On 30 June 2021, Martijn Bart will defend his PhD thesis
The deep sea forms by far the largest part of our biosphere. Yet, the majority of deep-water ecosystems is still relatively undescribed. In recent years, the seafloor of the North-Atlantic Ocean has been found to be abundantly inhabited by sponges. Sponges are unique animal-microbe symbioses that form large deep-sea sponge grounds, create sponge reefs and are major components of deep‐sea coral reefs. These ecosystems increase local biodiversity and productivity, and have been suggested to play a crucial role in the (re)cycling of elements such as carbon, nitrogen and silica. The aim of this thesis was to assess the carbon and nitrogen cycling through deep-sea sponges, and to evaluate the ecological impact of resource cycling within deep-sea sponge ecosystems. Using ex situ experiments, in situ incubations and linear inverse modelling, we found that the majority (92-100%) of the diet of deep-sea sponges consists of dissolved organic matter (DOM), and that the processing of food varies between deep-sea sponge species. The capacity of deep-sea sponges to take up DOM in large quantities makes them important players in the recycling of energy in deep-sea ecosystems. We found that DOM assimilated by sponges is eventually released as particulate detritus (‘sponge poo’) which is in turn consumed by other deep-sea organisms. By transferring the energy and nutrients stored in DOM to other organisms in the food chain, sponges bring life to the perpetually cold and dark ecosystems in the deep sea.