Sponge-Microbe Interactions

Sponges (phylum Porifera) are amongst the most basal metazoans and emerged over 650 million years
ago (Hooper and Van Soest, 2002). Today, more than 8,600 sponge species are formally described
with the majority of them inhabiting marine ecosystems, extending from intertidal zones to deep
ocean trenches in tropical, temperate, and polar regions (Hooper and Van Soest, 2002). These sessile
filter feeders are important constituents of benthic ecosystems, as they can occupy up to 80% of the
available substrate, provide habitat for a wide range of infaunal species, be a vital trophic link between
the benthos and the pelagic realm through their remarkable filtration capacity, and mediate
biogeochemical fluxes through the uptake of organic matter and the recycling of nutrients (Webster
and Thomas, 2016, Bell, 2008). In coral reef ecosystems, for example, sponges are part of a highly
efficient recycling pathway (the sponge loop), as they take up dissolved organic matter from
oligotrophic waters and convert it into cellular detritus that becomes food for higher trophic levels
(De Goeij et al., 2013). Furthermore, sponges are known for their ubiquitous production of bioactive
metabolites with a broad range of biochemical and biomedical applications (Taylor et al., 2007).
Besides their evolutionary, ecological and biotechnological importance, sponges are also prominent
examples of highly complex microbial symbioses (Webster and Thomas, 2016, Thomas et al., 2016).
The microbial consortium of sponges can make up to 35% of the host’s biomass, is known for its
remarkable diversity, and has a fundamental role in the sponge’s physiology and ecology (Hentschel
et al., 2012). Despite the constant exposure to seawater microbes, sponges can maintain a highly
stable and species-specific microbial community (Glasl et al., 2018). Due to the integral role of
microbes, sponges are considered holobionts, encompassing the sponge host and a diverse
consortium of bacteria, archaea, unicellular algae, fungi, and viruses (Webster and Taylor, 2012). The
evolutionary and ecological success of marine sponges, as well as the production of natural bioactive
compounds, is underpinned by their intimate associations with microbial symbionts. However,
despite recent progress in research on sponge-microbe symbioses triggered by the advent of (meta)-
omics methods, the mechanistic interactions of these ancient multi-partner symbioses are still very
poorly understood.