Nitrogen cycling in modern sponges with clues about their role in past oceans

forthcoming

Sponges are the oldest metazoans on our planet and can be found in all aquatic environments, from canals in Dutch medieval towns to deep-sea trenches. Nevertheless, we lack a deep understanding of nitrogen (N) cycling processes in sponges from different taxonomic classes. In fact, glass sponges (class Hexactinellida) I collected in the abyssal Central Pacific have a bulk δ15N signal which is inconsistent with their feeding type and trophic level. Due to the large biomass that sponges reach at some sites and their filtration capacity, they can have large impacts on local biogeochemical cycles. Even during earth history, sponges formed assemblages that likely transformed paleoenvironmental conditions in their direct surroundings. Therefore, I propose to study the N cycle in selected species of the four sponge classes and to solve the mystery of N cycling in glass sponges. By studying N cycling in fossilized sponges, I will infer in which N processes the sponges were involved when they were alive. Based on these results, I will derive the environmental marine conditions present during the time of the emergence of sponges. In pulse-chase labelling experiments, the project SPYCLING will feed shallow- and deep-water sponges different 15N-enriched substrates to decipher their diet preferences using bulk and compound-specific stable isotope analyses. These analyses will be supplemented by DNA-stable isotope probing. We will use metagenomics and metatranscriptomics to determine the symbionts involved in N cycling and to constrain the relevant N processes. SPYCLING will identify the areas inside sponges where N is processed by high resolution mapping of 15N-enriched sponges. N cycling in ancient sponges will be studied by measuring fossil-bound N isotope. This ambitious and ground-breaking interdisciplinary frontier research will constrain the role of modern sponges in N cycling and it will examine how past oceanographic conditions and ancient sponges influenced each other.