Written by: Abhishek Chari
In searching for topics to discuss in the ‘Life is Rough’ series, I’ve come across some very fascinating phenomena. Each of these is fascinating on its own, but the deeper I dug into them, the more I realized that they are all connected. And, the connections are just as interesting as the individual phenomena. I can only hope that my narration of the first of these issues does justice to its metaphorically, and literally, gripping reality.
(Image of barnacles Balanus sp.)
(Ria Tan https://www.flickr.com/photos/wildsingapore/4124921756 (CC BY-NC-ND 2.0))
They’re not the kind of animals that are ever going to be featured high on a list of nature’s sexiest spineless creatures. They are mostly marine, filter-feeding arthropods (which translates to: they live in the sea, they filter water to eat the microscopic plankton that are suspended in it, and they don’t have a spine or backbone but have an exoskeleton, a segmented body and jointed/paired limbs). The adult forms of barnacles are a few inches in size and attach themselves to hard surfaces in the ocean, sometimes even other animals, mostly in shallow and tidal water. The larval forms are motile and serve as an important source of food for fish. It is hard to understand what an adult barnacle looks like from just a picture. The best I can come up with is the image of a soft, miniature crab-like organism inside a shell, stuck by its head to a rocky surface, its feathery legs coming up and out.
(A video of barnacles feeding)
Barnacles contribute to a problem that costs at least 500 million US dollars per year, and that’s just for the United States Navy. Worldwide costs for dealing with the same problem are estimated to run to 10 billion pounds sterling per year. Barnacles do something called ‘fouling’. Fouling is the attachment and growth of marine organisms on man-made things that are submerged in water. They attach to ship and submarine hulls, oil rigs, and even the insides of water supply pipelines for power plants and water desalination plants. By attaching in large numbers, they compromise the functioning of all the vehicles, structures and systems mentioned above. Additionally, the increased fuel needed to move ships encrusted with barnacles, the costs of cleaning befouled structures, the loss in energy generation from power plants and the hazardous chemicals that are commonly used to prevent fouling are detrimental both to the environment and the economy. So, understanding how barnacles attach to surfaces is very important indeed.
Barnacles attach to a variety of surfaces using a glue that they secrete called barnacle cement. It’s difficult to analyze barnacle cement because it hardens in just about five minutes and is very resistant to being dissolved after that. Supported by decades of research in various fields of biology and an interesting evolutionary hypothesis, scientists uncovered evidence that points to the secret identity of barnacle cement: barnacle blood. (Note: Even though the fluid that functions as blood in invertebrates is called hemolymph, for ease of understanding I am referring to it as blood in this article.)
Lets forget about the fouling problem for a second, and take a look at barnacle adhesion itself. They secrete a glue that is rich in proteins, which forms a strong, insoluble bond with many surfaces, while submerged in water. Many man-made adhesives don’t work well on wet surfaces. This is because they usually end up sticking to the water rather than the surfaces they are supposed to be joining together.
So, barnacles make this crazy glue or cement that works underwater, to adhere them to many surfaces. But, this isn’t the only situation in which a living organism needs to bind wet surfaces together. When any animal is wounded, that wound has to be closed up quickly to prevent blood loss. Closing up the wound by blood clotting is basically gluing wet surfaces together. This phenomenon has been thoroughly researched in many animals. Scientists hypothesised that when barnacles adapted to a non-motile adult life, the mechanisms for cement-based adhesion may have been co-opted from the evolutionarily conserved blood clotting process.
We’ve all seen how blood clots. It works in just about the same way in lots of different organisms. So, what’s really happening at the molecular level? Lets retrace the steps from the end to the beginning, like reconstructing a car crash. The end is the clot. It’s a strong mesh filling up the wound. It’s made of protein fibers that are cross-linked to each other. These fibers are normally floating around in the blood, in a large, inactive form. They are activated by being chopped up into a smaller, active form. What’s doing the chopping? A biological nanomachine called an enzyme. Let’s recap. The clot is a mesh of linked fibers which are activated by an enzyme. The activating enzyme is itself activated by being cut into a smaller, active form. But, what’s cutting up the enzyme? Another enzyme! From here on, a series of enzymes are activated by the ones above them in the sequence by being cut up. In this way, we’ve retraced the process all the way back to a set of physical and chemical signals generated by the wound that needs to be closed.
Now let’s play it out from beginning to end, like it happens in real life. There’s the wound, gushing blood. The wound generates signals that turn on an enzyme cascade of activation, which leads to activated mesh-fiber proteins. A final bit of cross-linking and, voila! We have a clot to stop the blood loss from the wound.
Research until this point in time had already produced clues pointing to clotting-like reactions in barnacle cement hardening. Based on this, scientists came up with a set of experiments to test barnacle cement.Some of these experiments yielded results supporting the similarity of blood clotting to cement hardening.
Let’s look at a few of the most important results. Hardened cement under a microscope shows a fibrous structure similar to a blood clot. The hardened cement shows biochemical evidence of the cross-linking reaction end-products in its components. Barnacle cells, that are responsible for the final cross-linking step in blood clotting, are present in cement. Heparin, an anticoagulant which retards blood clotting, inhibits cement hardening to a degree. Enzymes of similar function and proteins of similar structure are seen in cement hardening and blood clotting. Finally, inhibiting enzyme activity changes the protein profile of the cement because the proteins haven’t been cut up as much as they would have been otherwise.
To sum up the results: they saw similar enzyme activity and protein structure, retardation by anticoagulant, the presence of cells responsible for the cross-linking reaction, appropriate reaction end-products, and the microscopic similarity of cement structure to a clot mesh.
(How blood clotting and cement hardening are similar)
Other experiments tested more intricate aspects of the enzymatic reactions. Rather than giving clear answers, they have provoked more questions. But, this is how research works. A research problem is never fully solved by a few experiments, however interesting or novel the hypothesis or idea may be. Scientific research uncovers evidence slowly and methodically. But every experiment that succeeds or fails allows scientists to ask better questions, and plan better experiments. There is a lot of ongoing research to nail down all the molecules and mechanisms involved in cement hardening. Not to mention the research on other marine organisms that contribute to fouling. It takes many scientists, working in different laboratories over the years, to piece together all the data needed to understand natural phenomena.
Researching underwater adhesion, in general, has led to the development of many biomimetic materials. There are a wide range of applications for such technology, from synthetic non-fouling surfaces, to improved medical adhesives for easily reconstructing broken bones and damaged tissue.
If nothing else, the research on barnacle adhesion demonstrates the power of applying evolutionary principles to better understand biological phenomena. Now that we understand barnacle cement hardening as a strange type of blood clotting, we can start to explore those interesting connections I talked about in the beginning of this article. Some of the previous research that helped scientists to understand blood clotting (and therefore, cement hardening) was done on marine animals like the horseshoe crab. Do you know what horseshoe crab blood costs these days? $15,000 per quart. That’s fifteen thousand dollars for about a liter of blood. Why is it so costly? Well, stay tuned for the next episode of ‘Life Is Rough’!