BIOMES

Biogeochemical Impacts Of Mixotrophy and Ecological Stoichiometry

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 Obiettivo del progetto (Objective)

'Marine phytoplankton and zooplankton communities mediate a large flux of carbon from the atmosphere into the ocean. This biogeochemical process has traditionally been modelled with the focus primarily on the supply and consumption of essential nutrients. I propose to broaden this approach to resolve important processes occurring at higher trophic levels. I plan to use observations and food-web models to examine how trophic interactions influence and control large-scale ecology and biogeochemical cycling. I will address two outstanding questions that have not yet been examined from this perspective.

1) Marine ecosystem models have traditionally classified phytoplankton as strictly photoautotrophic consumers of inorganic nutrients. This view is challenged by recent observations showing that the majority of photosynthetic plankton also graze upon a large fraction of the bacterial community. These ``mixotrophic' species combine autotrophic and heterotrophic nutrition. They have a large capacity to increase the flux of carbon through the marine biota, but their ecology is not yet clearly understood. I will explore the mechanisms by which mixotrophs are able to compete in different environments, and I will examine how their success affects the cycling of elements at the ocean basin scale.

2) Phytoplankton show large variability in their elemental composition. These changes are driven by adaptation and acclimation to different environments, and hence the changing balance of marine communities should affect elemental ratios across broad environmental gradients. This elemental variability is an important component of the global carbon cycle, but the underlying links between physiology, ecology and biogeochemistry are still poorly understood. I will ask how biodiversity can facilitate observed ecological and biogeochemical distributions on the global scale, and examine how physiological changes may affect the biogeochemical function of marine communities.'

Introduzione (Teaser)

Marine plankton influences the movement of carbon from the atmosphere to the ocean. This biogeochemical process has traditionally been modelled on the supply and consumption of essential nutrients, but an EU-funded project took a new approach.

Descrizione progetto (Article)

Conventional models show marine plankton belonging to one of two groups: tiny plants known as phytoplankton or the zooplankton that feed on them. However, this approach is challenged by the existence of mixotrophic organisms, which can use different sources of energy and carbon. For example, the majority of photosynthesising plankton also graze on bacteria.

The 'Biogeochemical impacts of mixotrophy and ecological stoichiometry' (BIOMES) project studied the ecological mechanisms that influence mixotrophy. Observations and food web models were used to investigate how interactions between trophic levels in the food chain affect large-scale ecology and biogeochemical cycling. The trophic level of an organism can be described as the position it occupies in the food chain.

A computer model was created to show an accurate representation of mixotrophy based on a continuous spectrum from phytoplankton to zooplankton. It was developed using a 1D water column model before being extended to a full global ocean model. The model will help to clarify the mechanisms that allow mixotrophs to compete in different environments and how their success affects the cycling of elements.

Furthermore, changes in plankton elemental ratios were investigated across large areas, and a model was developed to understand competition between marine nitrogen fixers and other phytoplankton. This elemental variability is an important component of the global carbon cycle. Therefore, the model will help to explain the links between physiology, ecology and biogeochemistry.

Results showed that mixotrophs can take advantage of resource pools that were previously thought of as unavailable to phytoplankton or zooplankton. This mechanism could have important implications, because the model results suggest mixotrophy enables much more efficient transfer of biomass to higher levels in the food chain.

The work conducted by BIOMES has provided scientists with new insights into how marine plankton affects biogeochemical cycles in the ocean. It will also help to increase understanding of the global carbon cycle.