Diel feeding rhythms in marine protistan grazers

Resumen

Protistan grazers are a key component of marine planktonic food webs. These marine protists are the major grazers of pelagic primary production in the oceans and, therefore, they display a crucial role in marine biogeochemical cycles as pivotal intermediaries of the energy and mass flux from primary producers to higher trophic levels. Notwithstanding their relevant role in the global pelagic system, some key aspects related to their trophic behaviour remain still poorly understood. Among these features, diel feeding rhythms are of relevant importance as they represent the coupling between the cycles of primary production and the feeding cycles of their predators and, consequently, they strongly condition the carbon flux mediated by marine protistan grazers and the dynamics of planktonic food webs. This Ph.D. Thesis aims to deepen our knowledge of the diel feeding rhythms in marine protistan grazers, exploring their occurrence and the mechanisms that generate and modulate this rhythmic behaviour. Accordingly, we first investigated the existence of diel feeding rhythms in diverse species of heterotrophic and mixotrophic protistan grazers (the dinoflagellates Gyrodinium dominans, Oxyrrhis marina and Karlodinium armiger, and the ciliates Strombidium arenicola and Mesodinium rubrum). Then, we evaluated how intrinsic characteristics of the prey (Rhodomonas salina), including the growth phase and the diel variations on its stoichiometric composition, as well as own characteristics of the grazers, such as their previous feeding history and the timing for cell division, may be causal factors or perhaps regulate the diel feeding activity of marine protists. We also assessed the effect of extrinsic factors, such as the prey concentration, the light and the risk of predation, on the feeding rhythm of marine protists. Finally, we conducted field experimentation to study the diel feeding rhythms of protistan grazers in a natural ecosystem, the Gullmar Fjord (Sweden). As major conclusions of the present Ph.D. Thesis, we found that there might not exist a unique underlying mechanism causing the different patterns of diel feeding rhythms we observed in marine protistan grazers. Instead, it appears that marine protists species might have developed feeding rhythms largely conditioned by their physiological and behavioural characteristics, as well as by the ecological conditions from their original habitat, which might determine the factors by which it is modulated.