The marine biosphere is well-endowed with functionally critical symbioses. Coral symbiosis research has principally been driven by the need to understand the fundamental importance of the photosynthesizing zooxanthellate symbionts to the survival of the coral animal host. In contrast, sponge symbiosis research has largely been motivated by the discovery of bioactive molecules (Taylor et al., 2007b). Thus, the microbial symbionts from sponges have been studied chiefly in relation to their biodiversity and biogeography. Two recent articles on microbial symbiosis in marine sponges (Taylor et al., 2007a; Vogel, 2008) emphasize the significance of the high density and diversity of resident microbes to sponge function and the utility of sponge microbial diversity to pharmacologists and biotechnologists. Focal points raised by these articles for future research include: ‘the evolution of symbiont diversity, microbial metabolism, host–microbe interactions and potential biotechnological implications’. As these are very important research directions, due consideration should also be afforded to additional key research areas, including (i) the significance of microbial associations to sponge health (by enhancing our understanding of the specific functions of sponge symbionts and investigation of other ‘symbiotic’ processes such as disease), (ii) the consistency and stability of the symbioses across wider environmental gradients and (iii) the sensitivity of the symbioses in the face of global change. Here we broaden the scope of the recent commentaries on sponge symbiosis to include these critical environmental issues.

To date, there is little direct evidence confirming the contribution of symbiotic microbes to sponge well-being or survival. An exception involves the cyanobacteria, in which earlier studies (comprehensively reviewed in Taylor et al., 2007b) have shown translocation of photosynthate from cyanobacteria to the host sponge (Wilkinson, 1979) and a decline in sponge health with a loss of cyanobacteria (Thacker, 2005). However, in general, the presence of microbes with certain metabolic phenotypes only provides circumstantial evidence that they are important for sponge health. Examples include ammonium-oxidizing archaea (Steger et al., 2008), the nitrite-oxidizing phylum Nitrospira (Hentschel et al., 2002), sulphate-reducing bacteria (Hoffmann et al., 2005), anaerobic phototrophs (Imhoff and Trüper, 1976) and vertical transfer of some microbial populations from parent sponge to larval offspring (for example, Steger et al., 2008 and references therein). However, additional research is required to reveal the cellular and molecular interactions (the critical functional aspects) of sponge–microbial symbiotic relationships. The interaction between the host and the symbiont has been identified earlier as a focal point for future research (Taylor et al., 2007a); however, to date, research into sponge microbiology has not elucidated the roles bacteria play in sponge biology and ecology. As the concept of ‘who eats what, where and when?’ is considered a Holy Grail in studies of microbial ecology (Neufeld et al., 2007), an appreciation of symbiotic function requires a grasp of the communication between the host and its symbionts: understanding the language spoken between the host and symbiont should reveal how the symbiosis occurs and what compels the association to persist.

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