AEGIS: Assessing the interactions between microalgae, microbial grazers, and viruses in a changing ocean

Phytoplankton – single celled plants – are the base of aquatic food webs and play important roles in controlling carbon dioxide concentrations through photosynthesis. In marine ecosystems, there are two main ways phytoplankton can die. One is being eaten by microbial predators, the other is infection and death due to viral infection. In 1 teaspoon of water there are 10 million viruses, infecting phytoplankton, bacteria, and other marine organisms. These viruses have important functions for population dynamics, nutrient cycles, and evolution. How phytoplankton die will have important effects on the fate of elements inside the cell. For example, whether it enters the food web (grazing), or if it is released into the surrounding water due to cells bursting open (viral infection). A major question in marine ecology is what controls the balance between these two pathways, and how climate change will influence this. In nature, phytoplankton can be under attack from predators or viruses at the same time, however in lab experiments typically only one process is studied at a time. In AEGIS, we aim to conduct lab experiments to determine the interactions between phytoplankton, predators, and viruses in different types of phytoplankton. We will also investigate how future climate change scenarios (e.g., temperature increase) will affect the dynamics of phytoplankton growth, predation, and viral infection. To extend our results from the lab to whole ecosystems, we will conduct mesocosm experiments to observe how temperature increase and changes in nutrient flow to marine ecosystems change microbial communities. A mesocosm is an enclosure of water from the environment, which is changed in some way (e.g., temperature increase) allowing us to see the effect this has relative to a control (unchanged) enclosure. These types of experiments allow us to determine the effects in a more natural setting than lab experiments.

In aquatic environments, mortality is a driving mechanism of evolution, population dynamics, biodiversity, and the fluxes of nutrients, organic matter, and energy. The major mortality processes influencing populations of microbial plankton are grazing by predators and lysis by viruses. Climate change, and associated effects, are impacting aquatic ecosystems, including the microbial communities which support these systems. However, there are fundamental questions regarding the interactions between microorganisms and their mortality agents which remain unknown. These interactions are occurring against a background of a changing climate. Due to the essential role mortality plays in ecosystems, any changes in these interactions will significantly impact marine ecosystem functioning. In AEGIS, we seek to understand the interplay between phytoplankton, microzooplankton, and viruses in keystone phytoplankton species, and unravel how this influences microbial population dynamics and biogeochemical cycles in marine ecosystems. Alongside this, we aim to define how climate change impacts, such as rising temperature and altered nutrient dynamics will influence these microbial interactions. Our interdisciplinary approach includes laboratory, field, data synthesis, and modelling methodologies to understand interactions between phytoplankton, microzooplankton and viruses at different temporal and spatial scales. The knowledge gained within AEGIS will revolutionize our understanding of complex marine ecosystems, significantly enhancing our ability to predict the impacts of climate change on our oceans and assess mitigation strategies.