IN SEARCH OF A PHYSIOLOGICAL BASIS FOR COVARIATIONS IN LIGHT‐LIMITED AND LIGHT‐SATURATED PHOTOSYNTHESIS¹

Abstract

The photosynthesis‐irradiance (PE) relationship links indices of phytoplankton biomass (e.g. chl) to rates of primary production. The PE curve can be characterized by two variables: the light‐limited slope (αb) and the light‐saturated rate (Pbmax) of photosynthesis. Variability in PE curves can be separated into two categories: that associated with changes in the light saturation index, Ek (=Pbmax/αb) and that associated with parallel changes in αband Pbmax (i.e. no change in Ek). The former group we refer to as “Ek‐dependent” variability, and it results predominantly from photoacclimation (i.e. physiological adjustments in response to changing light). The latter group we refer to as “Ek‐independent” variability, and its physiological basis is unknown. Here, we provide the first review of the sporadic field and laboratory reports of Ek‐independent variability, and then from a stepwise analysis of potential mechanisms we propose that this important yet largely neglected phenomenon results from growth rate–dependent variability in the metabolic processing of photosynthetically generated reductants (and generally not from changes in the oxygen‐evolving PSII complexes). Specifically, we suggest that as growth rates decrease (e.g. due to nutrient stress), reductants are increasingly used for simple ATP generation through a fast (<1s) respiratory pathway that skips the carbon reduction cycle altogether and is undetected by standard PE methodologies. The proposed mechanism is consistent with the field and laboratory data and involves a simple new “twist” on established metabolic pathways. Our conclusions emphasize that simple reductants, not reduced carbon compounds, are the central currency of photoautotrophs.

Computation of primary production from remote sensing of ocean colour at the northwestern Atlantic C-SOLAS Lagrangian site

“…Because we were not studying discrete water samples from specific depths, but rather the entire water column over a wide range of incident irradiances, light history cannot be treated in the same way as other physical factors such as temperature. As reported by Platt & Jassby (1976) and Behrenfeld et al (2004), the 2 photosynthetic parameters are correlated in many aquatic systems. Thus, temperature probably could be a good indicator of α B in large scale studies (Fig.…”

Photosynthetic electron turnover in the tropical and subtropical Atlantic Ocean

“…Phytoplankton communities adjust their photochemical constituents and pathways towards an acclimatory state that can be characterised by their rate of cellular light absorption, saturating light-intensity and maximum rate of cellular production (Allen, 2002;MacIntyre et al, 2002;Behrenfeld et al, 2004). This dynamic process of photoacclimation is described here by s PSII,478 and s max PSII;478 , E k and ETR max RCII .…”

“…8, Sections 4.1 and 4.2.). As such, ETR max RCII Behrenfeld et al, 2004;Moore et al, 2006) and in turn community metabolism (Suggett, Maberly and Geider, unpubl. ) will be determined by changes in PSII light absorption.…”

“…Autotrophic O 2 evolution and C i assimilation are governed by the rate of electron turnover by the photochemical 'light' reactions (Allen, 2002;Behrenfeld et al, 2004;Kramer et al, 2004). Absorbed light energy promotes the splitting of water to release O 2 , protons and electrons.…”

The relationship between light intensity and nutrient uptake kinetics in six freshwater diatoms

“…Previous studies indicated that acclimation to a given irradiance of phytoplankton depends on the intrinsic capacity of the algae species concerned and nutrient availability (Behrenfeld et al, 2004;Falkowski and Raven, 2007). Light intensity and the efficiency of nutrient uptake were strongly coupled at lower substrate availability (Krause et al, 2012).…”