ABSTRACT
Sponges live in tight associations with associated microbes thriving within their bodies. In this chapter, we discuss these intertwined host-symbiont relationships and how microbes influence multiple features of sponge physiology. We start by describing the vast and specific microbial diversity harbored by sponges, which include archaea, bacteria, eukaryote, and virus taxa; how this diversity is influenced by the mode of symbiont transmission; and the environmental factors that shape these microbial communities. Furthermore, we showcase the links between symbiont density and sponge physiology by comparing distinctive traits (e.g., pumping rates, feeding strategies, nutrient cycling) across the continuous spectrum of high (HMA) and low microbial abundance (LMA) sponge holobionts. Second, we present the functions performed by the 130sponge-associated microbes, focusing on models for metabolic exchanges and nutrition specialization that illustrate the intricate and specialized interactions for the efficient recycling of resources within the host. We also addressed the repercussions of the microbiome-mediated functions at the ecosystem level and the concept of sponges as “nested ecosystems” by giving examples of enhanced productivity and nutrient cycling, which determine the ecological role of the holobiont. From the host perspective, we introduce the immune system as the essential force for establishing and maintaining sponge-microbe interactions, and as the guardian of homeostasis within the holobiont. We summarize the current knowledge on the immune cellular and molecular mechanisms that regulate the crosstalk between the sponge host and its symbiotic partners and identify future perspectives on the emerging research field of sponge immunity. Finally, we examine how sponge holobionts face a changing environment, highlight the relevance of understanding the mechanisms controlling homeostasis and dysbiosis in the holobiont, and how changes in the sponge-associated microbial community can, either lead to disease outbreaks or aid sponges to adapt quickly under stress-related scenarios. Understanding the complexity of sponge holobionts is a major challenge, but establishing sponge model systems and adopting holistic approaches will allow us to continue deciphering the interplay between sponges and microbes.
