My latest paper has just been published in Proceedings of the Royal Society B! My colleagues and I describe how a partnership between a group of ciliates (a type of single-celled organism) called Kentrophoros and their bacterial symbionts had a single evolutionary origin. This is despite the fact that different species of Kentrophoros can look very different from each other and are found all over the world. The bacteria are also a lineage that is new to science, and that as far as we know is only associated with these ciliates. This means that after the first Kentrophoros and its bacterial partner got together tens or hundreds of millions of years ago, their descendants have diversified into different species and spread themselves throughout the globe, all the while remaining true to each other.
Still have questions? Read more below…
Why is this called a symbiosis? What do they do for each other?
In short, Kentrophoros carries bacteria on its back that it eats. Most ciliates are grazers or phagotrophs – they prey on other microbes like bacteria and algae. Kentrophoros, on the other hand, doesn’t need to go hunting, because it carries around its food – a dense “lawn” of symbiotic bacteria – on its back! Carrying your entire food supply with you is no joke. Around 50% of the total volume is occupied by bacteria, which is considerably more than other symbiotic species.
These symbionts are sulfur-oxidizing bacteria, which can produce energy from sulfide, the stinky gas that smells like rotten eggs. That makes sense because Kentrophoros lives in marine sediments, e.g. at beaches, mangroves, and seagrass meadows. In these habitats, sulfide is produced by other kinds of bacteria that break down organic debris that has settled and been buried, so it is a convenient energy source for many bacteria including the symbionts of Kentrophoros.
What do they look like?
I think that these ciliates are beautiful and elegant, and if you don’t believe me, you can check out this short movie I compiled some years ago with video clips of live Kentrophoros:
From the video you can see that Kentrophoros comes in all sorts of shapes and sizes. There are flat ones, tubular ones, big ones, small ones… We found one (undescribed) species that we think is the biggest one so far. It’s the one that occupies the first 30 seconds of the video above.
Remember that the bacteria are carried by the ciliate on its back, and that they are 50% of the total biovolume (a ratio which is pretty much consistent between different species). So if the ciliate cell gets bigger, while the bacteria remain pretty much the same size and shape (little rods), it will need to increase the surface area that’s available for the bacteria to attach to it.
This brings us to the first remarkable thing that we found. In this large species, the surface that carries the bacteria is folded into rows of pouches, which keeps the bacteria:ciliate volume ratio consistent, even as the host cell gets really large. Despite being one single cell, it can be several millimeters in length, large enough to see with the naked eye. In fact, when I first saw this species, I was convinced that it was actually a small animal of some sort, and had to check under the microscope to be sure that it was a ciliate!
My collaborator Thomas Schwaha from the University of Vienna made a 3D virtual model of this species by slicing one individual into thin sections only 1 micron thick, and then putting together the images on the computer. From this model we could calculate the volume ratio, and also better visualize how the organisms are structured:
What is new about this research?
The first species of Kentrophoros was described in 1928, and people figured out pretty early on that the weird “spines” they have are actually bacteria that they eat. One paper (Fenchel & Finlay, 1989, Ophelia 30:75) called this symbiosis a “kitchen garden”, which I find to be a really picturesque name.
Despite this, no one had bothered to find out what kind of bacteria these are. There are many animals that have similar sulfur-oxidizing symbionts, like the deep sea tube-worms from hydrothermal vents, or mouthless and gutless oligochaete worms related to earthworms that also live close to seagrass meadows. This sort of symbiosis – making energy from sulfide to feed a host – has independently evolved several times in different groups of animals and bacteria, a pattern which we call “convergent evolution”. The question for us, going in, was whether the bacteria living with Kentrophoros were from one of these lineages that people already knew about, or if it was something new.
What we found was that this was a new lineage of bacteria that had not been found before, in animals or ciliates or anywhere else. This is another independent instance of convergence, except that Kentrophoros is of course not an animal but a single-celled ciliate.
Their diversity is also amazing. In a single bucket of sand from the Mediterranean or Caribbean we can sometimes find several species of Kentrophoros living together (of course some patience is required because many buckets will turn up empty). For this study we collected a total of 17 different species and I’m sure that there are more sitting in our freezers. To put this in perspective, only about 15 species have been formally described in the past 90 years, most of them collected during general surveys or by people who were looking for something else. This was also the first time anyone had looked specifically for them at the two sites in the Mediterranean and Caribbean where we did our field work.
Each ciliate species had its own specific symbiont species, and when we compared their respective family trees (phylogenies), their branching pattern although not perfectly matching was still significantly similar. This means that for the most part when a new species of Kentrophoros evolved, its bacterial symbionts also evolved into new species along with it. This was also something we did not necessarily expect, because as they are extracellular symbionts one might naively think that they could hop around between different host species quite easily, especially since different species frequently co-occur.
Given how diverse and widespread they are, we think that this is an example of a symbiotic evolutionary radiation. This symbiosis must have been pretty successful in order to survive for so long and for the partners to remain so loyal to each other.
How do you find these organisms?
If they’re so widespread and wonderful, why haven’t we heard more about them already? One problem is that they live inside sand/sediment so unlike fishes or corals you don’t see them when you go snorkelling or diving. A typical habitat is sediment near seagrass or mangroves, like in this picture below:
To find them, we first have to scoop up some of this sediment into a bucket, and then bring it back up on land, where we can slowly sift through it by decanting it with clean seawater into shallow trays. Large species can be spotted with a careful eye this way. However smaller species have to be extracted by other methods and sorted under the microscope, which is much more challenging. The ciliates are also soft-bodied and quite fragile, so it is no surprise that methods that are used to collect hardier animals will not be suitable.
Can they be eaten, be used as medicine, solve climate change … ?
Unfortunately, not any time soon. They may be big for ciliates, but they’re still really small and not so easy to find. Compared to the total mass of bacteria in the sediments where they live, they are make up only a small part of the biological community with a correspondingly modest contribution to its ecology. What they can help us with is to understand how these sorts of partnerships between different species function and evolve. This is something that we still don’t know much about except in a very few species, despite the fact that almost all “higher” organisms, including humans, are intimately involved with bacteria. By studying these symbioses in different kinds of host species, we are gradually learning which are the general biological principles that govern them.
Nonetheless it is pretty cool to know that when you go to a nice tropical island for a holiday, or stand on a muddy shore in higher latitudes, that somewhere beneath your feet is an ancient partnership that is still going strong after millions and millions of years.
Seah BKB, Schwaha T, Volland J-M, Huettel B, Dubilier N, Gruber-Vodicka HR. 2017 Specificity in diversity: single origin of a widespread ciliate-bacteria symbiosis. Proc. R. Soc. B 284 : 20170764. http://dx.doi.org/10.1098/rspb.2017.0764