[Coral-List] light and temperature in bleaching

[Thomas Goreau]

Begin forwarded message:

> From: Thomas Goreau <goreau at bestweb.net>
> Date: February 18, 2009 12:39:06 AM EST
> To: coral-list at coral.aoml.noaa.gov
> Subject: Re: Coral-List Digest, Vol 6, Issue 19
>
>>
>
> Richard Dunne's comments below are a valuable summary and raise many  
> interesting points. There is no doubt high light speeds the rate of  
> bleaching in warm water. It is certainly true that solar radiation  
> causes warming of the ocean and that high light and high  
> temperatures very often, in fact usually, covary. It is also true  
> that there are inadequate long term surface light measurements, but  
> this ignores the excellent satellite cloud data, from which valuable  
> semi-quantitative inferences regarding irradiation can be drawn. The  
> work being done with NOAA's buoys measuring real time light and  
> temperature will do much to clear this up. In addition work underway  
> on gene expression patterns by people like Craig Downs should  
> clarify which biochemical and biophysical pathways kick into action  
> first, and then trigger others.
>
> But it is NOT true, in our experience in the field, as he states,  
> that:
> "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"
>
> We have seen numerous cases where bleaching took place when water  
> temperatures were high but when light levels were not elevated due  
> to high cloud conditions and rain, that is to say reduced solar  
> irradiation. That is because the reefs were shaded by high cloud  
> cover around islands caused by local heating, while the ocean  
> temperatures were due to warm water carried in by currents from  
> remote (and less cloudy) areas not affected by local cloudiness as  
> the reef were. But in such cases the "rate" of bleaching is less  
> than in full sunlight, as we notice experimentally.
>
> But note the precise wording of Dunne's statement above. He does not  
> say they do not bleach in "reduced light" but "the absence of light"  
> that is to say total darkness (conditions known to cause bleaching,  
> more than 50 years ago my parents kept corals bleached for years in  
> the dark for physiological controls on studies of the physiology,  
> carbon translocation, and calcification of corals with and without  
> zooxanthellae). But he explicitly rejects this from consideration in  
> the exception in the parenthesis in the first part of the sentence.  
> So the statement is inherently contradictory.
>
> It remains the case that there are no known mass bleaching events at  
> lower than bleaching threshold temperatures with simultaneous high  
> light (discounting local bleaching caused by cold temperatures!), in  
> which light could unambiguously be regarded the primary cause of  
> bleaching and not temperature. Furthermore there is every reason to  
> believe that light levels in reefs have been declining worldwide due  
> to 1) clearly increased global ocean cloudiness documented by  
> satellite and surface meteorological data, and 2) greatly increased  
> turbidity of nearshore reef habitats caused by eutrophication and by  
> erosion following deforestation. This simply does not fit the global  
> increase of light on reefs that Dunne's hypothesis of light as a  
> PRIMARY factor in the global increases of mass coral bleaching would  
> require.
>
> Thomas J. Goreau, PhD
> President, Global Coral Reef Alliance
> Coordinator, United Nations Commission on Sustainable Development  
> Partnership in New Technologies for Small Island Developing States
> 37 Pleasant Street, Cambridge MA 02139
> 617-864-4226
> goreau at bestweb.net
> http://www.globalcoral.org
>
>> Message: 2
>> Date: Mon, 16 Feb 2009 18:23:17 +0000
>> From: Richard Dunne <RichardPDunne at aol.com>
>> Subject: [Coral-List] Coral Bleaching and Solar Radiation
>> To: Coral List <coral-list at coral.aoml.noaa.gov>
>> Message-ID: <4999AF15.1010103 at aol.com>
>> Content-Type: text/plain; charset=windows-1252; format=flowed
>>
>> WIDESPREAD CORAL BLEACHING AND THE ROLE OF SOLAR RADIATION
>>
>> 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.
>>
>> Scenario
>> 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.
>>
>> Conclusion
>> 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
>
>
>

Thomas J. Goreau, PhD
President, Global Coral Reef Alliance
Coordinator, United Nations Commission on Sustainable Development  
Partnership in New Technologies for Small Island Developing States
37 Pleasant Street, Cambridge MA 02139
617-864-4226
goreau at bestweb.net
http://www.globalcoral.org