Coral Bleaching (Part 1) Flashcards
What is bleaching?
Loss of colour – a visual phenotype, not the functions behind this
What are the different colours within corals associated with?
The brownish coral colour is mostly due to photosynthetic pigments within the symbiont algae.
However, the other colours (pinks, greens, yellows etc.,) are unrelated to the symbionts (only the brown), instead related to the coral animals.
White is associated with the coral skeleton
What are two of the main photosynthetic pigments in corals?
- Chlorophylls (green)
- Peridinin (typically brown in dinoflagellates)
What can be concluded of a coral is white visually?
This coral is not definitely dead – just has lost its symbionts, resulting in transparent tissues so the skeleton is visible.
What is “end of summer bleaching”
Natural seasonal variation where the corals become paler in the summer (when they lose some of their symbionts). This is hard to determine if they are fully bleached or not, as it varies greatly by colony.
What is the ultimate cause of bleaching?
An accumulation of heat stress.
This is associated with high anomalous temperatures (important to specify “anomalous” as what is deemed high can vary by organism).
What average temperature can be used to define anomalously high temperatures/hot spots?
The average temperature of the hottest month (i.e., the maximum value for each location).
What is the definition of a hotspot?
A temperature one degree above the “average temperature of the hottest month” (however this definition is often debated).
What is a benefit of using hotspots to understand hot temperatures?
Accounts for regional variations in “normal temperatures”, meaning regions can be discussed collectively.
What is the bleaching threshold?
It is typically where temperatures exceed 1°C above the maximum monthly mean sea surface temperature (SST), causing corals to experience heat stress and potentially expel their symbiotic algae, leading to coral bleaching
What does DHW stand for?
Degree Heating Week
How are DHWs calculated?
0.5 x (the sum of hotspots over a 12 week period)
Note: temperature is measured 2 times a week by NOAA as a satellite passes regions bi-weekly, so this is why it is multiplied by 0.5
Who is the main coral reef watch?
NOAA - have data biweekly for most regions
What are the key intervals when forecasting bleaching?
- No stress (hotspot <= 0)
- Bleaching watch (0 < hotspot < 1)
- Bleaching warning (1 </ hotspot and 0 < DHW < 4)
- Bleaching alert level 1 (1 </ hotspot and 4 , DHW </ 8)
- Bleaching alert level 2 (1 <= hotspot and 8 </ DHW)
How does normal light-dependent photosynthetic reactions occur and where?
In the thylakoid membrane:
- PSII absorbs photons.
- This light is directly used to split a molecule of water (photophosphorylation), producing protons and electrons.
- The electrons are passed on down the chain (ETC), with reduction reactions facilitating the transport of protons across the membrane into the thylakoid lumen, generating a gradient.
- This gradient causes protons to move back down the gradient across the membrane via ATP synthase, which synthesises ATP with this transportation.
- The electrons end up reducing NADP+ to NADPH
Which are the two crucial outputs of the light-dependent reactions of photosynthesis?
ATP and NADPH (and oxygen)
What does the D1 protein do in PSII?
The D1 protein splits water into protons and electrons, removing O2. As O2 is a gas, it passes the membrane and is eliminated out.
What happens to the D1 protein under elevated temperatures and irradiance? What does this cause?
- The D1 protein denatures
- Photons continue to be absorbed by the pigments, but there is no way to channel this energy into photolysis.
- The cell, with highly excited pigments, releases that energy, typically to any nearby molecules such as oxygen, producing radicals and superoxides that are very reactive (absorbing more energy) - known as ROS
- Those ROS start destroying other enzymes and lipids (cell membranes functionality loss etc.)
What functions does RuBisCO carry out?
RuBisCO is involved in the light independent reactions and fixes CO₂ into organic molecules in the Calvin cycle.
However, it also has an oxygenation activity, leading to photorespiration (this occurs as it cannot distinguish between CO2 and O2, so accidentally fixes O2 instead of CO2).
What is special about the RuBisCO in Symbiodiniaceae and what can this lead to?
Symbiodinacea -> less efficient Class 2 RuBisCO with a higher tendency for oxygenation rather than carboxylation.
This is especially true under heat/oxidative stress, causing reduced C fixation and potential bleaching.
What can temperature also affect in terms of molecules? What does this mean for photosynthesis?
Lipids -> increasing temperature can therefore “melt” the phospholipid membrane.
This can break down the ETC, disrupting the light dependent reactions.
In summary, what are the main impacts of increasing temp/light on photosynthetic reactions?
D1 protein denaturation (ATP and NADPH for next stage not produced)
Membrane disturbances (lipid disruption - e.g., affecting ETC)
Enzymes of Calvin Cycle disrupted (e.g., RuBisCO increasing oxygenation and reducing carboxylation).
What increase has been seen in symbiont cells prior to bleaching? What cascade does this relate to?
Increase in ROS (reactive oxygen species).
Loss of photosynthetic pigments (damage by ROS and heat stress)
Loss of symbiont cells (cell death, expulsion or detachment from host cells)
Which four other unusual factors can cause bleaching?
- Cold stress (slows metabolism)
- Low salinity (disrupts balance of stable ions)
- Diseases (toxin production can disrupt corals or symbionts)
- Nutrient enrichment (imbalances)
