A stepping stone of rotting wood

Many of the animals living at hydrothermal vents and cold seeps carry chemosynthetic bacterial symbionts in their body, which convert methane or hydrogen sulfide into food. Some have lost the ability to feed on anything other than what the bacteria living inside their tissues provide them. Almost all cannot survive without a sufficient supply of methane or hydrogen sulfide. One hypothesis is that decomposing organic matter that has sunk from the surface, like whale carcasses, seaweed, and wood could serve as a food source, providing stepping stones between vents or seeps.

A field of mussels at a cold seep (photo Wikipedia)

Animals more typically found at vents and seeps are known to colonise the remains of whales in the deep sea. Smith et al. (1989), were the first to report vent animals colonising whale skeletons. They also provided a conservative estimate of the distribution of whale carcasses on the ocean floor and suggested that they would provide a persistent and abundant habitat for cold seep and hydrothermal vent animals. Adding sunken wood and seaweed to the list only increases the amount of available habitat.

A whale skeleton in the deep sea with patrolling hagfish (photo Wikipedia)

A gap in our understanding, though, is how enough methane or hydrogen sulfide is produced to support populations of chemosynthetic animals by decaying wood. The deep sea is a cold place, which is not conducive to the rapid breakdown of organic material. A large amount of wood would be required to provide the surface area necessary to produce enough gas. One hypothesis though, is that the surface area of wood might be increased by larger organisms, such as wood-boring bivalves, breaking it up first.

Researchers tested this idea by depositing wood logs on the Eastern Mediterranean seafloor at 1700 meters down and returned a year later to examine the animals and bacteria the had colonised the wood. They also measured the chemicals in the water released by the bacteria breaking down the wood. Using underwater robots, they observed that wood-boring bivalves had indeed broken the wood into smaller pieces, which were further broken down by other organisms. 

The activity of the organisms digesting the wood reduced the amount of dissolved oxygen, resulting in anoxic conditions that allowed sulfate-reducing bacteria to move in and produce hydrogen sulfide. The hydrogen sulfide then attracted a species of mussel, which usually found at cold seeps where it gains energy from symbiotic chemosynthetic bacteria. The mussels seemed to preferentially colonise cavities under the bark of the wood, presumably because sulfide levels were higher there.

The chemosynthetic mussel Idas modiolaeformis was found in the sunken wood piles (photo from Bienhold et al. 2013)

So it appears that wood boring organisms are able to pave the way for chemosynthetic organisms to colonise sunken wood in the deep sea. Their burrows, feces and the chips of wood that they produce all increase the surface area of material available for hydrogen sulfide producing bacteria to digest. Moreover, their activity and the activity of other organisms produce the anoxic conditions required for sulfate reduction, which is necessary to support chemosynthetic life.

A hypothetical succession of animals on submerged wood in the deep sea over a year. Initially wood-boring bivalves move in, followed by predators and detritus feeders (e.g. polychaetes and sipunculids). The respiration of the colonisers creates anoxic niches that allow the chemosynthetic mussels to move in (diagram from Bienhold et al. 2013).

Smith, C., Kukert, H., Wheatcroft, R., Jumars, P., & Deming, J. (1989). Vent fauna on whale remains Nature, 341 (6237), 27-28 DOI: 10.1038/341027a0  

Bienhold, C., Pop Ristova, P., Wenzhöfer, F., Dittmar, T., & Boetius, A. (2013). How Deep-Sea Wood Falls Sustain Chemosynthetic Life PLoS ONE, 8 (1) DOI: 10.1371/journal.pone.0053590