FROZEN HEAT | Volume 1

Chemosymbiotic animals at methane seeps can be large or small, form bushes, dense beds, reefs, or live alone, and they can grow very quickly or exceptionally slowly. Animal commu- nities at methane seeps include single-celled organisms (pro- tozoans) and multi-celled animals (metazoans). Most of the metazoans are invertebrates. Many are sustained, one way or another, by microbial activity linked to methane. Common ex- amples include vestimentiferan tubeworms (Fig. 2.5, A), crabs (Fig. 2.5 B, E), and a diversity of clams (Fig. 2.5, C, F). All of these taxa are relatively large compared to non-seep, deep-sea fauna. Many seep-endemic organisms have reduced or absent digestive systems. Instead, they provide homes to symbiotic chemoautotrophic bacteria that provide the host with nutrition through aerobic sulphide and/or methane oxi- dation (Fig. 2.6). The seeps and seep organisms support a wealth of grazing, predatory, and deposit-feeding taxa by providing substrate for attachment, access to reduced compounds, entrainment of organic-rich particles, and access to microbial protozoan or metazoan prey (Carney 1994; Cordes et al. 2010). Additionally, the carbonates (limestone is a type of carbonate) precipitated by microbial AOM consortia form crusts, rocks, boulders, and even vast landscapes at seeps (Teichert et al. 2005). These seeps can support high densities of mussels, tubeworms, and grazing gastropods (Olu-Le Roy et al. 1996; Levin et al. 2010). Because the chemosynthetic life forms described here re- quire different chemical balances and concentrations of methane and sulphide (Sibuet and Olu-Le Roy 1998; 2003; Levin 2005), distinct habitat patches form in response to the fluid chemistry and fluid flow rate (flux). Generally, sedi- ments covered with mats of sulphur-oxidizing bacteria are as- sociated with the strongest fluid and methane fluxes or near- surface gas hydrates. Mussel and vesicomyid clam beds are associated with high to moderate fluxes. Solemyid clam beds, as well as vestimentiferan frenulate tubeworm fields, are as- sociated with lower oscillating fluxes or deeper gas hydrates (Fig. 2.7) (Sahling et al. 2002; Sibuet and Olu-Le Roy 2003; Levin 2005; Sommer et al. 2006). Such connections have been documented in several methane-seep environments (e.g. Van Dover et al. 2003; Olu-Le Roy et al. 2007; 2009). The combination of microbial mats, the beds, bushes, and

Morphology of a tube worm hosting sulphide-oxidizing symbionts

O

2

HS -

CO

2

Plume

Heart like structure Dorsal vessel Ventral vessel

Vestimentum

Trophosome

O

2

Coelomic Cavity

HS - CO

Bacteria

2

Figure 2.6: Symbiotic relationships for obtaining energy from sulphide. Morphology of a tube worm (top) and photo of a clam hosting sulphide-oxidizing symbionts (bottom, photo courtesy of Greg Rouse, Scripps Institution of Oceanography). Tube worms host their symbionts in the trophosome, a specialized organ. Oxygen (O 2 ), sulphide (HS – ), and carbon dioxide (CO 2 ) are taken up from the surrounding water through the animal’s plume and delivered via the blood stream to the symbionts. Clams harbour their symbionts in their gills. Oxygen and carbon dioxide are available from the surrounding water, and sulphide is taken up from the sediment through the clam’s foot.

A GLOBAL OUTLOOK ON METHANE GAS HYDRATES 45