The Function and Behavior of Fluorescent Proteins

[cobra2326]

I recently had my most reliable coral, presumably Acropora yongei, decide to completely brown out on me. At first I was pretty annoyed. But that annoyance quickly turned into fascination and questioning, so I've been digging in to some articles and literature about fluorescent proteins. What was interesting is I could literally see the fluorescent protein disappearing from the surface, revealing only the brown zooxanthellae beneath it. I wish I had taken some better pictures to document it, but here's one where you can see some patchiness where the fluorescence was disappearing:

I know Dana's article [1] makes a mention that it's theorized the fluorescent proteins serve as a sort of sunscreen for the coral:

Role of Fluorescent and Non-fluorescent Proteins: Kawaguti (1944) made remarkable observations and believed coral colors were natural sunscreens. Roth et al. (2010) have confirmed this in a tangential sort of way. The latter scientists subjected Acropora yongei (the Bali Slimer Acropora, if you will) fragments to various light intensities using T5 fluorescent lamps. PAR values were categorized as follows: Low Light (30 µmol·m²·sec), Medium Light (300 µmol·m²·sec) and High Light (900 µmol·m²·sec -a remarkable feat considering the light source). They observed the green fluorescence under all lighting conditions. In low light conditions, the density (number) of zooxanthellae fell to very low levels, as did the level of GFP. This is contrary to what many hobbyists believe - that zooxanthellae numbers would increase under low light (causing a 'browning' of the coral thus masking the GFP). These researchers believe the loss of zooxanthellae might be in an effort to avoid self-shading of the dinoflagellates, or perhaps due to a reduction in volume of coral host tissues. GFP content increased substantially at 300 µmol·m²·sec, and increased by 1.6X at a light level of 900 µmol·m²·sec. See Figure 13.

I also found a nice publication with some details of fluorescence changes due to temperature stresses. [2] My current theory about the cause of the browning is due to a temperature drop to about 73F. This particular coral is most acclimated to temperatures in the 80-84F range, this winter being more 77-78F. This lines up with the papers observations of a loss of fluorescence but not zoox. Another theory I have was the carbon I brought online for a few days, at half strength, but in the presence of very low nutrients (I have no fish in my nano).

I would love to have a deeper discussion about the role of FP in corals.

Why, for instance, would a stressed coral shed its protective "sunscreen". Perhaps Dana answered that in his article already, stating "However, various factors (pH, light, ultraviolet radiation, infrared radiation, metal ions, etc.) can alter the relationship between the charge of amino acids and the fluorophore/chromophore, thus causing it to twist (called a 'hula twist')", but I'm not sure why that is? Wouldn't this cause further light stress?

Also, what have your observations been of browning events, or other general losses of fluorescence? I was fascinated by the loss of what appeared to be a "layer of fluorescence" in my coral, revealing a layer beneath it. Have you observed something similar? And what were the events that transpired afterwards, or caused it?

[1] http://www.advancedaquarist.com/2012/12/corals
[2] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3594756/

 

[cobra2326]

I thought this was a pretty good summary from [2]:

Despite the prevalence of fluorescent proteins in corals, their function remains ambiguous and controversial. The most parsimonious hypothesis is photoprotection27, and experimentally, corals photoacclimate by changing fluorescent proteins concentrations; however, in the field there is a lack of correlation between fluorescence intensity and level of ambient light (depth). A role in photosynthetic enhancement seems unlikely due to inefficient energy transfer between endosymbiotic dinoflagellates and host proteins33. Additional hypotheses include antioxidant capacity, regulation of dinoflagellate population, coral innate immune response, and camouflage. It is probable that different fluorescent proteins will have different functions, thus providing corals with a mosaic of functional opportunities, such as suggested for other organisms41. However, the evolution of GFP-like homologues seems to indicate that the absorption properties of fluorescent proteins might be critical to their ecological function and/or that distinct types of fluorescent proteins could have different and possibly complementary functions. While the role of fluorescent proteins in corals remains unresolved, changes in fluorescent protein expression are one of the quickest and largest responses to changes in light and heat.

 

[taricha]

Wow. That's pretty relevant. I have a couple of green fluorescent corals that have browned hard in the past week (acropora humilis most dramatically) as my tank had gotten down to 72-74, it's almost always a hot tank running 80-84.
Never would have connected that.

 

[A. grandis]

I'll put some more wood into the fire:

"In the same vein, coloration was wrongly attributed to natural sunscreens that absorb and mitigate the effects of damaging UV radiation. By extension, the idea was formed within the hobby that ultraviolet energy is necessary for colorful corals (we know now that these sunscreens – MAAs for mycosporine-like amino acids – are colorless and do not lend color."

FROM:
http://www.advancedaquarist.com/2006/9/aafeature
Grandis.

 

[cobra2326]

Wow. That's pretty relevant. I have a couple of green fluorescent corals that have browned hard in the past week (acropora humilis most dramatically) as my tank had gotten down to 72-74, it's almost always a hot tank running 80-84.
Never would have connected that.

Yes, it surprised me too as this coral has seen all sorts of wild swings, but never on the cold side.