**4.4 The sponge holobiont – Functional aspects**

The sponge microbiome is a prime example of natural chemodiversity, occupying an extensive range of functions in primary (C and N cycling – see e.g Li, 2009) and secondary metabolism (thousands of original molecules of various classes and modes of action) which has been studied worldwide by natural product chemists.

#### **4.4.1 Functional "primary" aspects of symbiosis**

The biochemical nature of sponge-microbe symbioses is largely unknown, and ideally requires investigations at single strain microniche consortium and whole microbiome levels (Kamke et al., 2010) for a given host to obtain a better insight into the functional dynamics of the holobiont. Several basic (primary metabolism) sponge-associated microbial processes have been described, including:


Coral Reef Biodiversity in the Face of Climatic Changes 91

About half of the 1,300 scleractinian coral species are reef-building, largely colonial, zooxanthellate (hermatypic) and occurring in the clear, shallow and oligotrophic waters of the tropics. The other half of the order is largely solitary and azooxanthellate, occurring in all regions of the oceans, including the greatest depths (Budd et al., 2010). The reef-building corals function as primary ecosystem engineers, constructing the framework that serves as a habitat for all other coral reef-associated organisms (Wild et al., 2011). Scleractinians are actively engaged in the production and transformation of mineral and organic materials. Coral limestone structures are broken down by bioeroding organisms and abiotic processes into sand, itself acting as a natural biocatalytic filter for the cycling of organic matter by

The term holobiont, sometimes collectively defined as biota engaged in a host-symbiont partnership (Santiago-Vázquez et al., 2006), has been borrowed by coral researchers to conveniently include the coral host and all of its associated interactive life forms (reviewed in Rohwer, 2010). This includes tissue-associated symbiotic photosynthetic microalgae, surface and mucus-dwelling bacteria and archaea (Siboni et al., 2008) and recently investigated viruses (Vega Thurber, 2008). Endolithic algae and fungi that bore into the mineral skeleton (Wegley et al., 2004) may be included in this definition as permanent associates (as in Bourne et al., 2009). Very recently a true symbiotic relationship has been described between the acroporid *Acropora muricata* and the hydrozoan *Zanclea margaritae* (Pantos & Bythell, 2010). "Mobile" associates (crustaceans, mollusks, polychaetes etc.) provided they have developed a specific niche or trophic preference with the host and developed a cryptic or aposematic appearance as a result, should logically be included in this definition in that the host's disappearance would probably signify a loss of this

The coral holobiont is now regarded as a functional unit by scientists who are interested in physiology, pathology, biochemistry and environmental issues of reef-buiding anthozoans. A review of the functional microbiota associated with corals is provided by Laming (2010). Fig. 1 illustrates the typical coral holobiont with its associated macro- and micro-organisms.

**5.3 The coral holobiont on autotrophic, heterotrophic and mixotrophic feeding modes**  Mixotrophic organisms can functionally combine different modes of nutrition: (i) by using photosynthesis for inorganic carbon fixation; and (ii) by taking up organic sources. Coral polyps are diploblastic and hence have no mesoderm-derived digestive tract or specialized respiratory organ. Nutrient and energy requirements of the whole colony must depend (i) on direct diffusion of dissolved gases and simple organic molecules across the polyp body wall, (ii) on "assisted metabolism" with pseudo-respiratory and pseudo-digestive functions in association, respectively, with symbiotic macroalgae sequestered in the endoderm, and mucus-bound bacterial consortia, (iii) during the night, on heterotrophy, i.e. ingestion of bacteria and planktonic particles that are digested in the coelomic cavity, the organic products being further broken down by other bacteria. During the day, polyps function in autotrophic mode, i.e. relying on oxygen production and carbon photosynthates provided by the symbiotic zooxanthellae. Other commensal members of the "extended" holobiont, i.e. crustaceans, echinoderms, polychaete worms, mollusks, etc., live mostly off the food

resident heterotrophic microorganisms (Wild et al., 2005).

particles trapped in the coral mucus, or as parasites.

additional biodiversity.

**5.2 The coral holobiont: An example of 3-way functional integration** 

zooxanthellae (Schönberg et al., 2008) and when present actively contribute to carbon supply to the host (Weisz et al., 2010),

iv. methane oxidation (Vacelet et al., 1996) and sulfate reduction (Hoffmann et al., 2005).

### **4.4.2 Communication "secondary" aspects of symbiosis**

So-called secondary metabolites may be produced and released according to age and reproductive status, but also as a response to abiotic and biotic stresses, against predation and microbial infection, for resource defense against competitors, etc. The communication chemistry of soft-bodied sessile invertebrates can indeed be regarded as a vocabulary of molecular words, and its transcriptomics can be equated to proper syntax, in order to respond as exactly and as economically as possible to an identifiable conflict. According to their mode of action, these molecules can be volatile (short MW halocarbons), surface or tissue bound, or mucus-borne. The participation of the microbiome to the biosynthesis of sponge metabolites has been established in a number of cases in natural conditions, but cultivated individual strains or functional consortia of interest may not express the desired phenotype (production of a specific molecule), or may not be cultivatable outside their host. Aside from possible applications in human welfare, bacteria provide prime examples of prokaryote-metazoan coevolution which have endured an estimated 600 million years of existence and survived major biogeoclimatic changes.

The sponge mesohyl provides a broad variety of ecological microniches that host bacterial consortia (Thiel et al., 2007), with varying degrees of dependence to the host, while cortical regions tend to be dominated by cyanobacteria (Li, 2009). Both components are known to be involved in the synthesis of bioactive secondary metabolites which are naturally produced (or prompted) in response to microbial pathogens (antibiotics), space competitors (allelopathic substances), epibionts (antifouling molecules), and predators (antifeedants, intoxicants, serine protease inhibitors). This chemical arsenal, together with the presence of structural (sharp mineral sclerites, or tough spongy texture) and visual (warning or cryptic colors and patterns) defenses, are necessary to the survival of these non-motile and often exposed invertebrates. Most classes of so-called secondary metabolites are represented, making sponges a treasure trove for the discovery of new drugs. Here we are concerned with the global chemodiversity aspect and the reader is prompted to consult updated reviews (for example in the dedicated issues of *Natural Products Reports* since 1977), or texts such as Kornprobst (2010b) that provide a user-friendly review of sponge-derived metabolites, addressing their possible biosynthetic origins and their potential applications. Metagenomic screening to identify key polyketide synthase (PKS) and non-ribosomal peptide synthetase (NRPS) genes, and new cloning and biosynthetic expression strategies may provide a sustainable method to obtain new pharmaceuticals derived from the uncultured bacterial symbionts, e.g. with cyanobacteria (Li, 2009). Novel culturing techniques (e.g. Selvin et al., 2009), including co-culturing of microorganisms modulating the proliferation (through quorum sensing) and the expression of strains of interest are now actively investigated (Dusane et al., 2011).
