Corals are structure building invertebrates
which create a habitat for many organisms. Within the corals live photosynthetically
active symbionts like dinoflagellates (Symbiodinium)
which provide up to 85 % of the host’s nutrition. Stress factors like high
temperature, salinity stress or high light intensity trigger the loss or death
of Symbiodinium (coral bleaching) resulting
in coral death. Many studies investigated the effect of high temperature stress
(32 °C or more) on corals and their symbionts. The study conducted by Fujise
et al. (2014) assesses the effect on
expulsion of Symbiodinium of moderate
and more realistic thermal stress.
Two coral species (Acropora selago and Acropora
muricata) were collected at 2 m depth in Japan. Measurements were
performed at normal conditions (27 °C), at transition conditions (27 – 30 °C)
and at moderate thermal stress conditions (30 °C). To determine
differences between Symbiodinium in hospite and expelled Symbiodinium the PSII maximum quantum
yield (Fv/Fm) was measured by using a
PAM fluorometer. Additionally Fujise et al.
used transmitted light and fluorescence microscopy to count the expelled
symbionts.
Fujise et
al. found that the expulsion rates in both A. selago and A. muricata
were constant at normal conditions, under transition conditions and under
moderate thermal stress conditions. The amount of expelled normal Symbiodinium cells did not increase at
30 °C, but the expelled degraded cells increased in both coral species. The
in hospite cells have constant Fv/Fm values despite the increasing
temperature. The Fv/Fm values of
normal expelled cells decreased with an increase in temperature.
Combining previous findings Fujise et al. suggest that the expulsion of
degraded and normal Symbiodinium cells
is a normal regulation mechanism to maintain the density of symbionts in the
coral and to preserve a constant amount of fixed organic matter within the host
by releasing surfeit fixed carbon. They suggest that corals digest more
degraded cells under stress conditions and also releases more degraded Symbiodinium under moderate thermal
stress conditions. More normal but photosynthetically damaged cells are
expelled under stress conditions and a decrease of the Fv/Fm values of the expelled cells at 30 °C was detected.
Furthermore, elevated temperature leads to reactive oxygen species (ROS) which
have a negative influence on the photosynthesis and the symbiont in general and
seem to lead to an increase of expulsion rates of Symbiodinium. Normal but photosynthetically damaged symbionts are
expelled to avoid an accumulation of ROS.
In conclusion Fujise et al. suggest that there are two different mechanisms in
expulsion. One seems to be the normal regulation mechanism of the symbiont
density within the hosts tissue. The other one can be an adaptive mechanism as
a response to moderate thermal stress. So coral bleaching can occur even after
long moderate thermal stress and might be a result of an accumulation of damaged
Symbiodinium in the tissue.
All in all this study gives a very important
insight in the mechanisms of coral bleaching and we can infer that even with
moderate but prolonged thermal stress corals will have big issues to adapt and
the world is about to lose important habitats. Consequently it would be
interesting to conduct similar studies with different coral species to see if
they react the same way or if they have developed other mechanisms for dealing
with thermal stress maybe without expelling their symbionts.
Reviewed paper:
Reviewed paper:
Fujise, L., Yamashita,
H., Suzuki, G., Sasaki, K., Liao, L. M., & Koike, K. (2014).
Moderate thermal stress causes active and immediate expulsion of
photosynthetically damaged zooxanthellae (Symbiodinium) from corals. PloS one, 9(12), e114321.
Hi Eleni,
ReplyDeleteThanks for your review. This is a really interesting study, and I agree that looking at other coral species would be a good way to go in the future. I wonder if there's been any indication as to what might happen to these Acropora sp. at the high temperature of 32 degrees celsius that you mention has been conducted in other studies on corals and their symbionts. Do you think an increase to 32 degrees would have an effect on the Acropora sp. and their symbionts in this type of study, and if so what type of effect do you think it would have?
Thanks,
Amy
Hi Amy,
ReplyDeletethey mention that there is a high expulsion of Symbiodinium at 32°C due to host cell detachment which leads to a loss of the symbionts and to coral bleaching. So it is a different mechanism than at 30°C but the effect is the same.
I hope this answers your question.
Eleni
Hi Eleni,
ReplyDeleteWhat do the authors mean by 'transition conditions'? Were they fluctuating during the experiment or slowly increasing?
Thank you,
Chloe
Hi Chloe,
DeleteTransition conditions are during an increase of temperature from 27 to 30°C over some days.
Once reached the temperature used it was constant.
Eleni