Trees in the ocean
As deforestation and the frequency of extreme weather events increase, it is expected that the amount of wood that is transported into the ocean will significantly increase. Once in the ocean, some remains floating for several months forming a hazard to shipping, and leaving unsightly deposits on shore, before sinking to the ocean floor. For example, Typhoon Morakot transported between 3.8-8.4 gigatonnes of coarse woody debris into the ocean during August of 2009 in the Philippines, leading to a significant fall of organic material into the nutrient-limited deep waters that surround the country. For many organisms in the deep sea, organic matter is limiting, but consuming delicious logs is challenging.
Not all logs are equally diverse. Some logs are hubs for deep sea creatures whilst others just meters away are barren. One reason for the disparity between the logs is the species of tree. On the wreck of the Titanic the pine and oak furnishings were almost completely destroyed after 70 years whilst the teak was “Almost mint.” Another difference would be the size of the logs. Larger logs provide a larger resource, a more complex habitat and more niches and therefore these logs have higher diversity and are more likely to be colonised. There seems to be another factor that affecting the likelihood to be colonised that remains unknown.
Unlocking the Cellulose
The first problem for any organism wanting to consume a piece of wood is how to process it. Step one is being able to mechanically break down the wood. Lots of animals found on the logs have the ability to break up the wood into smaller pieces. There are worms that bore into the wood, such as the subfamily Xylophagainae. Crabs such as the squat lobster Munidopsisandamanica andamanica use their claws to grab splinters of the wood and then have strong calcified gastric teeth that can grind up these splinters into a fine flour so it is less likely to abrade the gut.
The next stage in being able to consume the wood is to access the cellulose. It is very rare for an animal to be born with the enzymes that can break down the cellulose. The squat lobster is able to unlock the cellulose as it has specially adapted gut flora. Other species that can break down cellulose include fungi and prokaryotes.
The subfamily Xylophagainae is well known to bore into woods. The purpose of this boring could be to gain nutrients from the wood (meaning they have the ability to digest cellulose), to shelter from predators, or to feed on other organic matter on the wood’s surface? One member of the subfamily lives in a protective shell suggesting that it is not boring for protection. They have also been observed boring very deep into the wood where the only organic matter present is the log itself. This all goes to suggest that they are able to break down the cellulose found in the wood.
After a couple of months the cellulose-digesting organisms have unlocked the energy trapped in the wood and attract new types of organisms which can feed on waste or the organisms themselves. The mechanical break down of the wood provides new niches for organisms. Many polychaete worms use the holes bored by Xylophagainae as shelters.
Six to twelve months after it first landed the elevated respiration rates and production of sulfides means the log has attracted a new type of visitor – chemosynthetic bacteria. There is plenty of information about the mechanics of chemosynthesis and the organisms that benefit from it on this site alone, but the shorthand version is that these bacteria can synthesise energy without photosynthesis. These bacteria often form a symbiotic relationship with larger, more complex organisms. The bivalve Idas modiolaeformis can act as an indicator for the presence of high sulphide conditions as they use sulphide oxidising symbionts to acquire oxygen.
The Idas modiolaeformis does not only occupy areas of wood fall but also hydrothermal vents and whale falls. It has been suggested that these different habitats can act as stepping stones for the organisms. It might be able to hop from a hydrothermal vent to a piece of sunken wood then onto a whale carcass and then back to another bit of wood (if not the individual then genetically speaking.) This was certainly true historically, the Mytilidae mussel species have been introduced to hydrothermal vents via these whale carcass and wooden stepping stones. This makes even more sense when considering that logs are not randomly distributed in the sea but instead form pathways that depend upon the river outflows and current in the sea that affect the log’s routes.
The analysis of symbionts in mussel shows that there are two types of bacteria found in mussels, ones that specialise in whale carcasses and others that specialise in wood. This contradicts the stepping stone hypothesis, showing that the two mussels have separated and had diverging evolution. This is significant as it shows that they are traveling far further than previously thought.
Sunken wood logs form part of unique deep-sea systems playing an important role in the diversity and distribution of organisms. The other main constituents of these communities are hydrothermal vents and whale falls, the role of wood is an overlooked and understudied aspect. I hope that in the future the role of sunken wood is better understood by the scientific community.