[Coral-List] P:R ratios as measures of daily productivity

[Thomas Krueger]

Hi everyone,
over the last couple of months I came across multiple instances where 
P:R ratios as approximate measures of a coral's autotrophic state (based 
on the original work by Odum & Odum 1955) have been calculated the wrong 
way. Only a few groups have the capability to measure oxygen 
productivity over a whole light or dark period in the field, so most of 
us rely on lab respirometry, where short-term incubations in sealed 
chambers are used to derive values for coral productivity. The hourly 
oxygen production or consumption rate can then be extrapolated to a 24h 
period to gain some indication about general productivity based on 
oxygen metabolism and to some degree on the dependence on autotrophy 
(expressed as P:R ratio). However, I have encountered multiple cases 
where authors have simply divided the measured Pgross (gross 
photosynthesis) directly through R (holobiont respiration). This is not 
a P:R ratio in the traditional (and I guess intended) sense as it does 
not consider the circadian context of productivity. Hourly Pgross needs 
to be multiplicated with the number of daylight hours and then divided 
by the product of 24h times the hourly respiration value. I believe part 
of the confusion comes from the fact that this variable is simply called 
"P:R ratio", but should be called "daily P:R ratio". There is merit in 
comparing hourly rates of Pgross and R in the light directly, but this 
would be better served as a "light R:Pgross ratio", representing the 
percentage of produced oxygen used for holobiont respiration in the 
light. As part of an ongoing manual for students and tutors, I have 
decided to already upload the few pages on measuring oxygen metabolism 
via respirometry here 
https://www.researchgate.net/publication/335125289_Measuring_coral_oxygen_metabolism_via_respirometry. 
It contains also a few questions for the wider community regarding best 
practice in obtaining reliable respirometry data (e.g. minimal 
saturation changes in the chamber, upper and lower limits for saturation 
changes to ensure coral health). The manual I am working on is still in 
progress, but I am helpful for any feedback in general and on the issue 
of P:R ratios in particular.
All the best,
Thomas

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*Thomas Krueger */Research Associate/

Waller Lab | Department of Biochemistry | University of Cambridge

*P: *+44 (0)1223 768942