Differential DOM-cycling through Microbes and Sponges

ended

Anthropogenic disturbances, such as nutrient pollution and overfishing, threaten coral reefs by facilitating community shifts from reef-building corals to algae, increasing production of algal-derived dissolved organic matter (DOM). DOM is the largest food source on reefs, but cannot be consumed by most reef animals. Instead, microbes and sponges transform DOM into particles that re-enter the food web. These recycling pathways, termed the “microbial and sponge loops”, are understood as crucial ecosystem adaptations for reefs to thrive as biological hotspots under nutrient-poor conditions. However, the increased load of algal-DOM on degraded reefs has been associated with the inefficient growth of pathogenic microbes—the “microbialization of reefs”. Sponges have been hypothesized to further stimulate this microbialization by feeding on increased algal-DOM and accelerating algal proliferation via the subsequent release of nutrients. Through this “vicious circle”, microbes and sponges may jointly promote the degradation of coral reefs even faster than the usual, and much more studied, suspects of climate change (i.e., rising seawater temperature and ocean acidification). I aim to determine whether planktonic microbes and sponges compete over DOM (i.e., feed on same or different DOM components) under past, present, and future projections of available DOM. Thereto, I will characterize the composition of different DOM types (e.g., coral- and algal-DOM) and mixtures of DOM representing past, present and future DOM-producing communities and assess their availability (uptake) and utilization (metabolic processing) by planktonic microbes and sponges. I will further assess the ability of planktonic microbes to utilize sponge-processed water and vice versa. With this project I will lay the foundation to integrate microbes and sponges as understudied key ecosystem engineers into coral reef food web frameworks that will allow to elucidate the fate of future coral reefs.