burden of free ferric ions.Superoxide dismutase(SOD),catalase(CAT)and peroxidase(POD)activities,together with bacterioferritin and total protein contents up-regulated in response to iron elevation,which forms a third line of defense to mitigate ROS.The fourth line of defense is formed by the whole metabolic system of JSC-1 cells.Differential gene expression analysis of JSC-1 indicated up-regulation of 94 and 125 genes and down-regulation of 89 and 183 genes at low(4 ?M)and high(400 ?M)iron concentration,respectively.The differentially expressed genes(DEGs)were enriched in 100 KEGG pathways and were found to be involved in lipopolysaccharide and fatty acid biosynthesis,starch,sucrose,chlorophyll and other metabolic pathways.Together with metabolic reprogramming,JSC-1 established a unique multiline defense system that allows JSC-1 to withstand severe oxidative stress.These findings have revealed potential survival strategies of ancient microorganisms inhabiting similar environment present in early earth history.In the second part of my study,I used cyanobacterium Leptolyngbya strain JSC-1 which served as a unique model for silver biomineralization and its bioaccumulation.Ionic silver is a potential hazard to aquatic life forms with the increasing use of silver based materials.The need for development of sustainable and ecofriendly processes to minimize the toxic effects of the free ions burden has now become a scientific consensus.Here I report our latest results in cyanobacterium Leptolyngbya JSC-1 research,including the tolerance towards toxic doses of silver,its extracellular biomineralization and silver nano-deposits formation inside the cells,with emphasis to its potential impacts on the environment.To implement this study,scanning electron microscopy(SEM)and energy dispersive x-ray spectroscopy(EDS)analysis were used to reveal the extracellular biomineralization of soluble silver(1-100 ?M)into corresponding nanoparticles(50-100 nm in diameter)by JSC-1,while X-ray photoelectron spectroscopy(XPS)examination divulged the presence of both Ag+and Ago in extracellularly biomineralized silver,depicting a mixture of both AgxO and elemental Ag.The scanning transmission electron microscopy(STEM),EDS and elemental mapping were employed to visualize the formation of intracellular silver nanoparticles.The silver tolerance in JSC-1 was further exploited to form a novel protocol for isolation and maintenance of axenic culture of this filamentous cyanobacterium.Consequently,this capability of silver biomineralization by JSC-1,both extra-and intra-cellularly might be useful for the investigation of Ag resistance in cyanobacteria.It might also be a sustainable alternative for heavy metals bioremediation in aquatic environments.The last part of this thesis focuses on dehalogenation of halogenated organic substrate(3-Chloropropiophenone)using both free and hydrogel entrapped microalgae Chlorella emersonii(211.8b)as biocatalysts.This study aimed at successful immobilization of Chlorella emersonii(211.8b)cells and to assess their biotransformation efficiency.Aquasorb(used as entrapping material in this study)was found to be highly biocompatible with no undesirable and adverse effects on the cell growth and viability.The results also showed a promising number of entrapped cells in terms of colony forming units(CFUs=2.1 × 104)per hydrogel bead with a comparable growth pattern to that of free cells.In comparison to free cells,an enhanced biotransformation of 3-chloropropiophenone into 1-phenyl-1-propanone by hydrogel entrapped 211.8b was observed,mainly due to high stability,better growth and improved tolerance towards stressed conditions.It was determined by GC-MS analysis that there is no activity of hydrogenase that could transform 1-phenyl-2-propenone into 1-phenyl-1-propanone because after 12 h the ratio between them(0.83±0.02)remained constant throughout.Furthermore,after 12 h of treatment,1-phenyl-1-propanone was found to be the major component(-84%)of the total dehalogenated product at every interval of time.The product,1-phenyl-2-propenone,was excluded from the biotransformation as it was also found in the control group(due to spontaneous generation).Hence,the enhanced synthesis of 1-phenyl-1-propanone can be ascribed to an enzymatic activity(a dehalogenase)from entrapped Chlorella(211.8b).Therefore,the aquasorb based immobilization of microalgae is highly recommended as an effective tool for exploiting their potentials of biocatalysis specifically when free cells activities are seized due to stress. |
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