Comment on “Measuring primary production rates in the ocean: Enigmatic results between incubation and non‐incubation methods at Station ALOHA” by P. D. Quay et al.

Abstract

[1] Quay et al. [2010, paragraph 52] state that “...an absolute standard for [primary production] does not exist [therefore] the accuracy of any method cannot be verified.” Further, after diminishing possible errors in their nonincubation methods (paragraph 53 in Quay et al. [2010]), they say that the difference they note with incubation methods “raises a cautionary flag about incubation-based [estimates]” of ocean primary production. [2] Here, I take issue with these statements, arguing (1) from current understanding of phytoplankton and photosynthetic physiology, and (2) from previous comparisons of in situ and incubation estimates of primary production, evidence that Quay et al. [2010] ignore. [3] That there is no “absolute standard” for the accuracy of any primary production method is true in the analytical sense, in that creation of a standard or calibration curve is not possible. However, there is substantial empirical, and strong theoretical, evidence for the limits of photosynthesis with regard to both the supply of irradiance, and with regard to the temperature at which metabolism proceeds. [4] Photochemical reactions in photosynthesis require 8 quanta to produce 1 molecule of O2 from the splitting of the water molecule, a quantum yield of 1/8 or 0.125. Measurements to support the theory are done under low irradiance where efficiency will be highest. Although difficult to confirm, it is generally accepted that a quantum yield for photosynthesis of 0.10 cannot be exceeded [Bannister and Wiedemann, 1984]. Falkowski [1981] extended these ideas to consider the limitations to photosynthesis under high irradiance. Falkowski argues from the actions of the photosynthetic unit in algal chloroplasts. The photosynthetic unit consists of antenna pigments to absorb irradiance, proteins, and a reaction center. The photosynthetic reaction center serves to stabilize, momentarily, the energy delivered from absorption of irradiance at the antenna in the form of electrons, and before those electrons are transferred along a chain of reactions leading to chemical energy. At high irradiance, electron transfer becomes saturated. A turnover of the photosynthetic unit can then be defined, representing the limit at which photochemistry can proceed. The number of photosynthetic units is proportional to the quantity of chlorophyll-a in the phytoplankton cell. The maximum rate of photosynthesis can be scaled, therefore, to the quantity of chlorophyll, and this is called the assimilation number. Falkowski [1981] concluded that 1 mg chlorophyll-a would correspond with fixing up to 25 mg C h . In the 30 years since that publication, I cannot recall data exceeding an assimilation number of 25 mg C (mg chlorophyll-a) 1 h 1 without there being serious questions. [5] The above limits to photosynthesis occur at the subcellular level, at the components of the photosynthetic units. At the cellular level, ambient temperature will ultimately limit the growth rate in phytoplankton, when light and nutrients are in abundance. Eppley [1972] compiled laboratory batch culture experiments conducted over a range of temperatures from near 0 C to >30 C. He found he could enclose virtually all the growth rate data with the equation,