12. Sponges and Their Microbial Symbionts: Nature’s Water Filtration System
Often disregarded in favour of more charismatic reef residents, sponges are vital in coral reef ecosystems mostly because of their symbiotic interactions with several populations of microorganisms. One of the most effective water filtering systems in nature is built from these interactions. Filter feeders, sponges pump vast amounts of water across their body to absorb nutrients. Still, the bacteria found in the tissues of the sponge are really what make this process amazing. Together forming up to 40% of the biomass of the sponge, these symbiotic bacteria—including bacteria, archaea, and even single-celled algae—form complex ecosystems. By effectively recycling and using different molecules, this cooperation lets sponges flourish in nutrient-starved tropical seas. Providing extra nutrition to their sponge hosts, the microorganisms help break down difficult organic debris, fix nitrogen, and occasionally even photosynthesis. To its microbial allies, the sponge provides a sheltered home and consistent source of nutrients in return. Wide-ranging consequences of this relationship affect the reef ecology. Filtering enormous amounts of water, sponges and their microbial symbionts eliminate particulate and dissolved organic materials, therefore cleansing the water and recovering nutrients otherwise lost from the reef system. Some sponge species are quite effective at filtering their own water volume every five seconds. Furthermore, the filtration and transformation of organic matter links the water column to the reef substrate, so enabling nutrient movement in the ecology. By building microhabitats for different small invertebrates and fish, the sponge-microbe symbiosis also helps to maintain reef biodiversity. Conservation of reefs depends on an awareness of these intricate interactions since changes in water quality or temperature can disturb these symbioses, so causing cascading effects over the reef ecology.