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Identifier 000430176
Title Metabolic processes and ecological adaptations of phytoplankton in oligotrophic environments : a theoretical and experimental approach
Alternative Title Διερεύνηση μεταβολικών διεργασιών και οικολογικών προσαρμογών του φυτοπλαγκτού στα ολιγοτροφικά περιβάλλοντα : μια θεωρητική και πειραματική προσέγγιση
Author Λιβανού, Ελένη Π.
Thesis advisor Λύκα, Κωνσταντίνα
Reviewer Ψαρρά, Στέλλα
Κοτζαμπάσης, Κυριάκος
Καρακάσης, Ιωάννης
Πήττα, Παρασκευή
Πετυχάκης, Γεώργιος
Χρηστάκη, Ουρανία
Abstract The present thesis investigates two aspects of phytoplankton ecophysiology, which are particularly important in oligotrophic aquatic ecosystems. The first aspect is related to the mechanism of excretion of dissolved organic matter by photosynthetic cells, under varying nutrient availability conditions. The second aspect refers to mixotrophy in pigmented nanoflagellates (PNF) which is defined as the combination of autotrophy through photosynthesis (phototrophy) and heterotrophy through phagocytosis of bacteria (phagotrophy). The results of the present study emerge from the combination of theoretical and experimental approaches. In order to better understand the processes of dissolved organic matter (DOM) release by photosynthetic cells under nutrient-replete and nutrient-limited conditions, a model based on Dynamic Energy Budget (DEB) theory is developed. In the context of DEB theory, two alternative pathways of DOM release emerge from the theory; one relates to growth and lysis of the cells and one to rejection of unprocessed substrates due to stoichiometric constraints. These pathways represent the two conceptual mechanisms of DOM release, which are the passive diffusion and active exudation, respectively. The relative contribution of the two mechanisms to DOM excretion depends on nutrient availability and affects the quality of produced DOM in terms of elemental and molecular composition and size fractionation, which, in turn, may have implications for the bioavailability of the produced DOM to bacteria. Focusing on mixotrophy among PNF, a general modelling framework is developed for describing the nutritional strategies in four types of mixotrophic PNF. The resulting mathematical expressions are incorporated in the aforementioned DEB model for photosynthetic cells. The resulting models describe explicitly, for the first time, the functional diversity of the PNF assemblage taking into account the dynamic interaction of phototrophy and phagotrophy within the four types of PNF. Simulations suggest that the growth dynamics of the four types of PNF are affected differently by the availability of resources (light, dissolved inorganic nutrients and bacteria). A comparison of the rates of organic carbon production and prey consumption by the various PNF types shows that their net ecosystem role, as producers or consumers, depends on the mixotrophic strategy and it can vary as a function of the prevailing environmental conditions. Moreover, in the present thesis, an in-depth investigation of mixotrophy among PNF in the ultra-oligotrophic, phosphorus (P)-limited Eastern Mediterranean Sea (EMS) is performed, employing a two-phase experimental study that combined both field and laboratory nanoflagellates grazing experiments. During the field experiments, the abundance, and grazing effect of pigmented (PNF) and heterotrophic (HNF) nanoflagellates on prokaryotic picoplankton stock (i.e., heterotrophic bacteria (HB) and Synechococcus) were assessed in April 2016 at four stations in the NW Levantine sea and at two selected depths representing the surface and the deeper (deep chlorophyll maximum) euphotic layer. Results showed that HNF dominated prokaryotic picoplankton consumption in the surface layer, whereas, PNF were the dominant grazers of prokaryotic picoplankton in the deeper euphotic layer. A negative relationship between phosphate concentration and ingestion rate of PNF on prokaryotic picoplankton was observed. Subsequent microcosm experiments were performed with nutrient depleted water from the Cretan Sea (EMS) during the late stratified season, in order to asses the responses of heterotrophic and pigmented nanoflagellates, in terms of bacterial grazing, when relaxing the observed P-limitation. In accordance with the results of the field experiments, PNF ingestion rate of heterotrophic bacteria (HB) was significantly reduced in the P-amended treatment, whereas for HNF this rates remained unaffected by P-addition. PNF dominated bacterivory in the unperturbed (control) bottles, whereas, in the P amended treatment HNF and PNF contributed equally to total bacterivory. The experimental results obtained in the present study emphasize the significant and P-dependent role of PNF as consumers of HB in the EMS. The results of the theoretical and experimental work undertaken in the present thesis are synthesized into an idealized microbial food web model. This theoretical analysis aims to further explore qualitatively the effect of phosphorus availability on the trophic interactions and physiological processes of heterotrophic bacteria, heterotrophic and pigmented nanoflagellates in the EMS. The model simulations suggest alternating pathways of P transfer through the microbial food web components during P-replete and P-limited conditions. Pigmented nanoflagellates hold a key role in P transfer through the trophic “by-pass” pathway sensu Thingstad et al. (2005) [Thingstad et al., 2005. Science, 309:1068–1071] under P-limited conditions, whereas the relative role of heterotrophic nanoflagellates in this pathway is more prominent at P-replete conditions. The complicated interactions between PNF and HB in the EMS could be conceptually described as an alternative microbial loop, in respect to the established paradigm involving heterotrophic bacteria and HNF. In this alternative microbial loop, PNF have the functional role of DOM primary producers, mainly through the active exudation mechanism, and, at the same time, share with HNF the functional role of bacterial consumers via mixotrophy. This alternative microbial loop in the oligotrophic system of the EMS operates in a phosphorus-dependent way. The results of this thesis emphasize the importance of the adaptive physiology of photosynthetic protists for the functioning of oligotrophic systems and the importance of taking into account this adaptive behaviour in biogeochemical models in order to predict the response of oligotrophic systems to environmental changes.
Language English
Subject Eastern Mediterranean
Mathematical modelling
Mixotrophy
Pigmented nanoflagellates
Ανατολική Μεσόγειος
Μαθηματική μοντελοποίηση
Μικτοτροφία
Φωτοσυνθετικά νανομαστιγωτά
Issue date 2020-06-17
Collection   School/Department--School of Sciences and Engineering--Department of Biology--Doctoral theses
  Type of Work--Doctoral theses
Permanent Link https://elocus.lib.uoc.gr//dlib/c/a/0/metadata-dlib-1593070529-216043-23454.tkl Bookmark and Share
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