Fluorescence dissolved organic matter (fDOM) analysis of inhalant and exhalent water samples from sponges studied at an artificial reef off the coast of the Florida Keys, USA in August 2021

Coral reefs are biodiverse and productive ecosystems that are found in typically oligotrophic (low nutrient) environments. The focus of research on nutrient acquisition and partitioning among coral reef organisms has, historically, been focused on corals and their symbiotic zooxanthellae. For example, different clades of zooxanthellae are adapted to different irradiance levels which facilitates coexisting species of corals. Comparatively very few studies have asked if and how coexisting sponges and their symbiotic microbial communities partition nutrients (i.e., utilization of different nutrient pools across species). To address this question, this study set up an artificial reef off the coast of the Florida Keys using dominant emergent sponges found in the Caribbean and Florida Keys. Inhalant and exhalent water samples were collected using Vacusip. Samples were filtered with a 0.2-micrometer supor filter and then processed for dissolved nutrients using fluorescence dissolved organic matter (fDOM). This is the first application of fDOM analysis to sponges.

In this study, we found that the microbial abundance (i.e. the commonly used high and low microbial abundance (HMA/LMA) classification) was not an effective indicator for the way in which nutrients are processed by these sponge species. Additionally, the use of fDOM analysis indicated nuance in DOM utilization across species with differential consumption of fDOM components across sponge species. In summary, rather than microbial abundance alone, a combination of sponge species identity and the composition of the symbiotic microbial community members (e.g., presence of photosymbionts) appears to explain the most variation in nutrient processing by sponges. These results provide the first support for resource partitioning of dissolved nutrients across coexisting sponge species and provide support for the evolutionary importance of microbial communities in sponges.