Cianobatteri e variazioni ambientali: meffetti sui lipidi di membrana e sulle desaturasi
Year: 2008
Type: dissertation
Abstract: Cyanobacteria are a group of prokaryotic microorganisms, including both unicellular and multicellular filamentous species, are able to perform oxygenic photosynthesis as algae and plants. Cyanobacteria constitute the phylum Cyanophyta, which is divided in four orders: Chroococcales, Oscillatoriales, Nostocales and Stigonematales. Their prokaryotic nature, the presence of many cryptic species and the coexistence of different classification systems make particularly difficult the taxonomic identification of these organisms. Currently, in order to characterize cyanobacteria, most authors use a polyphasic approach. Cyanobacteria have specific morphological and physiological features that enable them to colonize different types of substrates and to adapt themselves to great environmental changes. A mechanism by cyanobacteria to face extreme environmental conditions is the maintenance of the fluidity, and so of the efficiency, of biological membranes (homeoviscous adaptation) under variations of the environmental parameters. In fact, biological membranes are seats of several metabolic activities. Among the basic components of cyanobacterial membranes there are four glycerolipids (MGDG, DGDG, SQDG, PG) and several types of carotenoids. The fluidity and the efficiency of cyanobacterial membranes are strictly connected with the unsaturation degree of fatty acids esterified to glycerolipids and with the carotenoid types present. Fatty acid desaturases are enzymes involved in the regulation of fatty acid unsaturation degree. In fact, they are able to insert double bonds in the fatty acid acyl chains. So far, in cyanobacteria four types of fatty acid desaturases have been found (?6 desaturasi, ?9 desaturasi, ?12 desaturasi, ?3 desaturasi), that differ for the position in which they insert the double bond. This Ph.D. project had the aim to characterize a filamentous cyanobacterium isolated from the muds of the Euganean thermal District (PD). In fact, during the maturation process the mud is colonized by many microorganisms, but especially by cyanobacteria, which are considered responsible for most of the therapeutic properties of the mud. The characterization of this filamentous strain (strain ETS-02) was performed with a polyphasic approach, including morphological, ultrastructural, biochemical, physiological and molecular investigations. According to these analyses the organism results to belong to the Oscillatoriales order, to the Phormidiaceae family and to the Phormidium genus. So, it has been named Phormidium sp. ETS-02. This strain shows some peculiarities, as the ability of nitrogen fixation. Besides, it has been found the presence of phycoerythrocyanin, a phycobiliprotein restricted to few cyanobacteria, and a particular form of C-phycocyanin. The Euganean thermal spring environment is subjected to large diurnal and seasonal variations of temperature and light. So, another aim of this Ph.D. project was the study of the adaptation of Phormidium sp. ETS-02 at membrane level, maintaining it to different temperature and light conditions. Particular attention has been paid to the variation in fatty acid and carotenoid composition. Possible variations in the morphology and ultrastructure of ETS-02 at the different maintaining conditions were be considered too. The performed analyses showed that fatty acids and carotenoids contribute in a synergic and complementary way to the maintenance and functionality of Phormidium sp. ETS-02 biological membranes. Moreover, not only the temperature, as expected, but also the light influences the membrane fatty acid composition. In particular, the temperature affects the general fatty acid unsaturation degree, while the light acts on the presence of certain acylic species (?-linolenic acid). Regarding to carotenoids, the myxoxanthophyll content shows the greater variations between the different temperature and light conditions. The final part of this research project aimed to individualize and to determine the completed sequences of the fatty acid desaturase genes of Phormidium sp. ETS-02. The genome of this organism contains three fatty acid desaturase genes: desA, desB and desC, coding the ?12, ?3 and ?9 desaturases, respectively. The obtained amminoacidic sequences have been employed, together with those found in the Genbank database, for a molecular evolution analysis of these enzymes in the phylum Cyanophyta. Clades observed in the phylogenetic reconstruction, obtained using the maximum likelihood method, seem to reflect the phylogenetic relationships between the different taxa and the environment of origin of the cyanobacteria to which the sequences belong.
Author: Katia Sciuto
Language: English
Subfield: Biochemistry
Domain: Life Sciences
Open Access status: green
OpenAlex ID: W40625588