Recognition, signalling and response of the diatom Fragilariopsis to epibiotic bacterial colonization.

• Applicants: Professor Dr. Tilmann Harder; Jan Tebben, Ph.D.
• Subject Area: Oceanography
• Term: from 2020 to 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 442925794

The diatom Fragilariopsis dominates phytoplankton in the Southern Ocean and plays a central role in the significant (15% of global) primary productivity of Antarctic regions. Marine primary production is fuelled by spatially close interactions between microalgae and bacteria and driven by their reciprocal needs for essential trace elements, micro- and macro-nutrients. Several recent microbiome studies of diverse phytoplankton species suggest that they harbour unique microbiomes, which are consistent and specific within phytoplankton species and across temporal scales. Yet, the mechanistic processes causing these associations at different levels of specialization are poorly understood.The main hypothesis addressed in this proposal postulates that Fragilariopsis relies on, or employs comparable recognition and transduction strategies in response to environmental bacteria as animals and plants, despite their phylogenetic distance to the diatom. Thus, in theory this analogy allows a cross-disciplinary transfer of methodology from well investigated immunity and microbiome science in animals and plants to a phytoplankton species.The concept of active recognition, selection and filtering processes, which supposedly steer the assembly of Fragilariopsis-associated microbiomes, is well understood in the context of immunity in multicellular organisms, even in multicellular marine macroalgae, but not in diatoms. Such responses are triggered via receptor-mediated recognition of bacterial elicitors and mediated by well-investigated signaling cascades (e.g. via nitric oxide (NO)). As a consequence, this leads to the upregulation of a variety of quantifiable stress markers, such as intracellular reactive oxygen species or a shift of the glutathione to glutathione-disulfide ratio. Partially, these stress markers have been verified in microalgae and first evidence of NO signaling and NO synthase-like gene sequences in marine diatoms is emerging. This proposal thus addresses pertinent DFG SPP questions, such as the need of “improved understanding of polar processes and mechanisms'' and the “response [of SO organisms] to environmental change”. Specifically, we will isolate, culture and identify various bacteria from Fragilariopsis microbiomes. In parallel, different Fragilariopsis ecotypes are axenified. In co-culture assays, we will employ known and well established chemical and molecular tools to characterize and quantify intra- and extracellular responses of Fragilariopsis to bacteria and bacterial molecular elicitors, such as bacterial lipopolysaccharides.The short term goal of this proposal is to corroborate an active response of Fragilariopsis to bacteria or bacterial signaling molecules. The long-term goal of this work is to analyse the holobiont assembly and challenge the recognition process by experimentally adjusting future SO climate regimes to understand the susceptibility of the Fragilariopsis holobiont to predicted climate change scenarios.

• DFG Programme: Infrastructure Priority Programmes
• Subproject of: SPP 1158: Infrastructure area - Antarctic Research with Comparative Investigations in Arctic Sea Ice Areas
• Co-Investigators: Professor Dr. Bánk Beszteri; Professor Dr. Ralf Dringen; Dr. Scarlett Trimborn