How deep-sea wood falls sustain chemosynthetic life

Abstract

Large organic food falls to the deep sea--such as whale carcasses and wood logs--are known to serve as stepping stones for the dispersal of highly adapted chemosynthetic organisms inhabiting hot vents and cold seeps. Here we investigated the biogeochemical and microbiological processes leading to the development of sulfidic niches by deploying wood colonization experiments at a depth of 1690 m in the Eastern Mediterranean for one year. Wood-boring bivalves of the genus Xylophaga played a key role in the degradation of the wood logs, facilitating the development of anoxic zones and anaerobic microbial processes such as sulfate reduction. Fauna and bacteria associated with the wood included types reported from other deep-sea habitats including chemosynthetic ecosystems, confirming the potential role of large organic food falls as biodiversity hot spots and stepping stones for vent and seep communities. Specific bacterial communities developed on and around the wood falls within one year and were distinct from freshly submerged wood and background sediments. These included sulfate-reducing and cellulolytic bacterial taxa, which are likely to play an important role in the utilization of wood by chemosynthetic life and other deep-sea animals.

Figure 1. Wood experiments deployed in the Eastern Mediterranean deep sea during the BIONIL cruise in 2006 (RV Meteor) and recovered during the Medeco-2 expedition (RV Pourquoi Pas?) in 2007.

a, b) Wood#1 close to carbonate crust, c, d) Wood#2 on carbonate crust, e, f) Wood#5 on sediment. (Pictures a, c, e are courtesy of Marum, University Bremen, Germany; Pictures b, d, f are courtesy of Ifremer, France).

Figure 2. The Pockmark area in the Central Province of the Nile deep-sea fan in the Eastern Mediterranean with locations of the four wood colonization experiments: Wood#1 close to carbonate crust, wood#2 on carbonate crust, wood#5 on sediment, wood#6 close to carbonate crust, sampled after less than 1 day of submersion.

The maps were generated in ArcMap (ArcGIS Desktop 9.3) with continental margins provided by ESRI (Kranzberg, Germany) and bathymetry obtained from the 2-minute Gridded Global Relief Data ETOPO2v2 (2006, http://www.ngdc.noaa.gov/mgg/fliers/06mgg01.html). The bathymetry of the Pockmark Area was obtained during Meteor expedition M70/2 (BIONIL) using AUV Aster^x equipped with EM120 multibeam (IFREMER/Geosciences Azur).

Figure 3. Macrofauna colonizing the wood experiments after one year at the sea floor.

a) Xylophaga dorsalis, b) Idas modiolaeformis, c) Glycera noelae sp. nov. d) Cryptonome gen. nov. conclava, n. sp., e) Phascolosoma turnerae, f) Asterechinus elegans, g) Bathynectes piperitus, h) unidentified deep-sea fish, i, k) unidentified species of amphipods, l) unidentified species of Leptostracea. (Pictures h, g, f are courtesy of Ifremer, France (Medeco-2 expedition))

Figure 4. Ex situ microsensor measurements of oxygen, sulfide and pH at (0.5 m) and away (10 m) from wood experiments #1 and #5.

Note the different scales for sulfide concentrations.

Figure 5. Total bacterial cell numbers at (0.5 m) and away (10m) from wood experiments #1 and #5.

Figure 6. Non-metric multidimensional scaling (NMDS) ordination of relative ARISA OTU abundances with Bray-Curtis distance.

Each point represents the consensus of 2–3 replicate ARISA profiles. Colors and groupings indicate the origin of the samples. a includes a comparison only of the wood experiments, while b and c show bacterial community structure on the wood experiments in comparison to surrounding sediments at distances of 0.5 m and 10 m from the wood experiments. Asterisks (*) indicate the presence of wood chips in a sample.

Figure 7. Bacterial community composition of the wood experiments at the class level.

Wood#6 served as a control and was sampled after less than 1 day of submergence. Wood experiments #1, #2 and #5 had been submerged for 1 year.

Figure 8. Schematic illustration of the proposed succession of a wood fall during the first year at the deep-sea floor.

Organisms shown on inlets from left to right: amphipods and one other type of unidentified crustacean, wood-boring bivalve Xylophaga dorsalis, polychaete Glycera noelae sp. nov., polychaete Cryptonome gen. nov. conclava, n. sp., sipunculid Phascolosoma turnerae, chemosynthetic bivalve Idas modiolaeformis, DAPI-stained bacteria with pieces of wood (Illustration: Sabine Lüdeling, Medieningenieure).