Photosynthesis vs. Chemosynthesis

All living things need an energy source to power the chemical reactions that sustain life. Without such an energy source there could be no life. Most organisms on this planet get their energy, directly or indirectly, from the sun. Plants, algae, and some marine bacteria carry out photosynthesis, using the sun’s energy to produce sugars necessary for their survival. These organisms are called primary producers, because the energy stored in their organic matter (their primary productivity) fuels the rest of the ecosystem. Energy stored in plants is then used to power the planet’s herbivores, animals that eat plants. Finally, at the top of the food chain, carnivorous animals eat other animals, including herbivores, for their energy.

However, not ALL life on Earth depends on the sun for energy. More than 80% of the habitable space on this planet is in the deep, perpetually dark ocean, a region more than 1000m below the surface, where sunlight cannot penetrate and photosynthesis is, therefore, impossible. Most animals that live at these depths are detritivores, feeding on photosynthetically derived organic material (e.g. dead organisms, or fecal material, often called “marine snow”) that sinks down from the surface. But there are exceptions. In some places on the ocean floor volcanic activity causes heated and chemically altered seawater (or hydrothermal fluid) to flow out of large chimney like structures (often called “black smokers”; see figure 1). These fascinating areas are called hydrothermal vents, and some of the organisms that live around them derive their energy completely from non-photosynthetic sources.

Photosynthesis is analogous to the generation of electricity (useful energy) with a solar panel. In contrast, the primary producers in these vents function like a battery, harnessing the energy of chemicals from the hydrothermal vents and storing this energy in organic material that other organisms can eat. This chemically derived energy supports ecosystems that, in some places, have as much biomass (the total weight of organisms in a given area or volume) as found in tropical rain forests, notable among terrestrial ecosystems for their high biomass. This process is called chemosynthesis, and is carried out by microorganisms in the rocks, sediments, fluids, and even within some of the animals found at hydrothermal vents.

Figure 1: This image traces the pathway that seawater makes through the ocean crust as it is chemically and thermally transformed into hydrothermal fluid. This hydrothermal fluid carries the chemical energy that fuels hydrothermal vent ecosystems. This image also compares the chemistry of typical seawater with typical Mid-Ocean Ridge (MOR) hydrothermal fluid.

Why study vents?

Scientists study hydrothermal vents for many reasons. For one, the deep sea is very poorly described – we have better maps of the moon and Mars than we have of our own sea floor (this is because satellites can not see through water) – and we are curious about what lives in these depths and how they live down there. Another reason to study vents is that these studies may help us understand how the world’s first organisms evolved and functioned. Some scientists hypothesize that life may have begun at hydrothermal vents, because some of the most ancient organisms known seem to be heat loving hyperthermophiles (organisms that prefer temperatures above 80°C). However, this hypothesis has also been debated.

Additionally, organisms that live at hydrothermal vents are very unique because they can thrive in a chemical soup that would be toxic to most other organisms on Earth. These organisms, the so-called extremophiles, are adapted to these extreme thermal and chemical conditions, and they are of interest to those who are application-minded because they might make compounds that could benefit humanity, e.g., by being useful as drugs or by having chemical properties that make them valuable in some industrial settings.

Finally, astrobiologists who think about where we might find life beyond Earth are interested in vents because it is possible that volcanic activity on distant worlds such as Io, Titan, or Europa may form hydrothermal vents which would be potential energy sources for chemosynthetic life. By studying hydrothermal vents, these astrobiologists hope to gain a better understanding of what they should look for on other planets. Photosynthesis is less favorable as you get farther away from the sun. For example Europa (one of the planetary bodies hypothesized to be potential habitable) is 5.2 times as far from the sun as earth, and so the inverse square law tells us that sunlight would be 27 times weaker on Europa than it is on Earth. To return to the previous analogy, a solar panel would not work nearly as well that far away, but a battery would still work. This and the fact that many of these potentially habitable worlds have very extreme (cold, hot, or toxic) conditions on their surfaces may make chemosynthesis below the surface a more favorable energy source than photosynthesis.

For those of us focused on hydrothermal vents, it seems like there are new discoveries being made all the time.

Figure 2: This image (courtesy of Stace Beaulieu, Woods Hole Oceanographic Institution; http://www.whoi.edu/page.do?pid=83560&tid=4142&cid=71854&i=1) shows the distribution of known hydrothermal vent activity. Red icons represent vents that have been confirmed by visual exploration, and yellow icons represent vent activity that has been inferred from chemical or other signals, but not confirmed visually. Note the stars, which represent new discoveries in 2010 or 2011.

So, what have we learned recently about these alien worlds here on Earth?

Mid-Cayman Rise: the deepest and farthest off-axis vents discovered

In a recent paper published in the journal Nature Communications (Connelly et al., 2012; open access) an international group of researchers reported the discovery of two previously unknown hydrothermal vent fields on the underwater mountain range called the Mid-Cayman Rise in the Caribbean Sea (the unlabeled star between Mexico and Cuba in Figure 2). One of the vent fields (named Beebe after famed explorer and aquanaut William Beebe was found at a depth of 4,960m (or just over 3 miles) and represents the deepest vents known (20% deeper than any other known vent site at the time of publication). At the same time, the plume of hydrothermal fluids being emitted from the Beebe Vent Field chimneys rises far higher in the water column than the plumes from most known vent fields (1,100m as opposed to the typical 200-400m). This is interesting because it has implications for how far the metals and other vent-derived chemicals may be traveling away from the vent field. Scientists know that hydrothermal activity influences ocean chemistry, but these details and the implications of these findings are not yet known.

The other vent field, named after marine geochemist Karen Von Damm, is another noteworthy vent site because it is located in a region we active high temperature venting. The Von Damm field is the farthest off-axis (meaning away from the center of the ridge) high temperature vent that we are aware of. Most of the vents that have been discovered are fairly close to this axis. Because of this, we typically search for new vents on-axis. However, this new discovery means there might be many more high temperature vent fields out there waiting to be discovered than originally thought, because scientists have previously assumed that vents only occur close to the ridge axis. Additionally, this vent field seems to have a fairly unique combination of mineralogy and fluid chemistry. Most vent chimneys are composed largely of minerals that contain sulfide (S2-,) (such as pyrite, marcasite, chalcopyrite) and their vent fluids contain the toxic gas sulfide. Other types of vents tend to be low temperature, comprised largely of the mineral anhydrite that instead contains sulfate (SO4-2), with fluids very low in sulfide. Vent scientists think of these chimneys as being dominated by one mineral type or the other because of the chemical and thermal conditions needed for the different minerals to form and persist. The Von Damm vent field chimneys are very interesting because they appear to be built out of both mineral types simultaneously and their fluids contain sulfide, which is not usually found with anhydrite chimneys. All of this means that there is still a lot we don’t understand about how vents can form.

East Scotia Ridge: a new biological province discovered

As mentioned previously, vents tend to form along mid-oceans ridges, and all mid-ocean ridges are predicted to have hydrothermal vents. One of the least well explored ridges is the Antarctic East Scotia Ridge (marked in the lower right hand corner of Figure 2). Recent work on vents found at this ridge by a different international group published in PLOS Biology (Rogers et al., 2012; open access) indicates that the biological communities found here are distinct from those we know at other vent locations. It seems that there may be a group of organisms that only live at these Antarctic vents, possibly limited in their ability to disperse elsewhere because of how seawater currents flow around Antarctica and are somewhat disconnected from other ocean currents.

Vent biologists are very interested in figuring out how animals travel from vent to vent and determining how similar the species assemblages are at different vent sites, and this information about the Antarctic communities is a very interesting new piece of that puzzle. Instead of the characteristic tubeworms, mussels and shrimp found along the main mid-ocean ridge system, these East Scotia Ridge vents appear to be home to anemones, limpets, snails, stalked barnacles, and sea spiders. The dominant species is a new species of Yeti Crab in the genus Kiwa. This genus of crab is not only fantastic looking, it is known to literally dance in warm hydrothermal fluids in order to farm bacteria on its claws which it then eats (Thurber et al., 2011). You can see this behavior in this video!

One of the big questions in vent biology is how different species are distributed among the different vent sites and the ability (or inability) of different organisms to move among different vents and vent fields (all of this can be summed up by the term biogeography). This work adds new pieces into the vent biogeography puzzle.

The recent work on the Mid-Cayman Rise and the East Scotia Ridge highlights that we are still far from fully understanding even the basics of where vents are and what organisms colonize them. Work that in the coming years and decades will no doubt likely shed new light on to how these captivating environments influence the overlying ocean both in terms of their biology and their chemical influence on global biogeochemical cycles.

Heather Craig Olins is a doctoral candidate studying hydrothermal vent microbial ecology in Harvard’s Organismic and Evolutionary Biology department.

Technical References:

Connelly, DP et al. (2012). Hydrothermal vent fields and chemosynthetic biota on the world’s deepest seafloor spreading centre. Nature Communications 3:620. DOI: 10.1038/ncomms1636
Rogers, AD et al. (2012). The Discovery of New Deep-Sea Hydrothermal Vent Communities in the Southern Ocean and Implications for Biogeography. PLOS Biology 10(1). DOI: 10.1371/journal.pbio.1001234.
Thurber AR, Jones WJ, Schnabel K (2011) Dancing for Food in the Deep Sea: Bacterial Farming by a New Species of Yeti Crab. PLoS ONE 6(11). doi:10.1371/journal.pone.0026243

Further Reading:

2012 Cayman Rise cruise blog: http://www.oases2012.blogspot.com/
Hydrothermal vents and the origin of life: http://www.space.com/19923-hydrothermal-vents-origin-life.html
First pictures from the deepest vents: http://io9.com/5986874/the-first-pictures-of-life-in-the-deepest-ocean-vents-ever-discovered
More thoughts on the Antarctic vent diversity: http://blogs.scientificamerican.com/artful-amoeba/2012/01/04/deep-sea-explorers-stumble-on-antarctic-lost-world/
“Lost World” discovered around Antarctic vents: http://www.sciencedaily.com/releases/2012/01/120103185246.htm
A free ebook chronicling some recent expeditions to hydrothermal vents: http://www.smashwords.com/books/view/129604

References for Figure 1:

Reysenbach, A. L., & Cady, S. L. (2001). Microbiology of ancient and modern hydrothermal systems. TRENDS in Microbiology, 9(2), 79–86.
Wankel, S. D., Germanovich, L. N., Lilley, M. D., Genc, G., DiPerna, C. J., Bradley, A. S., et al. (2011). Influence of subsurface biosphere on geochemical fluxes from diffuse hydrothermal fluids. Nature Geoscience, 4(6), 1–8.
Tivey, M. K. (2007). Generation of Seafloor Hydrothermal Vent Fluids and Associated Mineral Deposits. Oceanography, 20(1), 50–65

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