I’ve always, always, always loved animals. As a kid, I wanted to be a vet (“also an actor,” thought this 30-something genetics student, “kids are weird”). My Mum worked at a Safari Park in the UK, which didn’t help, as I grew up in a house of darling dogs, cute cats, silly snakes and lovable lizards. My own, personal, private zoo, and I loved it. Of course, as a kid I’ve also always, always, always, always loved Disney movies, particularly of the Pixar variety. Finding Nemo, then, holds an especially dear, squishy, jellyfish-shaped place in my heart.
If you don’t know anything about Finding Nemo , then you probably haven’t lived, but besides that our story begins in Australia’s Great Barrier Reef: 2000+ kilometers of astounding ecological diversity; 1,500+ species of fish; 3000 species of mollusks; 350 species of hard corals, and never-mind all the soft corals, clams, sponges, etc (Gutierrez, 2014). Despite being a computer animated film, every shade and hue you see in Finding Nemo is no exaggeration about what’s really out there, ready to astound you.
Finding Nemo released in 2003. I was around 15-years-old. Even now, however, I remember a thought I had while snug in those big cinema-chairs, holding the obligatory empty bucket of popcorn, eviscerated before the film even began:
“How much of it still looks like that?”
Yes, kids are weird, but I wasn’t an especially jaded 15-year-old. I’m sure a lot of people were aware of things like coral bleaching, but being the son of a conservationist perhaps made it more present in my mind. I’m sure a lot of people are still aware of bleaching events, because it’s still a thing. Climate change, among other things, contributes to these bleaching events, and refers to when the corals lose those fantastic colours. “Bleaching”, it turns out, is quite literal.
Coral bleaching boils down to an endosymbiotitic relations between coral polyps (which we’ll refer to as corals for the sake of simplicity), and their micro-algae Symbiodinium. Corals are immobile or “sessile” creatures often referred to as “reef builders”: they pump out the calcium-carbonate responsible for these hard coral reefs forming. The micro-algae are photosynthetic (hopefully not surprising anybody) and can live inside the corals, exchanging photosynthates (sugary treats from photosynthesis) for some of the corals’ own inorganic molecules (ammonium, nitrate, etc).
Increasing water temperatures, as a result of climate change, causes stress on both the corals and the algae (I empathize, being English makes me about as thermally intolerant as you can imagine). The corals may expel their algae as a short term survival tactic, reducing their burden as a host. Alternatively, the algae my decide to jump ship, making things easier for themselves. Sadly as the corals are sessile, they rely on their symbiotic relationships for up to 90% of their nutritional needs (Falkowski et al, 1984). Prolonged bleaching events means the corals eventually to starve, leading to a slight case of death. While coral reefs can recover, climate change has resulted in these recovery periods becoming shorter and shorter, with many organisms unable to keep up (Hoegh-Guldberg and Bruno, 2010).
Disney is going to end up having a hard time Finding Sequels. Aha.
As mentioned, the thermal tolerance of the algae, as well as the corals, contribute to these bleaching events (Pandolfi et al, 2011). As the algae themselves have much shorter life-spans (days, Wilkerson, 1988) than the corals (years, Babcock, 1991), perhaps the algae can evolve their thermal tolerance quicker than the corals? If this so, then perhaps the algae’s increased thermal tolerance can help prevent the corals becoming bleached?
Experimental evolution can be thought of as a kind of indirect artificial selection: as opposed to picking and choosing which organisms can breed based on a shared desirable trait, we instead change the environment that our organisms inhabit, favouring the traits we wish to develop. In terms of Symbiodinium, this would involve growing a strain at an elevated temperature and hoping to observe positive growth (ie growth that is equal to or better than growth observed at a more common temperature). If positive growth is observed, we can try to grow a sample of this strain at an even higher temperature, and so on and so on.
This is precisely what Chakravarti and van Oppen tried to achieve in their recent 2017 study. They took 5 genetically distinct Symbiodinium strains and attempted to grow them at 30°C, a temperature that Symbiodinium are known to tolerate in the literature. If “positive growth” was observed for these strains (explained below), then a sample of those strains were then grown at 31°C, and so on and, indeed, so on. This process was repeated over the course of a year.
“Positive growth” in this study was assessed in terms of “growth rate” and “photosynthetic efficiency”. Growth rate compared the initial cell density with cell density at a later time, as well as the cell doubling or generation time at a given experimental temperature. This makes intuitive sense: if an experimental strain’s rates of growth are positive after prolonged exposure to an elevated temperature (where the corresponding wild-type may struggle), it suggests a stable, adaptive change for the experimental strains’ thermal tolerance.
“Photosynthetic efficiency” was assessed using common plant stress measurements : the maximum and effective quantum yield. Maximum quantum yield assesses how well our plants can photosynthesize after being “dark-adapted”, whereas the effective quantum yield assesses photosynthesis when light is available at a steady state.
Without going too much into the biochemistry of it all, photosynthesis involves the movement of electrons, excited by light energy, or “photons”. Electron movement is facilitated by “reaction centres” that carry the electrons. A dark-adapted plant should have more available reaction centers, as there’s been no light available to excite electrons for them to be carried by the reaction centres. When light is available at a steady state, however, there’s a constantly back-and-forth between reaction centers picking up and dropping off excited electrons. A stressed plant will have fewer available reaction centres, resulting in reduced maximum and effective quantum yields.
The figure below from another study (Roth, Goericke, and Deheyn, 2012) helps us visualize the effect of both heat and cold on the photophysiology (ie photosynthetic efficiency) of the coral Acropora yongei.
Here, we see the stress caused by both cold (downward arrows) and heat (upward arrows) have detrimental effects on the effective and maximum quantum yields of the corals, compared to the control temperature (white circles). Heat is shown as being more deleterious over time.
(We also see the effects of maximum excitation pressure of photosystem II, Qm, which from what I can understand involves the availability of some reaction centers in response to environmental factors such as heat and light, Grey et al, 1996).
This again makes intuitive sense: if a strain, after prolonged exprosure to a higher temperature, can photosynthesis better than its wild-type at the same temperature, it also suggests a stable adaptive change has taken place.
It is important to remember that, while Charkravarti and van Oppen’s claim to have observed stable adaptive changes in their experimental strains (a great start), it’s certainly not the end of the line. Experimental evolution, as with experiments as a whole, involves controlling a number of variables. Controlling variables means it’s easier to understand the relationship between our results (stable adaptive change) and the experimental conditions we did change (temperature).
Only two traits (growth rate and photosynthetic efficiency) were assessed in this experiment. In a real world setting, these two traits will not be the only ones that matter. Chakravarti and van Oppen discuss in their paper future experiments that need to be considered. Do these evolved strains contribute to the corals during heat stress? Being photosynthetically efficient in vivo is a great start, as many species of coral are “broadcast spawners“. Adapted strains could be released during a spawning period, surviving elevated temperatures ex hospite before being taken up by the coral spawn. Being photosynthetically efficient, however, doesn’t mean the strains release photosynthates when in hospite (Stat, Morris, and Gates, 2008). We also need to ask if these evolved strains can and will infect the corals and be retained by them (Gabay, Weis, and Davy, 2018).
What about where these evolved strains exist on the symbiotic spectrum of parasitism to mutualism (Baker, 2018)? Studies have shown that an algae may be more or less beneficial to a coral depending on the coral’s life stage, such as one strain may be more symbiotic for juveniles (Suzuki et al, 2013), but more parasitic in mature corals (Stat, Morris, and Gates, 2008). Corals also rarely host only one strain of Symbiodinium, and so we ask how these corals perform with a single evolved strain compared to being given a “cocktail” of thermally tolerant strains.
What about the thermal tolerance of the corals themselves? Some recent papers by Liew et al and Yong et al have suggested bringing about epigenetic changes in the corals by using “coral nurseries”. It is thought that growing corals in nurseries at elevated temperatures (rather than relying on sporadic, “natural” temperature increases) may cause epigenetic changes that would increase expression of thermal tolerance genes. These young corals would then better react to haphazard temperature changes, rather than hoping these’s enough time between bleaching events for the corals to develop these changes more naturally. Yong et al go on to say that these epigenetic changes also causes changes in transcription that responds to algal symbiosis of the corals.
It’s all exciting stuff, as we consider the myriad of ways in which we might be able to help. Charkravarti and van Oppen have used experimental evolution here as a tool to improve the thermal tolerance of the endosymbiont Symbiodinium, claiming to have observed stable adaptive change in 3 of their 5 experimental strains. I believe combining some of the approaches discussed here, and other incredibly clever ways, is the best hope we have to aid these diverse ecological systems. I hope these efforts bare fruit. I hope there’s a time where I can watch Finding Nemo and no longer think: How much of it still looks like that?
(Different movie, I know. Thanks for reading!)