*This marks my 200th post. In preparation for comprehensive examination in less than 2 weeks and because its International Dissertation Writing Month, I will be posting my thesis proposal as I madly try to finish it all in time over the next few days. Feel free to question, correct, nitpick, criticize (constructively, I'm in a fragile state right now!), comment, praise me and make suggestions for improvement. And yes, I'm freakin' out!!!!
Biodiversity of Chemosynthetic Communities at the Eastern-Lau Spreading Centre
Many communities persist due the presence of a foundation species. Dayton (1972) defined a foundation species as an large, influential species that has a positive effect on community inhabitants through modifying environmental parameters, species interactions and resource availability. It is hypothesized that foundation fauna can ameliorate abiotic stress by modifying the chemical and physical environment (Bruno et al. 2003). The refuge provided by the foundation species increases abundance, biomass and diversity of other associated species (Bruno & Bertness 2001). Many chemoautotrophic megafauna at hydrothermal vents are foundation fauna that live in environments with high concentrations of toxic chemicals, heavy metals, high temperatures and low oxygen concentrations. Chemoautotrophs may modify the physical and chemical habitat (Johnson et al. 1994, Shank et al. 1998) and provide structural heterogeneity (Govenar & Fisher 2007), making the local environment more amenable to less tolerant colonizers (Shank et al. 1998). My research aims to test hypotheses relating environmental stress to the species composition and diversity of chemosynthetic communities at the Eastern-Lau Spreading Center (ELSC). Additionally, I will investigate the trophic ecology and food web structure between communities associated with three chemoautotrophic foundation species.
This thesis work is part of a larger project investigating the geology, chemistry, microbiology and ecology of deep-sea hydrothermal vents at the ELSC. Back-arc basins are the result of a complex plate tectonics setting where two or more oceanic plates converge with one plate subducting beneath the other(s). The friction caused by the subducting plates’ movement melts the rock along the plate boundary causing a volcanic arc to form on the overlaying plate. On the backside of that arc, a spreading centre is created as a result of the extensional forces of the subduction. Where back-arc spreading occurs, the crust is thin and fragile (see figure below). Seawater seeps down into cracks and fissures and comes into contact with super-heated rock. The seawater is heated and rises, carrying reduced chemicals and heavy metals in solution, which exits the crust at the seafloor and precipitates out of solution creating hydrothermal vent chimneys. Bacteria have evolved metabolic pathways to utilize the energy from oxidizing some of these reduced chemicals such as sulfides and methane. Furthermore, several metazoans have evolved a symbiosis with these bacteria with the result being abundant communities surrounding many vents worldwide.
The hydrothermal vent ecosystem is based upon chemosynthetic primary production and offers an opportunity to empirically test various measures of the diversity-stress hypothesis. At the ELSC, 3 major chemoautotrophic foundation species occur in dense aggregations: the snails Alviniconcha hessleri and Ifremeria nautilei and the mussel Bathymodiolus brevior. Henry et al. (submitted) have found clear differences in the thresholds for maximum H2S and minimum O2 concentrations and maximum temperatures for the three foundation species. Podowski et al. (submitted) have found significant differences in situ of H2S, O2 and temperature measurements between each of these community types, supporting the hypothesis that each foundation species occupies a specific thermo-chemical microhabitat. Furthermore, they occupy a gradient of stress, similar to a “bull’s eye” pattern, with A. hessleri in the highest stress environment (high H2S, low O2, high temp.), B. brevior in lowest stress environment (low H2S, higher O2 and near ambient temp.) and I. nautilei occupying an intermediate position with overlap on both ends. This well-defined gradient allows me to test the relationship between species diversity and stress.
At vents, stress can be ameliorated by chemoautotrophic foundation species by removal of sulfide from the environment (Johnson et al. 1988, Johnson et al. 1994). Mussel beds in low vent flow at the Galapagos Rift Zone vents have been shown to diffuse venting laterally and removal experiments have shown that sulfide concentrations above the mussel bed are lower pre-removal (Johnson et al. 1994). Stress reduction, by way of vent flow diffusion and sulfide removal, should increase with biomass of the foundation species if facilitation were an important factor in structuring communities associated with a foundation species. I hypothesize that diversity should increase with foundation species biomass (greater stress reduction) to a point where the response curve asymptotes at the maximum local species diversity. If competitive exclusion played a role in structuring these communities, then according to theory diversity is predicted to be greatest at intermediate levels of stress (Bruno et al. 2003). Since I. nautilei inhabits an intermediate position in stress tolerance, I would expect to see its associated community have the highest mean diversity.
The ELSC is further characterized by two different substrate types: lava tends to exhibit more basaltic characteristics in the north and andesitic in the south. The physical and chemical properties of these lava types are subtly different, but may have implications for how the hydrothermal venting regime is distributed over space. Preliminary evidence suggests that andesite, a more pliable and porous lava type, diffuses venting laterally more than basalt. This has implications for the fauna living on each lava type. This observation leads to the prediction that communities on andesite hosted substrate will have higher biomass and greater diversity than communities on basalt hosted substrate. The effect of substrate may be confounded by lateral diffusion by beds of mussels, and presumably snails. Even so, the greater diffusivity of andesite may act synergistically the diffusive activity of the mussel and snail beds to increase the amount of habitat that can support their respective associated communities.
The communities associated with these foundation fauna are poorly documented and the biological literature for this area consists mainly of taxonomic descriptions and video observations. Rapid progress is being made in understanding the ELSC communities due to the 2005 and 2006 expeditions to the area. Podowski et al. (submitted) built upon the limitations imposed by the laboratory setting of Henry et al.’s (submitted) physiological work of the chemoautotrophic species by measuring in situ chemical concentrations and temperature measurements of communities as a whole. Though their findings were novel and furthered our understanding of these communities, but Podowski et al.’s work was limited to the 2-dimensional view of the surface of the communities. Thus, diversity was biased against smaller fauna and those that lived within the foundation species’ matrix and not at the surface. Inferences on abundance, biomass, diversity and trophic interactions to the communities as a whole are thus limited. In order to understand how the communities associated with chemoautotrophic foundation species are structured, I collected 36 quantitative whole-community samples nested within site, which is nested within substrate (basaltic or andesitic). Each community is based on collections of one of three chemoautotrophic foundation species, confirmed upon retrieval as dominants in terms of abundance and biomass. Mixed communities, defined arbitrarily as the dominant foundation species composing 75% or less of the total foundation species abundance or biomass, were collected a priori and discovered a posteriori upon retrieval. I recognize that sampling may be biased by collecting from sites where the research team concentrated most of their multidisciplinary studies and by collecting the communities observed to be best collected by our sampling device. Sampling is thus haphazard and not random.
The hydrothermal vent food web is structured upon bacterially-derived chemoautotrophic primary production (Jannasch 1985). Bacteria may be free-living or in an endosymbiosis with a wide range invertebrates (Childress & Fisher 1992). There is evidence for the cycling of chemoautotrophic-derived nutrients through consumers at vents and exportation of these nutrients outside the vent system (Van Dover & Fry 1989, Fisher et al. 1994, Van Dover & Fry 1994, Bergquist et al. 2007). Tunnicliffe (1991) constructed the first general food web model of the vent ecosystem which was later expounded upon by Bergquist et al. (2007) for the Juan de Fuca vent ecosystem. Their models included 3 producer pools (subsurface bacteria, bacterial mats on the surface and symbiotic bacteria) and 4 consumer pools (grazers, scavengers/detrivores, suspension feeders, predators and endosymbiotic hosts) interacting through four trophic pathways. Bergquist et al. (2007) recognized the utility of investigating trophic structure in a quantitative community setting in order to compare those results to community characteristics, but their analyses were limited to only one community. In order to better understand the relationship between community characteristics and trophic structure, I will analyze stable isotopes of fauna from quantitative collections on both andesitic and basaltic substrates.
While identifying and enumerating species for the community ecology portion of my research, I became very interested in systematics. This is due to my own personal fascination with diversity and to the fact that many species remain undescribed. The ELSC fauna are taxonomically similar to fauna from other biogeographic provinces. One such example are species are shrimp in the family Alvinocarididae. They have so far been found only at gas/oil seeps and hydrothermal vents worldwide. I am describing, with a colleague from France, a new species of Alvinocaris from the ELSC that is morphologically most similar to A. lusca from the Galapagos Rift Zone and East Pacific Rise. This species phylogenetic placement, based only on the COI gene, is somewhat obscured and awaits further phylogenetic analysis with a wider array of nuclear and mitochondrial genes. Another interesting taxonomic group are the cnidarian fauna of the ELSC. The Cnidaria have not been reported to make up a significant portion of the community at vents in the Pacific and Atlantic Oceans. Eight species from five biogeographic provinces have been described. Only recently have they garnered attention as dense communities of anemones been observed at Central Indian Ridge vents (Van Dover et al. 2001). Our recent expeditions to the ELSC have discovered another densely populated community of mixed anemones and zoanthids. With colleagues from the Ohio State University, I am describing seven new species of anemone and one new species of zoanthid. The zoanthid and four of the anemone species live within the influence of hydrothermal vent flow. An additional species is found both near vents and in the ambient deep sea community. The remaining 2 anemone species are found in the ambient deep sea community only. The proximity of these cnidarians to vent effluent suggests a link to chemoautotrophic primary production. The additions of these new species of shrimp and cnidarians from ELSC fills gaps in the western Pacific back-arc basins that aid in our understanding of the biogeography of hydrothermal vent fauna.
An additional study I have undertaken is a stable isotope bioassay for primary production of particulate organic matter (POM) at methane seeps in the Gulf of Mexico using filter-feeding taxa. Previous studies have focused on primary production from the standpoint of the chemoautotrophic symbioses or free-living bacteria found in and on top of the sediment at seeps. Little attention has been given to the role of seeps in creating POM that enters the water column, despite the observations of suspension-feeding invertebrates at and near seep communities (i.e. tubeworm aggregations, mussel beds). Since filter-feeders directly uptake the POM in the water column, their tissue stable isotope values are point estimates of the available POM at that time. Stable carbon, nitrogen and sulfur isotope ratios for methanotrophy at seeps are distinct from photosynthetic-derived nutrients that fall down from surface water. We can exploit this difference in food source “signatures” to approximate the input of seep-derived primary production of POM relative to photosynthetic primary production of POM. I used a 2 end-member stable isotope mixing model to approximate the relative contribution of seep-derived nutrients to the POM pool at a methane seeps. Interestingly, about ¼ of the POM can be attributed to methane seep primary production of POM. This is my first chapter, so it will comprise the next edition of Dissertation Blogging!
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Bruno JF, Stachowicz JJ, Bertness MD (2003) Inclusion of facilitation into ecological theory. TREE 18:119-125
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Van Dover CL, Fry B (1994) Microorganisms as food resources at deep-sea hydrothermal vents. Limnology and Oceanography 39:51-57
Van Dover CL, Humphris SE, Fornari D, Cavanaugh CM, Collier R, Goffredi SK, Hashimoto J, Lilley MD, Reysenbach AL, Shank TM, Von Damm KL, Banta A, Gallant RM, Götz D, Green D, Hall J, Harmer TL, Hurtado LA, Johnson P, McKiness ZP, Meredith C, Olson E, Pan IL, Turnipseed M, Won Y, Young III CR, Vrijenhoek RC (2001) Biogeography and ecological setting of Indian Ocean hydrothermal vents. Science 294:818-823
*This post was updated on Nov. 30 after I made substantial changes to my thesis proposal after reading comments from my advisor.