Under the sea

Due to an unexpected lack of internet, today's post was delayed somewhat. Sorry!

A few weeks ago, I alluded to deep sea ecosystems (a picture of the ever-popular tube worms featured rather prominently). So, as I'm sure you've all been waiting with bated breath, today we're going to venture into the great blue yonder. Actually, we're going to venture well below the depths at which light penetrates seawater, so really, “blue” is more a state of mind down here than anything else.

Yes, the first rule of life in the deep sea is that it is dark. The ocean is divided into three zones, based on availability of light. In the upper 200m, or the the euphotic zone, light is available, although it gets darker with depth. It also gets bluer. The efficiency with which water absorbs light varies with wavelength (remember Wednesday's visit to the EM spectrum?), and the blue wavelengths are the least efficiently absorbed. So, the deeper you go within the euphotic zone, less and less of the light making it down is any wavelength but that of blue (475 nanometers, if you're interested). Most ocean life lives in the euphotic.

 Note the dramatic difference in how well different wavelengths are absorbed

Below the euphotic zone is the dysphotic zone, or as the cool scientists call it, the twilight zone. The only reason this zone is separate is because while some light penetrates (so it can't technically be called dark), the amount of light is miniscule – not enough to allow photosynthesis, and increasingly scarce with depth. It takes until 1,000 m for there to be absolutely no trace of light in the water. At this point, the aphotic zone (“midnight zone”, to the poetic) officially begins.

The aphotic zone isn't just dark. It tends to be cold (more on that in a minute), with water temperatures close to freezing. With a 1,000+ m water column overhead, pressures are extremely high. The cold and pressure might be survivable to a well-adapted organism, but what about the lack of light? The base of any food chain is the autotrophs – those organisms capable of generating their own food, usually through photosynthesis. In the absence of light, just how are autotrophs supposed to get by?

Well, photosynthesis may be the most familiar means of autotrophy up here on the surface, but it isn't the only one. There is also chemosynthesis. Photosynthesis works by using light energy to split carbon dioxide and water molecules, and recombine them into molecules of glucose. Oxygen is a convenient side effect.

6CO₂ + 6H₂O + Light Energy = C₆H₁₂O₆ + 6O₂

Unlike the single equation of photosynthesis, chemosynthesis can happen in a number of ways. Here's one example. An organism combines two chemicals within the cell, and then uses the energy from that reaction to fix carbon dioxide into a usable form. Here's a form of chemosynthesis that actually occurs on the ocean floor.

CO₂ + 4 H₂S + O₂ = CH₂O + 4S + 3H₂O.

Here, the energy from the reaction of hydrogen sulfide and oxygen is sufficient to split the carbon dioxide model and combine it with some of the newly released hydrogen into a usable sugar. Pure sulfur and water are produced as a side effect.

So, does this actually happen in the deep ocean? You'd better believe it. Chemosynthetic bacteria make up the base of deep ocean food webs, in much the same way that plants make up the base of many terrestrial food webs. Of course, in order to carry out chemosynthesis, the bacteria need a source of something that can be combined with oxygen (or another chemical), and quite a lot of the deep sea consists of chemically boring seawater. Quite a lot, but not all. We'll start with the less exciting of the two best-understood environments – cold seeps.

Cold seeps, as the name indicates, are areas of the sea floor where a gas is seeping out. The seepage is the result of tectonic activity, but these are not violent, explosive gaseous emissions.

          

Compare the distribution of methane seeps with the distribution of tectonic plate boundaries

Instead, gases seep out in liquid form, or persist in solid form – pressure on the deep sea floor is sufficient to compress gases like methane and hydrogen sulfide into fluids, and even ice. Petroleum and natural gas may also be seeping out of these sites – some deepwater drilling exploits cold seeps. As the gases move through sediments, they are used by a wide array of bacteria take advantage of the available methane and hydrogen sulfides to conduct chemosynthesis. These in turn are preyed on by worms, nematodes, clams, mussels and snails, who are consumed by eels, and even well-adapted fish (as a consequence of the high pressure at depth, few if any deep-sea organisms have lungs or other internal air spaces, as those would be crushed instantly). Cold seeps promote the formation of carbonates in ocean water – since snails, clams, mussels and other bivalves build their shells from carbonates, this is actually a pretty favorable environment.

Probably the weirdest creature in the cold seep ecosystems is the ice worm. As mentioned above, at sufficient depth, methane can be compressed into ice. This ice is called a gas hydrate – gas hydrates have all kinds of interesting properties, and are involved in my personal favorite ecological doomsday scenario (the topic of a later post), but under stable conditions in the deep are just ice with a different chemical makeup. There are worms that burrow in this ice. Ice worms can get up to 4 cm long, and have been shown to build fairly extensive burrows. It's not exactly clear what they eat down there, but just try to wrap your brain around the fact that in the cold, dark, bottom of the ocean, there are worms happily existing in frozen methane, much in the way that worms burrow in the well-aerated soils at the surface.

  

Ice worms!

Ok, now on to the more exciting environment – hydrothermal vents. Like cold seeps, vents are connected to tectonic activity, and emit gases (usually hydrogen sulfide). Unlike cold seeps, hydrothermal vents explode out hydrogen sulfide, along with heat and heavy metals. Hydrothermal vents are also (descriptively) called black smokers.

 

Well, that's different from gas quietly moving through the sediments. Hydrothermal vents occur when seawater filters into the ocean crust, and moves through the rock, towards an area where magma is closer to the surface. As it moves through the rock and towards the magma, the water gets superheated, and picks up dissolved metals and gases. Eventually, the hot, metal-and-gas-laced water explodes from the crust, releasing a plume of chemicals. The black smoke is actually metals beginning to precipitate out in solid form. 

Hydrothermal vents provide a veritable all-you-can-eat-buffet for chemosynthetic organisms – bacteria similar to the ones inhabiting cold seeps are readily found around the vents. However, the vents also come with some unique challenges. Temperatures in vent environments range from the near-freezing seawater to the extremely hot vented fluid. The hydrogen sulfide that bacteria feast on has the effect of dropping local seawater pH (values as low as 2.8 have been recorded), to the point that it is nearly impossible for snails to form shells – shells dissolve in low pH water. The metals spewed out by the vents are often toxic. In spite of this, chemosynthetic bacteria thrive, as do the worms, snails and fish that eat them.

Comparable to cold seep ice worms in the weird category are another unique worm life form – tube worms.

Tube worms are some of the largest members of the vent animal assemblage – the very accurately named giant tube worms grow up to 3 meters in length, and reach that length quickly. A giant tube worm is capable of growing at a rate of 33 inches per year. Just what are they eating to fuel such rapid growth? Depends on your definition of “eat”. Tube worms have a symbiotic relationship with chemosynthetic bacteria – worms ingest hydrogen-sulfide, which is converted into sugar and energy in the gut by bacteria. The worms are fed, and the bacteria are protected from other invertebrates that would prefer to eat them, rather than host them. Tube worms live at depths of up to a mile, sometimes right on the edge of vents. Not bad for a creature with no eyes or proper digestive system, right?

Deep sea ecosystems are further proof that when the conditions are harsh, evolution excels. The creatures living down there are adapted to an environment unlike any other on Earth, and are doing just fine, thank you very much. Life way under the sea is one huge, chemosynthetic tribute to the mind-boggling diversity of nature.