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Close encounters in the phycosphere: Microscale syntrophic interactions between diatom and diazotroph populations

Periodic Reporting for period 1 - Microsyndia (Close encounters in the phycosphere: Microscale syntrophic interactions between diatom and diazotroph populations)

Reporting period: 2020-08-01 to 2022-07-31

Marine unicellular algae, also named phytoplankton, are key to the world’s oxygen production and are also de basis of the oceans’ ecosystems. These tiny creatures, however, do not exist on their own, but are surrounded instead by bacteria that can influence their activity: some can be pathogenic, but many are also beneficial, and can enhance their productivity in difficult conditions. It is very likely that the interactions between bacteria and unicellular algae are species-specific. However, in highly diluted aquatic environments (such as the open ocean), it is not understood how these interactions are established. This project focuses on those interactions that can alleviate a nutritional need for the algae, e.g. by bacterial supply of nutrients or vitamins in a nutrient-diluted environment. In particular, it addresses the spatial dynamics of bacteria and microalgae over time with the aim to understand 1) how these associations are established and 2) to quantify the consequences (in terms of growth and activity of both bacteria and algae) of these interactions. A parallel goal of the MSCA Individual Fellowship is to foster the development of the individual researcher.
During the project, most effort was directed at establishing a system to work with and answer the above questions. I aimed at establishing a simplified system consisting of one algae species and one bacterial species that complement each other’s nutritional needs (what we termed a syntrophic interaction). I isolated new species of bacteria from the microalgae’s microbiome and studied how they affect the microalgae’s physiology. One of the isolates is capable of supplying vitamin B12, a vitamin that the algae cannot synthesise, and supports its growth in vitamin-deficient media. In turn, the alga’s photosynthetically fixed carbon provides organic matter to the bacterial partner to survive. We developed a microfluidic chip to be able to monitor single-cell growth of both partners in these syntrophic setting. In addition, microscopy experiments were performed to observe the behaviour of the bacteria: i.e. its swimming and attachment dynamics to the microalgae. The bacteria appear to be capable of both, but more experiments are needed to confirm preliminary observations and conclude their effect on the co-culture. After optimization of both the microfluidic chip and the visualization approach, results will be disseminated in scientific manuscripts that will be published in open access journals.
Another isolated bacterial species is capable of attachment to the microalgae by long appendages, named stalks. In this case, the attachment dynamics were easier characterized, but we are still working on understanding the effect of this association. It is not a pathogenic one, but we cannot confirm a benefit for the algae yet.
So far, the results presenting the latter system have been presented in two international scientific conferences (both in 2022): I attended the “Microbial Ocean Biophysics” international workshop (Spain, May 2022) and the Gordon Research Conference “Marine Microbes” (Switzerland, June 2022). I have also participated in outreach activities to promote marine microbiology to children and teenagers: I participated in the planning and organization of a scientific workshop about marine microbes for teenagers, as part of the outreach event Scientifica (September 2021), organized by ETH and UHZ. I also co-led and presented a laboratory activity on marine phytoplankton and light microscopy to schoolgirls as part of the event Kangaroo goes Science (June 2022), organized by the Kangaroo Switzerland Association.
The co-cultures established are unique and hold a significant potential for foundational research on bacteria-diatom dynamics. The microfluidic chip and visualization approach has been developed to fulfil this potential and is innovative with respect to the state of the art, since this has not been attempted with bacteria-diatom co-cultures. Due to unexpected delays in producing results, further work is planned for, beyond the duration of the funding period. The aim of the extended duration of the project is to finalize experiments, write and publish scientific manuscripts to disseminate the insights from our investigations.
These insights offer unprecedented insights to microscale dynamics of microbial interactions at the onset of inter-kingdom symbioses in aquatic environments. The co-cultures established provide knowledge on the symbiotic potential of heterotrophic bacteria and will shed light to their potential lifestyle in marine environments. Our results also provide important information directly linked with low-nutrient algal biomass production that could be further developed for algae biomass production (section 2.1). Therefore, this project has the prospect to have an impact on two expert audiences: international scientific community and the biotechnology industry sector.