Community Research and Development Information Service - CORDIS

Periodic Report Summary 1 - 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 eukaryotes have recently been found as both abundant and important for key ecosystem processes in the ocean. This project aims at a mechanistic understanding of the role mixotrophs play in the marine carbon cycle. To achieve this, mixotrophs will be 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.
In particular, the objectives include elucidating the metabolic integration of their nutritional pathways in a molecular study using transcriptome analysis (objective 1), assessing the role of mixotrophs in the natural plankton community and ecological factors controlling their distribution and diversity in the North-East Pacific (objectives 2-4). This will be achieved through quantification of metabolic rates of mixotrophs in their natural communities, allowing to assess their contribution to biogeochemical cycles (objective 2), identification of the taxonomic composition of the mixotrophic community (objective 3), and isolation of mixotrophs into culture to enable a more in-depth study of these organisms (objective 4). Finally, from the combined evaluation of data collected for the individual objectives the relative importance of genetic diversity versus phenotypic plasticity in determining the nutritional strategy and variability of mixotrophs in the natural community shall be assessed (objective 5). This multidisciplinary approach will provide the foundation necessary for linking molecular processes to the contribution of mixotrophs to the marine carbon cycle.
In the outgoing phase of the project the physiological and transcriptomics experiments with the cultured marine mixotroph Ochromonas sp. CCMP1393 addressing objective 1 have been completed. Additionally, an in depth physiological comparison between the two strains of Ochromonas CCMP1393 and CCMP2951 has been performed evaluating differences in the nutritional strategy and phenotypic plasticity in two closely related organisms (objective 5). Objectives 2-4 were addressed on three oceanographic cruises off the Californian coast. This allowed 3 large scale on-deck incubation experiments to be performed with natural microbial communities originating from different oceanic regions. Stable isotope labeled potential prey organisms have been added in the experiment to identify both mixotrophic and heterotrophic grazers and to semi-quantitatively assess their relative importance (objective 2 and 3). A first attempt to isolate mixotrophs into culture (objective 4) has not been successful so far.
Very different physiological responses to resource availability have been found in the two closely related strains of Ochromonas sp. While CCMP2951 is a facultative mixotroph able to grow purely heterotrophically in darkness, CCMP 1393 is an obligate mixotroph that requires both prey and light for growth. These results highlight the large differences in nutritional strategies that can be found in closely related organisms that would likely result in very different impacts on the microbial food web and ultimately carbon cycling. The genome of the obligate mixotroph lacks genes involved in nitrate utilization and the transcriptome experiment identified 1090 genes that were differentially expressed in response to food availability. Many of these have unknown or poorly characterized functions. Further genome and transcriptome analysis are expected to identify metabolic strategies underlying obligate mixotrophy and specifically to identify a putative set of genes involved in phagotrophy. Information on nutritional strategies of mixotrophs in natural environments is very scarce and the identification of molecular markers for specific strategies as performed here will allow to detect nutritional strategies in nature. This is crucial information required for calibration of mathematical models of marine biogeochemical cycles.
Oceanographic data from the three cruises along a productivity gradient in the NE Pacific Ocean showed oligotrophic warm surface water masses reaching areas closer to shore in 2015, while in 2016 colder mesotrophic waters supported a large phytoplankton bloom in the near-shore regions. Rates of primary productivity spanned two orders of magnituted along this transect. Tiny photosynthetic eukaryotes (picoplankton <2µm in size) such as the prasinophyte Ostreococcus reached very high abundances close to shore in 2016, while open ocean communities where dominated by the picocyanobacterium Prochlorococcus. Incubation experiments showed that microbial grazers were actively feeding on both Prochlorococcus and Ostreococcus and environmental conditions were therefore ideal to address the objectives of this project. The experiments performed with natural plankton communities will further identify microbial grazers, including presumably important mixotrophic grazers, and quantify their grazing rates across a productivity gradient. These will be the first data to allow linking an in-depth phylogenetic analysis of active marine microbial grazers to in-situ activities of specific phylogenetic groups. The link between phylogenetic and functional information will form a basis to dissect the complex microbial interactions that control the marine carbon cycle.

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Life Sciences
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