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Effects of light and CO₂ on the toxic cyanobacterium Cylindrospermopsis raciborskii
thesisposted on 10.10.2017, 05:40 authored by Pierangelini, Mattia
Cylindrospermopsis raciborskii is a toxic freshwater cyanobacterium occurring in freshwater ecosystems, worldwide. Due to its toxicity, particular concerns arise when the proliferation of C. raciborskii occurs in water reservoirs destined to human consumption. In Australia, the species C. raciborskii is recognised to be able to produce a particular type of toxin, called cylindrospermopsin (CYN). The amount of CYN and the levels water toxicity, are hypothesized to be susceptible to environmental parameters which drive either the occurrence of C. raciborskii strains with different toxic ability (toxic or nontoxic) or cell physiology. In this work I aimed to describe how environmental drivers, mainly light and CO₂ , affect the growth and the photosynthetic performance of C. raciborskii, establishing their physiological relationship with the cell CYN toxic capability. For these purposes, particular emphasis will be addressed to: 1) Studies of the photosynthetic apparatus of both C. raciborskii toxic and nontoxic strains in order to investigate specific and intraspecific light physiological requirements; 2) Studies of the physiological acclimation of the photosynthetic apparatus in response to light and CO₂ perturbations in the environment, with the aim of understanding the ecological success of the species; 3) Studies of how both light and CO₂ perturbations affect the overall cell metabolism and the ability of C. raciborskii to produce CYN. The results of this work demonstrated that both light and CO₂ environmental conditions are mains drivers for C. raciborskii eco-physiology. Exposed to both different levels of light and CO₂ , C. raciborskii showed elevated physiological changes necessary to adjust and optimize growth and photosynthesis under the different conditions. In ecological terms, such elevated physiological plasticity allows C. raciborskii to tolerate a wide range of light and CO₂ , making the species highly successful. Moreover, the physiological comparisons between toxic and nontoxic strains of C. raciborskii, revealed that different strains of the species have different physiological characteristics and consequently, different environmental preferences. Different (toxic) strains of C. raciborskii showed also a differential ability to acclimatize to (light) environmental changes. These results suggested that that each strain of C. raciborskii is unique and that strains compositions in natural assemblage, must be taken into account. Despite the light and CO₂ affected cell physiology, no effects were observed on the overall CYN biosynthesis of the cell, indicating that the CYN “production” is a constitutive process. For Water Authorities and in relation to water quality management, we can conclude that the water toxicity related with CYN and C. raciborskii, is likely to be regulated by environmental parameters, such as light and CO₂ , which drives the ecological occurrence of C. raciborskii strains with different toxic abilities rather than cell physiology.