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Final Report Summary - MIXOCARB (The Role of Mixotrophs in the Oceanic Carbon Cycle)

Oceanic life plays a major role in the global carbon cycles. Primary producers convert inorganic carbon into their biomass, while consumers take up organic material to fuel their respiration and release CO2 back into the environment. The two key processes in this cycle, primary production and consumption of organic material, are usually considered to be performed by different organisms, however, some organisms can perform both of them simultaneously. These so-called mixotrophs can have opposing effects on the carbon cycle depending on their balance between autotrophic and heterotrophic nutrition. Mixotrophic microbial eukaryotes have recently been found as both abundant and important for key ecosystem processes in the ocean. This project aimed at a mechanistic understanding of the role mixotrophs play in the marine carbon cycle. To achieve this, mixotrophs have been studied at all relevant organizational levels, ranging from gene expression patterns at the molecular level to the diversity and activity patterns on the ecosystem scale.

Specific objectives entailed elucidating the metabolic integration of nutritional pathways in cultured mixotrophs, as well as assessing the role of mixotrophs in marine communities and identifying the ecological factors controlling their distribution, diversity, and grazing rates. Finally, the synthesis of lab and field work data allowed to assess the relative importance of genetic diversity versus phenotypic plasticity in determining the nutritional strategy of mixotrophs at varying environmental conditions. This multidisciplinary work provided a foundation for future approaches to assess the contribution of mixotrophs to the marine carbon cycle using molecular tools.

An in depth physiological comparison of two marine mixotrophic chrysophytes in controlled laboratory experiments showed striking differences in nutritional strategies of these two closely related organisms. One was a facultative mixotroph that can also grow purely heterotrophically in darkness and the other was an obligate mixotroph requiring both light and prey for growth. The different nutritional strategies resulted in opposing acclimatization responses to prey and light availability and hence would have very different impacts on microbial food web dynamics and ultimately carbon cycling. The genome of the obligate mixotroph provided potential hints for the metabolic reasons underlying this lifestyle and its gene expression response to prey availability revealed a distinct strategy of resource allocation, with upregulation of components involved in prey capture after prey had been depleted and upregulation of components involved in the digestion of prey when cells were actively feeding. Such transcriptional patterns might serve as indicators for the detection of in-situ feeding activities by microbial eukaryotes in the future.

Fieldwork was performed on three oceanographic cruises off the Californian coast spanning a gradient from productive mesotrophic coastal waters to very oligotrophic open-ocean waters at the edge of the North Pacific Gyre. During these cruises an RNA-stable isotope probing (RNA-SIP) approach was used in on-deck incubation experiments to identify both mixotrophic and heterotrophic grazers and to semi-quantitatively assess their relative importance in the microbial community. Incubation experiments showed that microbial eukaryotes were actively feeding on both Prochlorococcus and Ostreococcus, with the most important grazers belonging to MArine STramemopiles (MAST) clades 1,3 4, 7 and 9, chrysophytes and choanoflagellates. Mixotrophic grazers included dictyochophytes and haptophytes, as well as chrysophytes belonging to the same clade as the two Ochromonas isolates studied in culture. An additional enrichment experiment performed with a natural microbial community from the North West Mediterranean Sea targeted bacterivorous protists to study the gene expression of different phylogenetic groups in response to prey availability.

In conclusion this project provided mechanistic insights into the metabolic peculiarities underlying different mixotrophic strategies as well as molecular markers that might be used to assess grazing activities of bacterivorous protists in natural environments. This information will help the urgently required integration of widespread mixotrophic lifestyles into mathematical models of marine biogeochemical cycles. The RNA-stable isotope labeling data further allowed linking in-depth phylogenetic analyses of active marine microbial grazers to in-situ activities of specific phylogenetic groups. Such links between phylogenetic and functional information are fundamental to dissect the complex microbial interactions that control the marine carbon cycle.

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Life Sciences
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