Hydrothermal activity lowers trophic diversity in Antarctic hydrothermal sediments

Abstract

Abstract. Hydrothermal sediments are those in which hydrothermal fluid is discharged through sediments and are one of the least studied deep-sea ecosystems. We present a combination of microbial and biochemical data to assess trophodynamics between and within hydrothermal and background areas of the Bransfield Strait (1050-1647 m of depth). Microbial composition, biomass, and fatty acid signatures varied widely between and within hydrothermally active and background sites, providing evidence of diverse metabolic activity. Several species had different feeding strategies and trophic positions between hydrothermally active and inactive areas, and the stable isotope values of consumers were not consistent with feeding morphology. Niche area and the diversity of microbial fatty acids was lowest at the most hydrothermally active site, reflecting trends in species diversity. Faunal uptake of chemosynthetically produced organics was relatively limited but was detected at both hydrothermal and non-hydrothermal sites, potentially suggesting that hydrothermal activity can affect trophodynamics over a much wider area than previously thought.

Benthic carbon fixation and cycling in diffuse hydrothermal and background sediments in the Bransfield Strait, Antarctica

“…In the "algae" treatment, lyophilised algal cells (Chlorella, Cambridge Isotope Laboratories, CNLM-455-1) enriched in 13 C and 15 N (both ∼ 100 atom %) were allowed to settle on the sediment surface, giving a final dose of 436 ± 30 mg C m −2 . This was equivalent to ∼ 1.6 % of total OC in the surface 1 cm of sediment, or ∼ 9 % of the annual OC input (Bell et al, 2017b). It is recognised that such organic detritus is less degraded than the sinking photosynthetic material which normally reaches the depths of our study sites.…”

“…The respiration rate was calculated for each core by placing a line of best fit through the amount of added 13 C over time, and normalised to surface area. Bacterial incorporation of 13 C was calculated by first subtracting the natural abundance of 13 C from the isotopic signature of each PLFA (data published in Bell et al, 2017a), where the difference exceeded the precision of the analytical technique, to give the amount of added C in each compound. Bacterial incorporation was then calculated using the four bacteria-specific PLFAs isoC14 : 0, isoC15 : 0, antisoC15 : 0 and isoC16 : 0, following Boschker and Middelburg (2002).…”

“…Faunal uptake of added 13 C was calculated by subtracting 13 C attributable to its natural abundance in the appropriate taxon (data published in Bell et al, 2017a) from faunal isotopic signatures, where the difference exceeded the precision of the analytical technique, and multiplying by the quantity of organic C in each specimen. Specimens were summed for each core, and the value was multiplied by 2 to account for only half of each horizon being used for faunal extraction.…”

“…These hydrothermally influenced sediments are likely to be more spatially extensive than hard substrate vents, although their diffusive nature makes their extent hard to quantify. Sedimented hydrothermal vents have been shown to influence biological community composition and nutrition at adjacent sites which were otherwise characterised as "inactive" or "off-vent" (Levin et al, 2009;Bell et al, 2016aBell et al, , b, 2017a. However, the ecology of sedimented hydrothermal sites has received relatively little study.…”

“…However, the ecology of sedimented hydrothermal sites has received relatively little study. There is only one modelling study that has focused on the interaction between benthic ecosystems and C cycling at SHVs (Bell et al, 2017b), and there are no direct observations of SHV C cycling by components of the benthic ecosystem.…”