Reefs under stress: Temperature Flashcards
Case Study: The Persian/Arabian Gulf
How can you tell if a coral has died?
Algae will grow over it.
If it is white (bleached) it has lost its zooxanthellae but not yet died.
What happens in the summer for some corals?
“End of summer bleaching” -> seasonal bleaching
What is an example of a region with coral reefs that is unexpectedly hot? How hot?
The Persian/Arabian Gulf (PAG).
Waters here can exceed 32 degrees C.
What was found about the coral genera in the PAG that made it confusing as to why they were able to survive in these extreme conditions?
The corals were found to be a subgroup of those from the Indian Ocean. This was strange as why can these corals survive but not their conspecifics?
Is there significant seasonal variability in the PAG? How does Fiji compare?
Yes -> 20-34 degrees C across winter-summer
Fiji = 25-29 degrees C in seasonal variation
What would be expected about the Symbiodiniacineae in the PAG given past understanding about genera/clades?
Clade D/Genera Durusdinium were found to be more heat resistant, and therefore might be expected to be in the PAG.
What makes Symbiodiniacineae genera so important in determining heat resistance?
The Symbiodiniacineae carries out the photosynthesis – this relies on membrane integrity, which breaks down in higher temperatures. Some are more tolerant than others and therefore these variations determine likelihood of bleaching.
Which genera of Symbiodiniacineae was found in southern PAG?
Most had Clade C/Cladocopium - specially the sub-species C3
Limited Durusdinium - unexpected
What was found in the genetic sequence of C3 of southern PAG that made it different from other Cladocopium?
ITS2 analysis:
- a consistent difference in 8 nucleotides in the same position (known as “S. thermo indel”), suggesting genetic modification
Further psbA analysis:
- resulted in a new species found called Symbiodinium thermophilum/Cladiocopium thermophilum (new name!) [note this is more similar to C3 than D]
What is another abiotic feature of PAG other than extreme temperatures?
High salinity, especially in southern and western PAG (there is a gradient).
It can reach as high as 42 (remaining consistent throughout the year).
Is it salinity or heat that appears to select for Cladiocopium thermophilum in this region? How was this tested?
- Used colonies from Abu Dhabi to compare had to do lab experiments (natural mean they are always exposed to both temperature and salinity)
- Everything else was maintained
- Porites at salinity 42 coped with temperatures up to 32-33 degrees C, whilst those exposed to lower salinities died when exposed to higher temperatures.
-> therefore it is likely that salinity plays a role in tolerating higher temps.
Why is it useful to look at Porites in PAG?
- Very common in PAG
- Typically associate with Cladiocopium thermophilum – C15 -> This is a persistent symbiosis – some species are more flexible (e.g., summer C3, winter C15) but not porites.
However, in PAG they were instead associated with a thermophilum closer to C3.
What was found when comparing Durusdinium and S. thermophilum at different temps and salinities?
In summer with max temperature 32-33 dC = no Durusdinium
In summer with max temperature 31-32 dC = slightly more Durusdinium
Therefore, the lower the temperature and the lower the salinity, the higher the abundance of Durusdinium.
The higher the temperature, and the higher the salinity, the higher the abundance of S. thermophilum (better suited to PAG conditions).
Why do the conditions that S. thermophilum lives at mean that it cannot be easily introduced to existing communities?
S. thermophilum is not capable of surviving at natural ocean salinities – either they will die or be outcompeted by communities already in a region. The source population can also be damaged in this way.
What about the PAG makes it unlikely that S. thermophilum evolved here?
It is a young sea.
Was only inundated fully 8000 years ago and only recently (4000 years ago) warmed to the current climate.
Therefore, evolution would have to be very fast.
How were the origins of S. thermophilum tested?
ITS2 sequence analysis -> those containing “S. thermo indel”. If this was identified then S. thermophilum is present.
Compared symbiotic samples across different regions/latitudes to identify if thermophilum is present widely and not just in PAG:
- Highest in southern PAG
- A few colonies were also present in the Red Sea
This showed that S. thermophilum hadn’t just evolved in PAG.
What was identified in the genetic diversity of S. thermophilum in the PAG that wasn’t true for S. thermophilum found in other regions?
S. thermophilum in the PAG are very similar to each other, but are genetically different from S. thermophilum anywhere else in the world -> therefore definitely a separate species/group.
What can be concluded about the formation of heat tolerant S. thermophilum in PAG?
The genetic uniformity of this symbiont in the hottest coral reef ecosystem in the world may have resulted from a strong positive selection of a few of the most thermally tolerant genotypes from an old lineage with a more widespread but cryptic distribution.
What can we conclude after understanding the origins of S. thermophilum in the PAG?
“the example of the PAG suggests that protecting present-day biodiversity is of upmost importance to provide the largest possible gene pool from which more stress-tolerant species and genotypes may emerge and become more common under severe natural selection”
This requires protecting against stresses other than heat stress, such as habitat destruction, pollution, and eutrophication, to ensure that heat-tolerant species are maintained.
What is true about the diversity of the PAG vs the wider Indian Ocean for example?
PAG = very little diversity
Indian Ocean = much more diverse
