[Coral-List] Coral Bleaching and Solar Radiation

Richard Dunne RichardPDunne at aol.com
Mon Feb 16 13:23:17 EST 2009


Following the recent discussions on Coral List concerning the role of 
light in coral bleaching, I thought that a relatively brief and simple 
statement (scientifically valid nonetheless) of the processes involved 
might be of benefit for the wider readership of Coral List (both 
scientific and non-scientific). It graphically illustrates how solar 
radiation plays a crucial role in coral bleaching. Whilst it does not 
represent a comprehensive statement which applies to all circumstances 
of coral bleaching nor does it purport to contain every aspect of the 
science, it nonetheless describes the essential physical and biological 
interactions which are involved in the majority of circumstances where 
widespread bleaching is observed.

The opening scenario is a coral reef situated in the tropics at a time 
of year when the local weather pattern is changing either from 
winter/spring to summer, or from the wet to the dry season. The sun’s 
daily altitude is increasing or is near its annual maximum. 
Consequently, the path length through the atmosphere is relatively short 
so atmospheric attenuation and scattering of solar energy is at its 
minimum. Attenuation and scattering by the clouds is also reduced as the 
skies clear. Wind strengths reduce, and waves and the mixing of the 
surface layers of the ocean diminish.

Highest annual irradiances are now reaching the surface of the sea, 
mainly concentrated in the direct beam of the sun. 94% of this radiation 
penetrates through the surface, with the remainder reflected back into 
the atmosphere. The solar radiation penetrates downwards to depths where 
the corals are living. On its path through the seawater, solar radiation 
is scattered and attenuated. Scattering is a direct function of the 4th 
power of the wavelength, so that the shorter wavelengths (Ultraviolet 
radiation - UVR) are scattered into a near omni-directional light field. 
At the same time, these shorter wavelengths are absorbed to a much 
greater extent compared to the wavelengths of photosynthetically active 
radiation (PAR) both by the water molecules and by other dissolved 
organic compounds in the seawater. Within the PAR wavelengths 
(400-700nm) the red part of the spectrum (>650nm) is strongly absorbed 
by the water molecules within the first few metres of the water surface 
(hence the blue colour of the ocean). All of this means that the coral 
receives a directional beam of PAR whose energy is largely concentrated 
into the blue/ blue green end of the spectrum.

This energy spectrum is well suited to absorption by the photosynthetic 
pigments in the zooxanthellae. Each pigment has what is called a 
photosynthetic action spectrum, which defines how well the pigment can 
absorb energy of a particular wavelength and then pass this energy on 
for use in photosynthesis. In the case of the main pigment (chlorophyll) 
the action spectrum has a large broad peak in the blue region. Over the 
course of any day, the irradiance reaching the corals rises to a maximum 
at about noon. The photosystems within the zooxanthellae have a finite 
capacity to absorb and use quantum light energy, above which any excess 
solar radiation can cause damage. Various mechanisms exist to protect 
the zooxanthellae from damage, including the xanthophyll pigments which 
can absorb excess light energy and dissipate it harmlessly. During 
photosynthesis and also when these protective mechanisms are overwhelmed 
by the increasing irradiance, components of the photosystem become 
damaged (photoinhibition). However, there are cellular processes which 
actively repair this damage, restoring the photosystem to full function. 
Importantly, these pathways are inhibited and damaged by heat. On a day 
when irradiances and sea temperatures are not excessive, this leads to a 
cycle of damage and repair which is in balance. By each successive dawn, 
the coral is in a fit state once again.

Two main factors, therefore can lead to the damage accumulating rather 
than being repaired. Irradiance can increase to damaging levels (high 
solar elevation, clear skies, calm seas, reduced water sediment, lowered 
sea levels, low tides) and/or the sea temperature can rise to a value at 
which the repair cycles are overwhelmed. The former can happen 
relatively quickly, the latter generally more slowly. When this occurs, 
the photosynthetic pigments break down, together with other cellular 
damage (e.g., due to oxygen radicals), and the zooxanthellae may then 
either be digested by the host coral or expelled before further damage 
results – the coral visibly “bleaches”. In the worst case scenario this 
progresses further to mortality of the coral.

The crucial factor in all this (with the exception of extreme 
temperatures or many days in darkness) is that corals do not bleach in 
the absence of “light” even when sea temperatures are elevated: as was 
demonstrated in the elegant experiments of Takahashi et al. (2004 Plant 
Cell Physiol 45:251-255). However, the relative contributions of 
irradiance and sea temperature have not yet been determined. We thus 
have a scenario where a similar degree of damage can accumulate from a 
combination of high irradiance and moderate sea temperature, to that 
from a lower irradiance and higher temperature.

What causes the sea temperature to rise?
At the beginning of the dry season/ early summer, sea temperatures are 
slowly increasing from their annual minimum, and normally lie within a 
range to which the corals in that region are well suited. The clear 
skies allow increased solar radiation to reach and penetrate the sea 
surface. In the seawater most of this solar radiation is absorbed, 
leading to a warming of the surface waters. In the absence of water 
mixing (normally caused by wind and storms) the surface layer heats up 
and what is called a thermocline often forms at varying depths (normally 
20-100m). Below this depth, the cooler ocean water becomes trapped. The 
warmth in this surface water builds as the summer/ dry season 
progresses. Some of the heat is re-radiated back into the atmosphere, 
particularly at night, but this is limited by the air temperature and 
also ‘greenhouse’ trapping of this low wavelength radiation. The whole 
seasonal cycle of seawater warming and then cooling is therefore driven 
by solar radiation, and peak annual sea temperatures normally lag behind 
the annual peak irradiance by a factor of a month or so because of the 
thermal inertia of the seawater mass.

What part does ‘global warming’ play?
I prefer to use the term ‘global climate change’ rather than ‘global 
warming’ because the processes involved are much more complex than a 
simple ‘warming’ per se. The sea warming is a natural seasonal cycle 
which will be exacerbated by longer periods of dry calm, sunny weather 
in any given year. Superimposed on this is the ‘elevated baseline’ that 
represents the effect of increasing atmospheric greenhouse gases which 
traps more of the heat which would normally escape back into space. As 
each year goes by therefore we can expect the seasonal temperature cycle 
to rise by a small but finite amount. However, there may also be 
changing patterns in regional meteorology, where for example, the dry/ 
summer season becomes unusually prolonged, or is substantially less 
cloudy than usual. Both of these will contribute, not only to elevated 
sea temperatures, but also to higher damaging irradiances. The coral is 
thus doubly imperilled.

In addition, these climate driven changes in regional meteorology can 
also protect the corals from bleaching and mortality. If for example the 
onset of the wet monsoon becomes progressively earlier or the skies are 
cloudier, then there is an instantaneous reduction in solar radiation, 
notwithstanding that the sea temperature may still remain at critical 
levels. Although repair mechanisms are still impeded, there is now 
insufficient energy to cause further photoinhibition. At the same time, 
the heat input to the ocean is also reduced, and there may also be 
stormy weather which further cools the ocean by mixing the deeper cooler 
water with the warm surface water.

Why does UVR not matter as much as PAR to coral bleaching?
UVR represents the shortest wavelengths of solar radiation that reach 
the earth’s surface (approx 390nm up to 400nm). Solar energy at the 
lowest wavelengths (UVB) is potentially highly damaging to living cells. 
At the longer wavelengths (UVA) it is known to cause photoinhibition. 
Thankfully, the shortest wavelengths are strongly absorbed by the ozone 
in the upper atmosphere and relatively little penetrates to the surface. 
Also, the atmosphere scatters UVR more strongly than PAR, as does 
seawater. The UVR energy distribution both above and below water is 
therefore more omni-directional compared to the longer wavelengths in 
PAR, effectively distributing the total available UV energy onto a much 
greater surface area and so reducing the irradiance (energy per surface 
area). Underwater, UVR is also heavily attenuated, so that high 
irradiances are limited to very shallow depths (a few metres at most). 
In addition, corals have evolved very potent protective mechanisms to 
avoid UVR damage by the presence of Mycosporine-like amino acids (MAAs), 
found in the zooxanthellae and host and which selectively absorb solar 
radiation at different UVR wavelengths. Corals posses a suite of these 
compounds at concentrations which vary from species to species, place to 
place, and seasonally. Although the production of new MAA is relatively 
slow, it is nonetheless possible for corals to add to their protection 
as seasonal solar irradiance increases. To date, direct evidence of 
coral bleaching which can be attributed to UVR in the natural 
environment has not been found.

Solar radiation has a pivotal role in the occurrence of both local and 
widespread bleaching of corals. Indirectly it causes the cycle of sea 
temperature warming which can lead to the disruption of photochemical 
and other cellular processes. Directly, it damages the photosystems of 
the intracellular zooxanthellae and also leads to the production of 
damaging oxygen radiacals. Arguments which attribute a primary role to 
sea temperature and a secondary role to “light” in coral bleaching are 
therefore irreconcilable. Solar radiation is the principle driving force 
in both cases, and the resultant bleaching is a complex interplay 
between its immediate and/or delayed effects. The fact that to date we 
have only been able to evolve models that correlate widespread bleaching 
in terms of sea temperature (Hotspots or DHW, etc) simply reflect our 
inability to accurately measure solar radiation across large areas and 
interpret and use that data in a meaningful way.

Richard Dunne

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