Coral reef ecosystems were originally described as enigmatic hotspots of biodiversity, as seemingly paradoxical highly productive ecosystems residing in marine deserts, and from a purely anthropomorphic viewpoint: just marvels of natural beauty (Darwin 1842). In the year 2016, 174 years later, it is crystal clear that coral reef ecosystems around the world are rapidly declining because of the combined effects of human activities (e.g. coastal development, overfishing, and eutrophication), including the most detrimental, longer-term effects of climate change (e.g. ocean acidification, increases in seawater temperature, and the intensity and number of hurricanes and tropical cyclones. While the overall decline of reefs in response to these stressors has received significant attention, our understanding of ecological processes that shaped coral reefs in the first place, and how such processes change with shifting reef states have not received similar attention. Consequently, changing patterns of reef community composition have been well described, but processes shaping these patterns remain poorly understood. In this ERC project we will recognize a so far largely neglected key ecosystem driver in the cycling of nutrients and energy on coral reef ecosystems: the sponges.
The discovery of "sponge loop" pathway (De Goeij et al. 2013, Science), in which sponges efficiently shunt a significant proportion of the reef's food and energy to higher trophic levels in an otherwise low-food environment, has provided new insight into how sponges are key ecosystem drivers that act as ecosystem “engines”: by efficiently retaining, transforming, and allocating food and energy, they drive communities within the food web of coral reefs. Current reef ecology models, without the inclusion of sponge-driven resource cycling, are therefore incomplete and need be redeveloped. These models are a much-needed foundation to predict future scenarios for tropical, temperate, and cold-water reef ecosystem ecology. However, mechanisms that determine the capacity of sponge engines, how they are fuelled, and how they drive reef communities within the food web are at present largely unknown. Moreover, the sponge loop sparked significant interest, controversy and discussion in the scientific world. In this project, we assessed critical knowledge gaps at the organismal and ecosystem level concerning the integration of sponges as ecological drivers of shallow and deep-sea ecosystems.
We conclude that sponges are important ecological drivers of coral reefs and many other ecosystems where they are abundant. On the Caribbean reefs of Curacao, we estimated sponges to be the largest group of benthic reef organisms. More than half of the biomass and diversity on these reefs is usually missed through traditional 2D photosurveys performed around the world, since these organisms largely live under and within the 3D reef framework. Sponges possess different strategies to feed on the largest source of food in marine ecosystems: dissolved organic matter. DOM produced by algae is processed better than coral-DOM, and with a more prominent role by the microbial symbionts. But, sponge cells are very important in the uptake, the “drinking”, of DOM and translocation to their symbiotic microbes. Additionally, algal-DOM causes a higher stress and immune response than coral-DOM and increases local eutrophication. Sponges are very abundant in our oceans, even form sponge reefs in the deep-sea, and likely influence the world’s carbon cycle. But to confirm the global role of sponges, to manage and protect them, we need much more precious data from ocean floor.
