White Paper Title: 
Examining Links among Symbiont Physiology, Holobiont Distribution and Physicochemical Habitats at Hydrothermal Vents

Deep-sea hydrothermal vents are dominated by symbiotic associations (Cavanaugh et al. 2006), which represent the majority of biomass and, in aggregate, are often more productive than rainforests and kelp beds (Girguis and Childress 2006).  Invertebrates from three phyla host chemoautotrophic bacteria, primarily γ-Proteobacteria, which harness energy from chemical oxidation to fix inorganic carbon (Cavanaugh et al. 2006).  These symbionts are often the primary source of nutrition for the host species.  While most are thought to be autotrophic sulfide oxidizers, little is known about the particular pathways, intermediates, and alternate substrates that are part of their chemolithotrophic metabolism.  Moreover, few connections have been made between the metabolic capacity of chemosynthetic symbionts and the specific chemical regimes occupied by their hosts. 

Given the importance of the Proteobacteria to these partnerships (as the primary source of nutrition), physiological differences among the symbionts of chemosynthetic vent fauna may have profound effects on the distribution of the holobionts.  Indirect evidence for this comes from previous studies that suggest that certain host animals associate with specific physicochemical regimes at vents (Le Bris et al. 2003, Le Bris et al. 2006, Luther III et al. 2001, Matabos et al. 2008, Moore et al. 2009, Podowski et al. 2009, Waite et al. 2008), though the fundamental cause for this zonation remains almost entirely unexplored (Colaco et al. 2002). While the role of symbiont physiology has been studied to some extent in other symbioses-dominated ecosystems (e.g. its role in the zonation of cnidarians (Thornhill et al. 2008, Verde and McCloskey 2007)), it has never been systematically examined or characterized among the chemoautotrophic symbioses.  In order to better understand faunal distribution in vent ecosystems, it is crucial to investigate the underlying differences in symbiont physiology that may play a role in the niche differentiation of holobionts within vent habitats. 

Gastropods of the genus Alviniconcha are found at hydrothermal vent fields in the Western Pacific  and Indian Ocean and have been reported to host intracellular γ- or ε-Proteobacterial endosymbionts individual (Suzuki et al. 2005a, Suzuki et al. 2005b, Suzuki et al. 2006, Urakawa et al. 2005).  An extensive sampling effort from a recent expedition to the Eastern Lau Spreading Center (ELSC) has revealed that Alviniconcha with symbionts of both types are found at vent fields along the ridge and that the relative abundance of individuals with each type at a vent field relates to the geology and geochemistry of that site.  It is likely that the metabolic differences between the γ- and ε-Proteobacterial endosymbionts are influencing this distribution pattern.  This work suggests that endosymbiont metabolism plays an important role in the distribution of host animals at hydrothermal vents and should be investigated in other chemosynthetic associations.  It is imperative that we bring together studies of the physicochemical habitat (e.g. geology and geochemistry) and holobiont distribution with studies targeting symbiont physiology.  Ultimately, this will facilitate a better understanding of the role of the symbiont in the ecology and evolution of many vent invertebrates.