The marine world is a microbial one. Unseen, but nevertheless present in vast numbers in seawater, microbes colonize a range of eukaryotic organisms, such as seaweeds, corals and sponges. They may be small in size, but they have a huge impact on the functioning of these hosts. Together the host and associated microbiome are often thought to form a holobiont: a functional unit that is based on a symbiotic relation. When studying the effects of environmental change on eukaryotic organisms, the interactions with microbes are often overseen. Especially in relation to human-caused environmental change, the holobiont concept may become increasingly important. To investigate the role of microbes in the reaction of the host to elevated temperature and increased nutrient loadings, we propose to study the green seaweed Ulva, Sea lettuce. These seaweeds are important for the food and biotech industries, but can also have a wide impact on the ecosystem by the formation of huge blooms coined “green tides”. In nature, seaweeds never occur without their microbes, so distinguishing between cause and effect is difficult. In a series of field and laboratory experiments, we aim to test if and how the response of an axenic host (a sterile seaweed lacking microbial associates) differs from the response of seaweeds plus their microbes (the holobiont) to climate change and attempt to establish causal relations, as well as testing if the same mechanisms apply to natural systems.