Feeding in mixoplankton: from lack of a standardized field technique to the impact of abiotic factors

Memoria de tesis doctoral presentada por Guilherme Duarte Ferreira para obtener el título de Doctor por la University of Copenhagen, realizada bajo la dirección del Dr. Albert Calbet del Institut de Ciències del Mar (ICM-CSIC) y del Dr.Per Juel Hansen de la Københavns Universitet.-- 148 pges, figures, tables, appendixThe purpose of this PhD thesis is to contribute to the understanding of the recently formulated “mixoplankton paradigm” (Appendix). This paradigm recognises the importance of phago-mixotrophs in marine waters and stresses the importance of including them in biogeochemical models in the future. The first objective was to study techniques to quantify grazing by different types of mixoplankton. Therefore, I explored three independent approaches to quantify the grazing impact of mixoplankton in the field and attempted to characterise the consequences of light and temperature on the obtained rates. I started by following the incorporation of Live Fluorescently Labelled Algae (LFLA) over time (Paper I) in several protistan grazers, attempting to determine possible caveats of the technique in the determination of mixoplanktonic herbivory. The results indicate that LFLA can be used to track diel ingestion and digestion rates in several species. However, I also showed that LFLA must be used with caution in the field, due to issues surrounding selectivity and feeding mechanisms. Therefore, as this method was far from perfect, I also explored a different approach, which relied on the selective inhibition of the mitochondria by rotenone (Paper II). According to the mechanism of action, rotenone was expected to display a stronger effect on protozooplankton than on mixoplankton. The results demonstrated that, at low concentrations of rotenone, mixoplankters survived better than protozooplankters, despite not being able to feed. As the latter was still able to eat at inhibiting concentrations for mixoplankton, rotenone could not be used to discriminate between trophic modes of nutrition. The final attempt focused on the ubiquitously used dilution technique (Paper III). The results of a laboratory food web simulation, including mixoplankton as well as protozooplankton as grazers, evidenced that chlorophyll is an inadequate proxy for phytoplankton biomass when mixoplankton are present. In addition, the results showed that selectivity and intraguild predation affect the outcome of the technique. Still, the data produced may be of much help for building in silico models able to disentangle the grazing impact exerted by mixoplankton and protozooplankton in the field. The second objective of the thesis was to understand how light and temperature modulate grazing in mixoplankton and protozooplankton. The former was addressed by conducting day/night incubations with a few species (Papers I and III), but also in the complete absence of light during an entire day (Paper III). The results support the existence of diel feeding rhythms, which are partly retained in the absence of light, even though this factor is critical for the correct expression of feeding for both protozooplankton and mixoplankton. The experiments from Paper IV enabled the determination of the effect of temperature on mixoplankton and protozooplankton in terms of growth and grazing, but also in terms of carbon dioxide sequestration and production. The results of this last paper showed that mixoplankton (but not protozooplankton) deviate from the canonical Metabolic Theory of Ecology. Indeed, the former become more phototrophic in a warming scenario, unlike past assumptions, which supported a higher degree of phagotrophy at higher temperatures. Protozooplankton displayed higher optimal temperatures for both growth and ingestion than their mixoplanktonic counterparts did. Altogether, this thesis contributed to the ongoing process of unveiling the ecological role of mixoplankton, in particular in the situations where their grazing impact is concernedPeer reviewe