Visualizing the process of digestion in the oldest known animal-microbe symbiosis (https://phys.org/news/2021-02-visualizing-digestion-oldest-animal-microbe-symbiosis.html)

 Marine biologists have been able to visualize for the first time how tropical sponges and their symbiotic bacteria work together to consume and recycle organic food. Sponges are the oldest known animal–microbe symbiosis and are found abundantly across the globe, from the canals of Amsterdam to kilometer-deep canyons on the ocean floor. They play an essential role in recycling resources in nutrient-poor ecosystems, such as coral reefs.
"The collaboration between the sponge host and their abundant microbiome in processing food was a mystery," says researcher Meggie Hudspith, who performs her Ph.D. research at the UvA Institute for Biodiversity and Ecosystem Dynamics. "Our study used cutting-edge imaging techniques (NanoSIMS) to trace the uptake of dissolved and particulate organic matter by sponges and their symbionts over time."
The authors found that microbial symbionts were actively involved in the processing of dissolved organic nutrients, but that the sponge filter cells were the primary sites of eating and drinking nutrients. Over time, nutrients were transferred from the sponge cells to their symbionts. This indicates that microbes recycle the waste products of the host, ensuring that limited nutrients are retained by the sponge and not expelled into the environment.