Chromera: Coral friend or foe?
Transcriptomic impact of Chromera infection on the coral
New research from my PhD work with Prof David Miller, the ARC Centre of Excellence for Coral Reef Studies (Coral CoE) at James Cook University has been published. We found that the coral-associated alga Chromera, despite being a near-relative of Symbiodinium (the photosynthetic symbiont essential for the survival of reef-building corals), is not beneficial to corals.
Previously it had been assumed that Chromera was an alternative coral photosymbiont that was more tolerant of heat stress than is Symbiodinium. The new study, published in Nature’s ISME Journal, comprehensively debunks the myth that Chromera is benefical to corals. Rather than being greeted as a friend, corals mount a hostile response when exposed to Chromera.
Corals live in close association with a number of microorganisms, amongst which Symbiodinium is critically important as it can supply the bulk of the coral’s energy needs. Chromera is a very similar alga to Symbiodinium, present in smaller numbers but more stress-tolerant, leading to the idea that it might be able to sustain corals that had undergone “bleaching”; it was first discovered in corals from Sydney Harbour in 2008, but has subsequently been found in association with a variety of corals from all around the world, including the Great Barrier Reef.
As well as the association with coral, Chromera is fundamentally interesting because it occupies a unique phylogenetic position –between free-living, photosynthetic algae such as Symbiodinium, and parasites such as Plasmodium (the malaria parasite). Technically, Chromera is the closest known photosynthetic relative of the apicomplexan parasites. This position led to the nature of the Chromera - coral relationship being questioned - is it mutualistic, in which both participants benefit, or is it a parasitic relationship?
To better understand the association, we studied the response of every gene in the coral genome when the baby coral (larva) starts to interact with Chromera. Coral larvae have been previously used to understand coral responses to beneficial Symbiodinium strains. Understanding the infection process by which algal symbionts gain entry into their coral hosts is very important specially when trying to engineer coral symbiosis with thermotolerant symbionts.
Having a clear and detailed understanding of the coral responses to beneficial symbiont was the first step in answering the question of whether Chromera is a beneficial symbiont or a parasite.
In the case of Chromera, we saw very different responses from those described for a beneficial type of Symbiodinium. The coral response to Chromera more closely resembled the response of vertebrate hosts to parasites and pathogens.
With Chromera, phagosome maturation (a host defence mechanism to kill invading pathogens) was strongly enhanced, whereas, in the case of beneficial Symbiodinium, phagosome functions were interrupted, allowing the Symbiodinium to establish itself within the coral’s tissue.
Our own attempts to survey Chromera distribution on the GBR support the fact that Chromera isn’t beneficial to corals, presumably as a consequence of hostile response of the coral to Chromera infection.
Why should we care about this work?
Coral-algal symbioses are the foundation for coral reef ecosystems. Flexibility in this mutualistic relationship might pave the way for corals to survive in the rapidly changing ocean. Unfortunately, the host-symbiont combination is complicated with the host compatibility. Consequently, understanding the molecular basis of algal symbiont compatibility is critically important. Here we investigated the coral response to Chromera and showed that the coral response is consistent with vertebrate host responses to parasites and coral responses to incompatible Symbiodinium. Such responses are of critical importance for elucidating coral interaction with symbionts with different host compatibility.
Chromera is highly thermo-tolerant in vitro? Can we make use of that?
Chromera can grow up to 35 C, a lethal temperature to corals and their algal symbionts in most parts of the world. Understanding the genetics underlying their high thermal (stress more generally) might inform genetic manipulation approaches in Symbiodinium to try to build up their thermal tolerance.
Apicomplexan-like alga in corals, so what ?
Research on Chromera may have medical implications. Chromera is a close relative of some nasty human parasites, but is much more convenient and safer to work with in the laboratory. Its close relationship to apicomplexan parasites might help, for example, in development of anti-malarial drugs. Coral associated microbes not only have ecological benefits they can also be used for screening for novel therapeutics for human healthcare.
Paper link:
https://www.nature.com/articles/s41396-017-0005-9
Paper metrics:
www.altmetric.com/details/31495010
On the media:
https://www.coralcoe.org.au/blog/nature-of-coral-chromera-interaction-revealed-photosynthetic-symbionts-are-not-always-beneficial-to-corals
Data links:
https://research.jcu.edu.au/researchdata/default/detail/1b595d251e523b5260931fc09cd49db5/
https://www.ncbi.nlm.nih.gov/geo/query/acc.cgi?acc=GSE102664
Transcriptomic impact of Chromera infection on the coral
New research from my PhD work with Prof David Miller, the ARC Centre of Excellence for Coral Reef Studies (Coral CoE) at James Cook University has been published. We found that the coral-associated alga Chromera, despite being a near-relative of Symbiodinium (the photosynthetic symbiont essential for the survival of reef-building corals), is not beneficial to corals.
Previously it had been assumed that Chromera was an alternative coral photosymbiont that was more tolerant of heat stress than is Symbiodinium. The new study, published in Nature’s ISME Journal, comprehensively debunks the myth that Chromera is benefical to corals. Rather than being greeted as a friend, corals mount a hostile response when exposed to Chromera.
Corals live in close association with a number of microorganisms, amongst which Symbiodinium is critically important as it can supply the bulk of the coral’s energy needs. Chromera is a very similar alga to Symbiodinium, present in smaller numbers but more stress-tolerant, leading to the idea that it might be able to sustain corals that had undergone “bleaching”; it was first discovered in corals from Sydney Harbour in 2008, but has subsequently been found in association with a variety of corals from all around the world, including the Great Barrier Reef.
As well as the association with coral, Chromera is fundamentally interesting because it occupies a unique phylogenetic position –between free-living, photosynthetic algae such as Symbiodinium, and parasites such as Plasmodium (the malaria parasite). Technically, Chromera is the closest known photosynthetic relative of the apicomplexan parasites. This position led to the nature of the Chromera - coral relationship being questioned - is it mutualistic, in which both participants benefit, or is it a parasitic relationship?
To better understand the association, we studied the response of every gene in the coral genome when the baby coral (larva) starts to interact with Chromera. Coral larvae have been previously used to understand coral responses to beneficial Symbiodinium strains. Understanding the infection process by which algal symbionts gain entry into their coral hosts is very important specially when trying to engineer coral symbiosis with thermotolerant symbionts.
Having a clear and detailed understanding of the coral responses to beneficial symbiont was the first step in answering the question of whether Chromera is a beneficial symbiont or a parasite.
In the case of Chromera, we saw very different responses from those described for a beneficial type of Symbiodinium. The coral response to Chromera more closely resembled the response of vertebrate hosts to parasites and pathogens.
With Chromera, phagosome maturation (a host defence mechanism to kill invading pathogens) was strongly enhanced, whereas, in the case of beneficial Symbiodinium, phagosome functions were interrupted, allowing the Symbiodinium to establish itself within the coral’s tissue.
Our own attempts to survey Chromera distribution on the GBR support the fact that Chromera isn’t beneficial to corals, presumably as a consequence of hostile response of the coral to Chromera infection.
Why should we care about this work?
Coral-algal symbioses are the foundation for coral reef ecosystems. Flexibility in this mutualistic relationship might pave the way for corals to survive in the rapidly changing ocean. Unfortunately, the host-symbiont combination is complicated with the host compatibility. Consequently, understanding the molecular basis of algal symbiont compatibility is critically important. Here we investigated the coral response to Chromera and showed that the coral response is consistent with vertebrate host responses to parasites and coral responses to incompatible Symbiodinium. Such responses are of critical importance for elucidating coral interaction with symbionts with different host compatibility.
Chromera is highly thermo-tolerant in vitro? Can we make use of that?
Chromera can grow up to 35 C, a lethal temperature to corals and their algal symbionts in most parts of the world. Understanding the genetics underlying their high thermal (stress more generally) might inform genetic manipulation approaches in Symbiodinium to try to build up their thermal tolerance.
Apicomplexan-like alga in corals, so what ?
Research on Chromera may have medical implications. Chromera is a close relative of some nasty human parasites, but is much more convenient and safer to work with in the laboratory. Its close relationship to apicomplexan parasites might help, for example, in development of anti-malarial drugs. Coral associated microbes not only have ecological benefits they can also be used for screening for novel therapeutics for human healthcare.
Paper link:
https://www.nature.com/articles/s41396-017-0005-9
Paper metrics:
www.altmetric.com/details/31495010
On the media:
https://www.coralcoe.org.au/blog/nature-of-coral-chromera-interaction-revealed-photosynthetic-symbionts-are-not-always-beneficial-to-corals
Data links:
https://research.jcu.edu.au/researchdata/default/detail/1b595d251e523b5260931fc09cd49db5/
https://www.ncbi.nlm.nih.gov/geo/query/acc.cgi?acc=GSE102664
