A recent paper published online in the Proceedings of the Royal Society B (I have no idea what the B means, but maybe one of you can clarify with a comment) draws an enticing connection between whale excrement and climate change.  Because this study involved whale poop, it even made the mainstream news.  The folks at MSN were even nice enough to provide an areal photo of what it looks like when a whale goes to the bathroom.  I have been lucky enough to experience this first hand on a whale watch when a whale shared the contents of its bowels with those of us aboard the ship, so this study had a little bit of a personal connection for me!  Hat tip to Chrissy at Working Through the Blue for pointing me towards this article.

The paper (entitled “Iron defecation by sperm whales stimulates carbon export in the Southern Ocean”) can be found here, and seems to be available for download without subscription.  You might be wondering whether this study draws a connection to the microbial world, or if I am just excited to write about whale poop… I assure you there is a connection.  In most environments certain nutrients are limiting, and therefore control the rate of primary production.  This means that no matter how much energy (food, sunlight, or chemical energy) is available, the microbes that form the base of the food pyramid can only convert that energy into biomass (storing carbon in the process) when that limiting nutrient is present.  In the case of the Southern Ocean, iron is the limiting nutrient.  The microscopic phytoplankton floating at the surface take CO2 out of the atmosphere and convert it to biomass that eventually falls through the water column and acts as food for other organisms.  However, they are restricted in how fast they do this because there is not much iron dissolved in these surface waters.

This is where the Sperm Whales come in.  These giants descend into the cold dark depths to feed (mostly on squid), and come up to the surface to breathe and relieve themselves.  It turns out that their waste is iron-rich.  By depositing this lovely substance in the photic zone, they apparently stimulate primary productivity.  This means that the photosynthetic plankton absorb CO2, grow, reproduce, are eaten, and so on, and eventually organic material sinks down and provides energy to life in the darker regions.  But wait you say (being the savvy reader that you are), don’t the whales exhale a lot of CO2 back into the atmosphere?  Yes, they do, but according to the modeling of these researchers, the plankton take up much more CO2 as a result of the iron released by the whale than the whale releases.  “By enhancing new primary production, the populations of 12,000 sperm whales in the Southern Ocean act as a carbon sunk, remivong 2 x 105 tonnes more carbon from the atmosphere than they add during respiration”.  They go on to suggest that the reduction of whale populations due to whaling is likely to have diminished this carbon sink.

How did the scientists calculate all of this?  They first assume that these whales eat only cephalopods (mostly squid).  I do not study whales, so I don’t really know how accurate of an assumption this is, but it seems to be true for this region (in other areas sperm whales eat more fish).  Then, they used published reports of the amount of iron in cephalopods, and then calculated the amount of prey consumed by the whales by averaging previous studies that had estimated how much these animals eat.  From what is known about iron retention in marine mammals (maybe as low as 10%, they used 15% as their estimate), they were then able to estimate how much iron must be released by the whales during defecation.  They then had to figure out how much of that iron stays in the photic zone long enough to initiate primary production.  They did this by assuming (based on observations) that undigested squid beaks sink almost immediately, but that the liquid material stays around for a long time (this is admittedly non-quantitative).  Finally, they know the form that iron is generally found in the gut (ferrous salts), and they know that that form  can be expected to dissolve quickly and therefore would hang around in the photic zone.  Their estimate was that 75% of the defecated iron would stay around in the photic zone.

I explain this because while headlines like “whale poop may help offset CO2 emissions” are certainly catchy, to gauge the accuracy of this claim it is important to know how the numbers were crunched.  Flashy findings like this can trickle into the mainstream news, but people generally don’t get an explanation of why they should believe this.  [SoapBoxAlert] I think this is part of why it can be so hard for non-scientists to determine which bits of “science” to “believe” and which to be critical of, especially when it comes to climate change [now back to the regularly scheduled science].

In this case the researchers have made some very interesting calculations (they also provide estimates of the losses in carbon sequestration that historical whaling may have caused).  I think it is important to know that their numbers are based on a model rather than actual measurements of the iron concentration of whale feces or what happens to that iron over time.  Models are only as good as the data that go into making them, and in this case it seems (to me, admittedly knowing very little about whales) that they have done a pretty good job.  Whales are very difficult animals to study, and something as simple as sampling their feces would take much more time, energy, and money than you might thing, so I am certainly not faulting the authors for this.  Similar calculations were done to estimate the rate of carbon withdrawn from the atmosphere, and then sunk, but since this is already getting long, I will spare you those details.

In summary, this study provided me the opportunity to write about whales, poo, microbes, the carbon cycle, and scientific methods at once… what better way to spend a Sunday morning!

Although I studied physics and chemistry in college, I have always held an inherent appreciation for how life works.  And now, researching microbes in action, I am continually amazed by their diverse habitats and metabolisms.  It is remarkable to think how despite the diminutive size of microbes, they are dominant members of our biosphere and play a key role in every known biogeochemical cycle.  This list includes carbon, sulfur, nitrogen, and oxygen as well as trace metals such as iron, cobalt and zinc.  I have started thinking that if there is an available compound on Earth, then microbes have found a way to use it.

What does it mean that microbes are involved in these biogeochemical cycles?  It usually implies that a microbial species has found a way to use a form of that specific element while the microbe undergoes cellular respiration.  Although respiration is complex, I tend to think of it in a simplified manner as a transport of electrons from one compound to another by a living organism.  The organism utilizes this electron transport to produce energy in order for it to stay alive and reproduce.  As humans, we take electrons from the carbon compounds we consume as food and transport those electrons to the oxygen we breathe.  Since we use oxygen, this is aerobic respiration. One can think of the food we eat as electron donors and oxygen as the electron acceptor.  Now, a fascinating aspect about microbes is that different species have evolved to use a variety of electron donors and acceptors in respiration process.  Not only is this how microbes are involved in Earth’s biogeochemical cycles, but this is why certain microbes can be found in such extreme conditions, such as hydrothermal vents which have been discussed earlier.

Since microbes can use a variety of compounds as electron acceptors, what if there was a way to harness these electrons to produce electricity?  In 1911, M.C. Potter was reported to be the first person to observe electricity production from microbes but it has only been within the past 30 years that this research has gained momentum.  In these systems, which are called microbial fuel cells, the material the microbes are using as an electron acceptor is called an electrode, more specifically an anode. By connecting the anode with a metal wire to a region with even higher electron affinity, termed a cathode, we can generate a circuit.  Oxygen is one of the best electron acceptors and typically these systems have oxygen present near the cathode and the microbes grow in an chamber without oxygen in order for electrons to flow from the anode to the cathode.

Personally, I find the mechanism for microbes to transport electrons to a solid surface fascinating as a basic science question and think that understanding this process better would shed some light on how microbes do this in nature.  Right now, there are a handful of microbes known to utilize a solid surface as an electron acceptor and I am curious about what others are out there that have not yet been identified.  What do these different species have in common?  What influences growth and thus electron generation?  How is the structure of the microbes growing on the anode as a biofilm influenced by the anode properties, such as material composition or surface patterning?  How does the internal properties within the biofilm correspond to electron generation, such as pH?  To address these questions, I perceive microbial fuel cells as a useful system to explore how microbes form biofilms on surfaces that act as an electron acceptor.

Further, as a new area of research with potentially huge contributions to be made more knowledge will improve how this mechanism is utilized in technology not only for energy production but also for organic waste removal.  An attractive aspect of this technology is that electricity generation by the microbes goes hand-in-hand with organic waste removal because the microbes use the organic compounds as their electron donors, or food.  In wastewater treatment, this would allow for the water to become purified at the same time electricity is generated.  Electricity production is still quite low with one report stating that their microbial fuel cell design could power 16 60-watt light bulbs.  There is still room for improvement in power generation and I believe that addressing the mechanistic questions about microbial electron transport to an outside electron acceptor will aid in both improving our basis for building these technologies and our understanding of how our microbes are major players in our world’s biogeochemical cycles.

A group of American and Canadian scientists (primarily based at McGill) published a paper recently in the journal of the International Society of Microbial Ecology (ISME) that offers insight into the possibility of life on Mars.  The paper is entitled “Microbial characterization of a subzero, hypersaline, methane seep in the Canadian High Arctic”, and you can read the abstract here.  The researchers found a spring that is below freezing, but liquid due to the high salinity (24%, for comparison the ocean is about 3.5%).  They found methane seeping out of this spring.  Methane seeps like this are extremely rare in the terrestrial world, but have been relatively well studied in the ocean.

In their words : “Here we report on the first microbiological and geochemical characterization of the only known terrestrial methane seep in a cryo-environment on Earth in the form of the hypersaline subzero spring, which arises through thick extensive permafrost in an area with an average annual air temperature of -15 °C and with air temperatures below -40 °C common during the winter months.  This site provides a model of how a methane seem can form in a hypersaline cryo-environment and can support a viable microbial community where methane itself may behave as an energy and carbon source for sustaining anaerobic oxidation of methane.”

Typically when methane is being produced it is either through biological or geological processes.  The recent discovery of methane plumes on Mars was exciting for this reason… it implied that either there was some unknown geologic activity on Mars (it is assumed to be inactive below the surface) or there was a community of microbes producing methane on Mars.  That is a maybe bit of an oversimplification, but I am not a planetary geologist.  Anyhow, folks in the astrobiology world have been proposing that we might be looking at evidence of methane producing microbes (methanogens) on Mars.  Other astrobiologists disagree with this contention because while there is water on Mars, it is very saline… so saline that some think life is impossible (technically it is the water activity and not the salinity that make the planet so inhospitable, but sanity is a good way to thing about it).  Two of the things that make this recent paper so cool is that it finds a location on Earth that could be analogous to the hyper-saline methane seeps on Mars, and that it provides evidence for an alternate scenario to methanogens being what we should expect to find if there is life on Mars.

The methane in the spring in this study is not likely to be produced biologically.  The difference between the carbon isotopes in the methane and the carbon dioxide indicate that it is not biological in origin (organisms preferentially use certain isotopes, and therefore leave an isotopic “signature” in the carbon that indicates when methane was formed biologically).  However, the scientists found a community of organisms living in this ultra-extreme environment.  The organisms they found (both bacteria and archaea) seem to be using the methane as energy and a carbon source, rather than producing it.  These metabolisms might be coupled with sulfur oxidation like it is at deep-sea hydrothermal vents and methane seeps.  Microbes that “eat” methane are interesting from a climate change point of view because methane is one of the major greenhouse gasses aside from carbon dioxide.

The archaea found in this study are related to those found in hypersaline deep-sea methane seep sediments, while the bacteria seemed to be related to those found in terrestrial Arctic and Antarctic environments.  Another cool thing about this paper is that the authors were able to culture 8 different types of bacteria from these samples.  This is exciting because more than 99% of all bacteria have not yet been cultured, and scientists can learn a great deal more about an organims once they are able to grow it in a lab.

The authors also suggest that this site might be a location where ancient DNA could be preserved.  This is because high salt tends to keep DNA from degrading over long periods of time.

In summary this study is very cool because…

  • It characterizes life from an extreme environment that has not been studied previously (terrestrial hypersaline methane seep).
  • It offers insight into what type of life we might expect to find in similar environments on Mars (not necessarily what we expected).
  • The authors were able to extract DNA from difficult samples, and was able to culture organisms from this extreme environment.
  • It suggests that this site might offer more cool discoveries in the future (like ancient DNA).
  • It draws a connection between the microbial ecology of the hot deep sea and the frigid Arctic.