As the
seawater surface temperature increases due to global warming, the existance of
our coral reefs is at risk. The zooxanthellae that provide the carbon needed
for the animal’s respiration, disappear
with increasing temperature and leave the coral in an ill, sometimes lethal
state. Since the emission of greenhouse gasses isn’t to decrease in the near
future, we can expect the loss of these microbes due to heat stress is to occur
more frequently. For example, a study from Van Hooidonk et al. (2013) predicts
annual bleaching events in some parts of the ocean by 2025. In this blog the paper of Grottoli et al.
(2014) is discussed, in which coral acclimation and recovery is studied in the
context of annual bleaching. The hypothesis predicted survival and acclimation
to annual bleaching. Acclimation was here defined as “the improved performance
of the physiological variables measured here from being significantly lower
than the control corals after bleaching to being no different from controls
after bleaching the following year”.
The
zooxanthellae, in the paper of Grottoli et al. (2013) more accurately called Symbiodinium (endosymbiotic
dinoflagellates), make use of photosynthesis to provide the coral’s host with
fixed carbon. However after bleaching, the animal gets more dependent on other
sources of carbon, such as from energy reserves or increased heterotrophy.
Also, the Symbiodinium community in a
certain coral colony might change after bleaching, by a shifting distribution
of Symbiodinium spp. These variables
(photosynthesis, heterotrophy, energy reserves and Symbiodinium type distribution) are, amongst others, measured over
a time span of two years. The non-control coral fragments were treated with an
artificially imposed bleaching event in 2009 and again in 2010. This was
executed by raising the water temperature approximately 1 degree over seven
days in a seawater filled tank, after which the increased temperature was maintained
for another eight and ten days in 2009 and 2010 respectively. Immediately after
the bleaching treatment, the coral fragments were used to measure respiration,
photosynthesis or ‘percent Contribution of Zooxanthellae to Animal Respiration’
(CZAR) and the feeding rate or ‘percent Contribution of Heterotrophy to Animal
Respiration’ (CHAR) and placed back in the reef. After six weeks on the reef
the following variables were measured or identified: Symbiodinium concentration, Symbiodinium
type distribution and change in energy reserves.
The
experiment was done with three different coral species: Porites divaricate, Porites astreoides, Orbicella faevolata. P. divarcata appeared to have the
highest acclimation capacity, since the coral only showed a significant decline
in calcification and Symbiodinium
cell density after the first bleaching event in 2009. After six weeks these
variables fully recovered and didn’t change after the following bleaching in
2010. The endosymbiont loss and decrease in calcification compared to controls
after the first bleaching was higher for P.
astreoides, but fully recovered too. However, after the bleaching treatment
in 2010, the decrease of these parameters was even worse and persisted
troughout the following six weeks. Like both Porites species, O. faveolata
showed similar bleaching effects and recovery of them in the first year,
but endosymbiont density and calcification did also decrease and fully recover
in the following year. Interestingly, the energy reserve of O. faveolata
didn’t change after bleaching in 2009, but did decrease after the treatment in
2010. The energy reserves for the Porites
species decreased after both the first and second bleaching in the first six
weeks back on the reef. As can be
expected with decreases in endosymbionts, all coral species showed a declined
CZAR, albeit not always significant. For P.
astreoides however, this loss in
fixed carbon supply was compensated for after the first bleaching event by an
increase of CHAR of 147%, although this trend wasn’t proven statistically
significant. These results for the fixed carbon acquisition imply that
heterotrophy doesn’t play a significant role in ability of corals to acclimate
to annual bleaching events. A factor that was associated with acclimation of
the corals, was the shifting in the dominant Symbiodinium species in a certain coral holobiont. For example, P. divarcata’s dominant endosymbiont was
C47 in 2009 and changed substantially to A4 over the next year. Of all three of
the corals, P. astreaoides showed the
weakest shift in dominant endosymbiont type. Together with having the highest
energy reserve, the change in distribution of Symbiodinium spp. might have caused P. divarcata to be the coral best resisting and recovering from
annual bleaching. With O. faveolata this
appeared to be more complex and P.
astreaoides just didn’t seem to acclimate.
What I find remarkable about this paper is that the results differ a lot per coral species.
This is useful information to show how acclimation capacity of corals is
species-specific. However, the coral species combination used here occurred to
me as if it was specifically chosen to show the distinction in the results. The authors of this
paper didn’t mention any reason for working with these species in particular,
but if they had more knowledge about them in relation to the research topic it
would have been interesting to read their expectations. Besides
that, some figures show results that are not proven to be significant, but do
comply with significant trends in other samples. In some cases this is
mentioned, but in my opinion the authors could have been more elaborative on
this. Ofcourse, these trends can be explained better with more extensive
research, using different coral species and working with a longer time scale.
However, this research gave an interesting insight in the abilities and
inabilities of coral species to survive and acclimate to more frequent
bleaching event. Some will win, some will lose.
Paper reviewed:
Grottoli, A. G., Warner, M. E., Levas, S. J., Aschaffenburg, M. D., Schoepf, V., McGinley, M., ... & Matsui, Y. (2014). The cumulative impact of annual coral bleaching can turn some coral species winners into losers. Global change biology, 20(12), 3823-3833.
References:
Van Hooidonk, R., Maynard, J. A., & Planes, S. (2013). Temporary refugia for coral reefs in a warming world. Nature Climate Change, 3(5), 508-511.
Hi Thyrza interesting read, I like how the paper looks at the effects of blenching over a two year period many of the papers I have read usually only focus on the immediate effect of blenching over a period of a few weeks and don't look at how the coral respond/recover and the microbes associated with the corals may change following the blenching event as the corals being to recover. It’s interesting how even if the coral are able to tolerate and recover from the first year they may not be recovered enough to tolerate a second blenching event the following year, something that appears to be becoming a much bigger issue with climate change. It’s also interesting how different species of coral are more tolerant and able to recover more effectively.
ReplyDeleteYou mentioned how the researchers appeared to choose 3 coral species that would show difference responses but didn't really explain or justify why they choose those species I was wondering if you had done any research into why there was a different between the species the paper mentions that P. divarcata appeared to have the highest acclimation capacity which was likely due to it having the highest energy reserve and being able to most effectively change the distribution of Symbiodinium spp. Have you come across any research which provides any suggestions/explanation as to why it has a higher energy reserve or is able to more effectively change its Symbiodinium spp. Distribution.
Thanks for your time
Alisha
Hi Alisha,
ReplyDeleteThanks for your comment. It's true that most papers on the subject of heat-stress survival by corals only researched single bleaching events or bleaching events with a longer time period in between. However the results from the paper I reviewed often comply with some results from this earlier research. For example, several studies already indicated that Porites species had the capacity to adapt to bleaching events seperated by more years (see Smith et al. (2013) for example). Furthermore, the feature of having a high energy reserve was also expected to play an important role in reducing coral susceptibility and increasing recovery rates (Anthony et al., 2009). From what I understood, this probably has to do with the fact that the coral still needs carbon resources to recover of course, if not more than it did before. However after bleaching, most corals lost a significant amount of their Symbiodinium population, which provide the coral host with fixed carbon. Therefore, other carbon resources need
to be used, like heterography or the energy reserves. In the paper I reviewed, it appeared that heterography doens't play as much of an important role as it was expected to do. Energy reserves did however and apparently P. divarcata has relatively more lipids, proteins and carbohydrates that can be used as energy reserves.
Why P. divarcata is able to more effectively shift it's Symbiodinium population is not really clear to me. I think it has more to do with the heat resistancy of the microbes themselves than it has to do with the coral host, but ofcourse there could be a question of animal-endosymbiont interaction that influences this, which is also put forward by the article reviewed.
I hope this answers your questions :)
Thyrza
Oh the articles mentioned in my comments are:
ReplyDeleteAnthony, K. R., Kline, D. I., Diaz-Pulido, G., Dove, S., & Hoegh-Guldberg, O. (2008). Ocean acidification causes bleaching and productivity loss in coral reef builders. Proceedings of the National Academy of Sciences, 105(45), 17442-17446.
Smith, T. B., Brandt, M. E., Calnan, J. M., Nemeth, R. S., Blondeau, J., Kadison, E., ... & Rothenberger, P. (2013). Convergent mortality responses of Caribbean coral species to seawater warming. Ecosphere, 4(7), 1-40.