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MARINFORSKHAV-Marine ressurser og miljø - havmiljø

MIXsTRUCT - Impact of MIXotrophs on the sTRUCTure of the marine pelagic food web

Alternative title: MIXsTRUCT - miksotrofe encellede organismers betydning for det marin pelagiske næringsnettet

Awarded: NOK 6.8 mill.

In a teaspoon of seawater, one finds a thousand algae, ten thousand bacteria, and hundred thousand viruses, which together constitute the basis for all life in the ocean. They are also pivotal for balancing climate on earth. We have models describing the role of microorganisms in the oceans but mixotrophs, organisms combining different ways of living, are not well accounted for. We therefore try to understand more about algae that combine photosynthesis with grazing. Climate changes include elevated temperature, increased precipitation, and riverine runoff. This result in increased input of colored organic carbon to the sea and may favor a mixotrophic lifestyle. MIXsTRUCT aims to find out how mixotrophy affects the marine food web´s structure and function, and how elevated organic carbon content in the ocean affects the mixotrophs´ performance and impact. To increase public awareness of the ocean and value science per se, we have collaborated with Christi Krybbe primary school. This has improved knowledge of and excitement over the diverse and beautiful marine microbial life for pupils, parents, and teachers, and challenged our own ability to communicate complex processes within the "invisible" part of the ocean. We have participated in four water enclosure experiments, two at coastal marine sites, one in a lake, and one in brackish water. In all experiments we added brown organic substance allowing for investigations of how brownification affect the aquatic microbial community. Our results indicate that many of smallest fotosynthesizers benefit from elevated levels of brown organic carbon. We have performed short- and long-term laboratory experiments using the established algal cultures which also contain bacteria, to compare growth rates and yield between an autotrophic (photosynthesizing only) and mixotrophic algae. Constantly higher bacteria concentration in autotrophic cultures compared to that of mixotrophic is an indication that the latter utilize bacteria as a food supplement, and it also seems like the mixotrophic algae remains healthier and have lower bacteria-algae ratio than the autotrophs. We are currently analyzing all these results to evaluate whether small additional lab experiments should be performed before we summarize the findings in a scientific paper. Another main objective has been, and still is, methodological development. It has been more difficult than anticipated to identify the organisms that both photosynthesize and eat other organisms, and measure what this means in terms of ecological success. To be able to utilize flow cytometry (well suited to count many and small organisms in aquatic samples) we have tested dyes that causes single celled organisms to change color when they eat. One of these was successfully applied to single celled cultures but did not give equally clear results when applied during a seawater enclosure. We suspected unspecific staining to be the reason. More thorough investigations using microscope confirmed that for some species only organelles (food vacuoles) were stained whereas in others several surfaces and membranes were unspecifically stained. The last water enclosure experiment we took part in (spring 2022) gathered international mixotrophy experts. Samples are currently being analyzed Two MSc students have been included in MIXsTRUCT. Benedikte Frøysaa studied how mixotrohic growth mode affected growth rates, biomass- and lipid production and finished in June 2020. Anna Grytaas defended her thesis «Does Brownification and Iron Affect Microbial Communities and Mixotrophic Activities? Experiences From a Mesocosm Experiment» Nov 11th 2020. In our collaborative work with Christi Krybbe primary school, we have introduced the ecologically important, but advanced, terms "trade-off" and "cascading effects". We made use of an ecology game «Run from the copepod” to enhance the understanding of these principles which are important to grasp in order to understand how mixotrophy and the microbial food web works. We presented and played the game for teachers in Vestland at Fagpedagogisk dag and gathered the activities in a training exercise called "Kom deg unna hoppekrepsen! En økologisk lek etterfulgt av dataanalyse" published at the UiB web page "Ekte data". This last year we managed, despite several periods with strict corona restrictions, to visit the school twice to finalize the collaboration (the pupils finished primary school in June). The class first listened to scientific talk given in English which gave them background information to play a computer game demonstrating aspects of a mesocosm experiment. Then they worked in groups to make a poster to describe the experimental set up and explain some results. We have continued this part of the work in subsequent projects.

Marine microorganisms constitute the basis for harvestable resources from the ocean, and are pivotal for balancing climate on earth. By combining theoretical models and empirical research, we have previously demonstrated how differences in the marine food web structure affect the biogeochemistry and functioning in the lower part of the marine pelagic food web, but also identified that we have not yet been able to account for the role that mixotrophic single celled organisms (those that combine phototrophy with phagotrophy) play. Elevated temperature, increased precipitation and riverine runoff are documented consequences of current climate change. Net result of such changes is increased input of organic carbon to marine ecosystems. We argue that this potentially favors mixotrophic organisms, and MIXsTRUCT will therefore combine molecular- and flow cytometry tools, and develop an approach suitable to quantify and identify such organisms in nature. Through different-scaled laboratory experiments and field observations, we will investigate how bacterivorous mixotrophy affects the marine pelagic food web´s structure and function, and to what extent elevated organic carbon content in the ocean affects the mixotrophs´ performance and impact. Through cooperation with nearby primary school (Christi Krybbe) and The Centre for Science Education (UiB ("Skolelaben")) MIXsTRUCT will raise the next generation´s level of knowledge about and excitement over, the diverse marine microbial life. This involvement will increase the awareness of pupils, teachers and parents of the value of science per se, and challenge and improve our own ability to communicate complex processes within the "invisible" part of the marine ecosystem with a non-scientific audience.

Funding scheme:

MARINFORSKHAV-Marine ressurser og miljø - havmiljø