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To finish up with benthic ecosystems of the ocean,
let’s review some of the special seep and vent communities that we first
discussed at the beginning of the semester when we studied hydrothermal vents and seafloor spreading.
These ecosystems are unique because they exist on deep seafloor where there’s no light, and
thus photosynthesis cannot be the energy source that serves as the foundation of the food web.
These ecosystems are also like oases – only this time in the darkness of the deep – they are found
where there is an unusually large concentration of energy gases percolating up from inside the
Earth – either methane or hydrogen sulfide or a combination of the two. Chemosynthesis is the
major energy transfer mechanism that supports this food chain, as bacteria use the energy stored in
these gases to produce sugars. This equation shows one example of chemosynthesis that happens at hot
vents atop seafloor spreading centers otherwise known as hydrothermal vents. Notice that in this
equation, sulfuric acid is the waste product. As this map shows, there are a number of
different vent or seep communities that have been identified and studied across
the world’s oceans. They can be separated into two major types – hot vents and cold seeps.
Hot vents are the hydrothermal vents that occur at seafloor spreading centers, where hot magmas under
the surface heat up seawater that has infiltrated the crust through cracks. This heated water is
now less dense, so it rises upwards. Enroute to the surface it leaches out metals from the crust
and when it is finally released through a vent on the seafloor, it is carrying many dissolved
ions and gases that are released into the cold water. The temperature of the water in the vents
can be as high as 400 degrees Celsius. Remember what the temperature of deep water is? Since most
of the world’s bottom water is from Antarctica, it’s likely it’s around 0 degrees Celsius. That
large drop in temperature that happens to these percolating fluids causes various ions to combine
and precipitate as solid minerals, creating progressively taller and taller chimneys around
the vents. The black material you see in the liquids that are leaving the vents are very tiny
particles of iron sulfide. These chimneys can be made up of many different kinds of metal sulfides,
including Zinc, Iron, Copper, and rare metals like gold and silver. A common animal found living near
these vents in the Pacific Ocean are tube worms. They are mixotrophs that contain gardens of
chemosynthetic bacteria, which they periodically move over the vent to give them access to the
hydrogen sulfide gases. Other animals that exist in these vents include giant clams, large mussels,
white crabs, transparent shrimp, microbial mats, and many others. There is a strong zonation of
life in these vents, because the ones that live right next to the vent must be extremophiles,
able to sustain extremely hot temperatures. As you move further from the vent, the temperatures
will cool off substantially, and the organisms will quickly change to those that cannot handle
high temperatures. These vents usually last for a few years before either an eruption barbecues
the residents, or the energy source disappears – the vent stops. Larval organisms are carried
by water currents and if they are lucky, they will arrive at and populate a new vent. Some organisms,
such as bacteria and certain larvae, can lie dormant in the sediment until a new vent appears.
Similar types of organisms exist around cold seeps as well as new ones. Microbial mats,
seastars, shrimp, crab, clams, mussels, fish, limpets, snails, brittle stars, anemones,
and tube worms. Chemosynthetic bacteria are also the base of the food chain. However extremophiles
are absent. The water is cold. And the energy source usually is methane exclusively or both
methane and hydrogen sulfide. To date, cold seeps have been found near hypersaline seeps,
subduction zone seeps, and hydrocarbon seeps. Hypersaline seeps are typically found at the
base of the continental slope, where it meets the abyssal plain. Out of the seafloor seeps
super-salty brines that have similar temperatures to the surroundings but salinities as high as 46.2
ppt. Because of the high salinity, they also are highly dense and don’t mix with the surrounding
water. They form an underwater lake. These seeps form because of buried salt beds that formed tens
to hundreds of millions of years ago along shallow seas and lagoons along a continental margin.
These salt-rich sediments have now been buried deeply and as seawater penetrates the crust,
the water dissolves some of the salt and becomes more saline. The water also picks up the gases
formed from decomposition of organic material that would have been deposited alongside. Therefore,
hydrogen sulfide and methane gases are associated with these seeps, and you will find communities of
tube worms and chemosynthetic bacteria and their predators lining the shorelines of these lakes.
There are some well-studied hypersaline seeps on the bottom of the Gulf of Mexico as shown here.
Hydrocarbon seeps are typically nearby hypersaline seeps, because hydrocarbons form when shallow
seas and intertidal wetlands, are buried and decomposed without oxygen. They also produce
primarily methane and hydrogen sulfide gases, but also oil and tar. These liquids and gases
are less dense than the surroundings, and they will rise up cracks and bubble out onto
the seafloor. When they do this on land, we call them tar pits. On the seafloor, they create large
asphalt strips that are surrounded by unusual seep communities, much like those found at hypersaline
seeps. Many of these seeps are currently found and studied near the oil and gas reservoirs in
the Gulf of Mexico.
Subduction zone seeps occur within trenches and
form as methane gas seeps up cracks in the rocks. Methane forms during decomposition, especially
decomposition that happens after rocks have been buried, and thus there is no oxygen. This gas is
low density, so it prefers to rise up cracks as they form. In areas where cracks concentrate,
there can be large amounts of seeping methane gas and thus, again, large communities of organisms
surrounding these seeps and basing their food web on chemosynthesis. Subduction zone seeps have
been found along the Japan Trench, Peru-Chile Trench, and just offshore of Oregon along the
Cascadia Subduction Zone.
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Please follow the website’s extra resource links to see live footage of hydrothermal
vents and cold seeps and see what are truly some of the most remarkable sights on the bottom of
the seafloor – including a lake, with a shore and waves – but on the bottom of the ocean,
and what looks like an underwater, naturally produced parking lot.
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