A Free Man

Great Interview Week continues in a scientific vein today. In last week’s Nature, a paper coming out of Dee Carter’s lab at the University of Sydney described the discovery of a previously unknown marine species. That, in itself, is noteworthy. However, the organism they found - an unremarkable unicellular brown alga - turns out to be an evolutionary “missing link”.

Bob Moore (the lead author on this study), Carter and their colleagues describe Chromera velia, now the closest-known photosynthetic relative to apicomplexan parasites - including the one that causes malaria. The discovery and phylogenetic characterization of Chromera illuminates a murky step in the evolution of photosynthesis. This close evolutionary relationship also means that Chromera will be a powerful model system for studying apicomplexan diseases.

Carter took a seat on the virtual couch to discuss her group’s recent discoveries:

CDV: My readers run the gamut from working scientists to lay persons. Can you clearly and concisely explain to the latter class why they should pay attention?

DC: Malaria and other diseases caused by organisms belonging to the class of parasites known as the apicomplexans kill and disable millions of people every year. Even in developed countries we see malaria in travelers returning from overseas, we have home-grown cases of toxoplasmosis and cryptosporidiosis (both apicomplexan diseases), and other parasites cause huge losses to our poultry and livestock industries.

CDV: In terms of the apicomplexa that are disease causing parasites, does your work bear any medical implications - improved treatment options for apicomplexan illnesses, for example?

DC: These parasites are difficult to treat and they are difficult to work on as they need to be grown in living host cells. Chromera can be grown on simple laboratory media and because of it’s close relationship to the parasites can potentially serve as a surrogate host for developing anti-parasitic drugs. It can also tell us something about how these parasites, which were in fact once algae themselves, evolved.

CDV: I noticed that Chromera velia was isolated from some stony coral in Sydney Harbour. I think it’s a great part of the story - you found what is essentially one of those holy grails of evolutionary biology - an extant missing link. Was this a serendipitous discovery? If so, what were you looking for out in the harbour?

DC: Yes, I have to admit that it was serendipitous! We were looking for the algal symbionts that inhabit corals and allow them to grow. These are expelled during coral bleaching and knowing their lifecycle and biodiversity is very important for reef conservation. Bob Moore, who was doing his PhD in my lab at the time, had a few samples from Sydney harbour and thought he would keep other algae that he came across as well – and one of these turned out to be Chromera. It was thanks to Bob’s scientific curiosity and his perseverance with this culture that this discovery was made.

CDV: Does Chromera velia tell us anything about the original endosymbiotic event that resulted in photosynthetic organisms? Perhaps more appropriately, was there one or multiple endosymbiotic events leading to the chloroplasts found in “higher” plants?

DC: No, not really about the original event –that’s going a very long way back. We do know there have been more than one event because extant organism are pigmented by different chlorophylls that have different origins.

CDV: For the technical wonks in the audience, how do you isolate a homogenous culture of a microscopic organism from a wild collection? It doesn’t seem a trivial task, particularly to get a pure enough culture to isolate genomic DNA. What controls do you employ to guarantee an axenic culture?

DC: We use standard environmental microbiology techniques of swabbing or streaking the material of interest onto an agar plate. We incorporated antibiotics in the plates to get rid of bacteria. Bob also developed a novel medium that contained bleach – he reasoned the algae would get eaten by reef fishes and would be able to tolerate high levels of hydrogen chloride – and this seemed to select for Chromera. Once the cultures are plated out we leave them to grow into colonies, pick off single colonies and subculture these until we are sure that they are pure.

CDV: If I read your results correctly, they suggest that the loss of photosynthesis led to the evolution of parasitism in Apicomplexa. This makes sense - you lose photosynthesis so have to find another energy source, like a host organism. Is this an evolutionary novelty? It seems that in most cases loss of your primary energy producing biological pathway would be an evolutionary dead end. Are there other examples of loss of photosynthesis?

DC: The apicomplexans are the only organism where we can say this has certainly happened, as they are the only ones yet found that have a relic chloroplast. I would say that the loss occurred during the transition to parasitism – the apicomplexans were already endosymbionts (that is, they could live in the cells of other organisms) and gradually evolved ways to extract more and more of their requirements from their host until making their own energy became unnecessary. Note that their relatives, the symbiotic dinoflagellates, live inside corals and are endosymbiotic –but the host controls the relationship so that they don’t invade and cause damage. It will be very interesting to see where Chromera fits on the spectrum from symbiont – free living – parasite, since it too was found in corals but has lost one of its photosynthetic pigments, so might be losing photosynthesis.

CDV: One of my scientific heroes, Edmund O. Wilson has suggested that we have identified only a fraction of the species on the planet, maybe as low as about 1.5% and that up to 50% of the extant species today could be extinct by the end of this century. Your discovery of a previously unidentified species supports Wilson’s idea that we’re missing out on a lot of life on earth. What do you think about his second idea - are we losing biodiversity that rapidly and how concerned should we be?

DC: I think we know a lot about the world’s big organisms but very little about the very small ones – there are plenty of studies that show you can only culture a small fraction of the microbial diversity that is present. I think we should all be very concerned about the loss of biodiversity that we are experiencing, not only because of the loss of important life forms but also because this can have other, more far-reaching ramifications. We see this on coral reefs where loss of microscopic algae leads to coral bleaching and coral mortality, with knock-on effects to the rest of the coral ecosystem including fish, anenomies, marine insects, other microbes, etc. Much of life is part of a delicately balanced system that has evolved over millennia – once gone they will take millennia to regenerate.

CDV: What’s the next Nature paper to come from the Carter Lab?

DC: The Chromera genome – if we can get the funds to do it!

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Thanks to Dee for being a sport. If you want to read more, check out the original paper here (or here) and Patrick Keeling’s review here.

Image Credits:

Sydney Harbour

Plesiastria

Apicomplexan

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