The role of chemosynthetic production in supporting the benthic food web at a shallow hydrothermal system (Comau Fjord, southern Chile)
Keywords: trophic ecology, chemosynthesis. hydrothermalism
The study of reducing systems, where carbon is fixed chemosynthetically by bacteria, i.e. methane, hydrocarbon and brine seeps, dead whale-falls, hypoxic sulfidic settings and hydrothermal vents, has produced some of the most exciting discoveries in the last decades in diverse areas of the marine biology, geology and chemistry. Biota at deep-water reducing settings is primarily composed of a small set of endemic large organisms relying on symbioses with chemoautotrophic bacteria, able to withstand extreme conditions and persist in a discontinuous and ephemeral environment. A larger set of accompanying species relying on heterotrophy are also conspicuous members of these systems. These environments are often considered hotspots of faunal abundance and/or diversity, mainly due to the local primary production and the environmental heterogeneity generated in an otherwise food-limited and homogeneous environment like the deep-sea. In contrast, shallower water counterparts of seeps and vents (<200 m water depth) are in general colonized by a subset of background species, and few or no endemics have been reported to thrive in these sites. In particular, in shallow water vents, hostile environmental conditions, like enhanced temperature and sulfide coupled with low pH, often have a negative effect over the fauna. However, opposite to the trend followed by eukaryotic communities, prokaryotes prosper at these environments and numerous groups of thermophilic bacteria and archaea have been reported. In spite of hostile conditions, it has been observed that at some locations eukaryotic heterotrophs benefit from production of sulfur-oxidizing bacteria. In fact at least at shallow vents off White Point, California, it has been proved that the abalone Haliotis cracherodii and a limpet Lottia limatula obtain substantial nutrition from bacterial mats of the genus Thiotrix. Similar microbial mats, but from coastal methane seeps, have been recently described, within the frame of FONDECYT project # 1080623, at Mocha Island, constituting a pioneer study for the region. At this site, stable isotope analysis of the local food-web indicates that these mats could constitute an energy source for the local heterotrophs. Recently, filamentous bacterial communities, similar in appearance to those at Mocha Island have been reported at the Comau Fjord (southern Chile, ~42°S). However, at this site, preliminary observations indicate that the fluid fueling these microbial communities could be of hydrothermal origin. Further investigation of white patches in one of the fjord walls suggested that the material corresponded to filamentous microbial communities, covering in general bare rock but also animals like chitons, mussels and dead corals, from 7 to 100 m water depth. Patches accumulate in places were sulfide-smelling water emanates from the rocks and it has been proposed that at least the sessile filamentous communities observed could be composed of the sulfur-oxidizing bacteria Thioploca or Beggiatoa. The main aim of the present proposal is thus to examine the environmental conditions and the marine invertebrate fauna inhabiting the vicinities of the Comau Fjord Hydrothermal Venting (CFHV), drawing comparisons with the “normal” conditions at a nearby site (control) with no evidence of fluids escaping from the substrate. We will address four main objectives: i) to perform a study of the benthic food web of the Comau Fjord using and stable isotope approach; ii) to identify the main sources of organic matter used by benthic communities at CFHV and control sites, using C and N stable isotope analysis, iii) to trace and compare, using quantitative descriptors, the structure of the local food webs at each site in order to asses the role of escaping fluids in fueling chemosynthetic production and its eventual transfer to the local benthic heterotrophs, and iv) to characterize the geochemical environment where the communities have been developing. This will be the first time that an isotopic approach will be used in the fjord area of Chile to describe in a detailed way the benthic food web of these delicate ecosystems. At a global scale, it will add to the scarce amount of available literature dealing with trophic aspects at shallow vent systems. Additionally, this particular setting provides exciting possibilities of discovering new trophic pathways and life styles, and ultimately understanding the functioning of a benthic system in a still relatively pristine area, which unfortunately is being increasingly menaced by anthropogenic disturbance (e.g. salmon culture, wood industry, tourism, hydroelectric undertakings).
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