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

Wed Aug 14 21:32:28 UTC 2019

Dear Thomas,

You have raised a very interesting discussion. One critical assumption in gas exchange based estimates of P/R (or just gross photosynthesis), is that dark respiration is representative of respiration in the light.
Our experience with symbiont-bearing corals and forams (but also in biofilms) is that respiration in light is NOT equal to respiration in dark. Typically daytime respiration is higher in light, and sometimes we see an increase with increasing irradiance.
See e.g.:
https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0110814
https://royalsocietypublishing.org/doi/full/10.1098/rspb.2010.2321
https://www.int-res.com/articles/meps/117/m117p159.pdf

Measurement protocols enabling determination of post-illumination respiration may give a proxy for how daytime respiration differs from dark respiration.

Cheers, Michael

Michael Kühl
Professor (Aquatic Microbial Ecology)

University of Copenhagen
Department of Biology
Marine Biology Section (MARS)
Strandpromenaden 5
DK-3000 Helsingør, Denmark

MOB +45 40 47 63 04
mkuhl at bio.ku.dk<mailto:mkuhl at bio.ku.dk>

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

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Hello Thomas:

You are absolutely correct that the P/R terminology has not always been used consistently (for corals or any other organism). The original concept was how much excess production does an organism have over basal metabolism at the end of the day (complete light-dark cycle), which of course presents some challenges to measure completely.

Years ago in a collaborative project between myself, Jim Porter, Peter Swat and Richard Dodge, we used Jim's respirometers to make semi-complete diel measurements of oxygen flux by doing back to back incubations from dawn to dusk that integrated all of the net production for our 18 experimental corals, and intermittent incubations over the nighttime to get snippets of respiration that we then interpolated for the hours when we didn't do back to back incubations (night work can be challenging especially in winter and during rough seas). With these values we calculated daily net production, and by doing the night-time interpolations we calculated how much O2 was consumed during the complete dark cycle. If one accepts the assumption that R is the same during the daytime while the corals are photosynthesizing, one can calculate a 24 hour P/R ratio.  I sadly confess these data were presented at a couple of meeting but haven't been published... I need to work on that.

But for most of the work I have done over the years, we have used short dark and light incubations back to back to get an idea of short term net and gross production, and P/R ratio, which if adjusted for the numbers of daylight and with the assumption that R at night is the same as R measured in the middle of the day, we can estimate a daily P/R ratio. These are short cuts because of the difficulty in making truly integrative measurements of diel R and P.  I think this is OK as long as one is clear with what one is reporting.

More recently with CISME (see information on website out our new diver portable underwater respirometer), we have observed that R is NOT the same all of the time, and it depends on the immediate light regime the coral was under before the R was measured. There are other reports that R is not the same day and night, so more work is needed to nail this down before we know how close we are to an accurate assessment of P/R with the methods that have been used.

I assume you are familiar with: Kinsey DW (1985) Metabolism, calcification and carbon production. I. System level studies. Proc 5th Int Coral Reef Congr 4:505-526, in which much of the older information is reviewed and summarized, and there are a number of more recent papers by Crossland and others. This topic of research has waned in recent years which is a pitty since there is still much to be learned. I hope that CISME will be of some assistance in furthering this research once it gets better known and in the hands of coral physiologists.

Alina

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-----Original Message-----

From: Coral-List <coral-list-bounces at coral.aoml.noaa.gov<mailto:coral-list-bounces at coral.aoml.noaa.gov>> On Behalf Of Thomas Krueger via Coral-List

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

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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