Exploring the mismatch between the theory and application of photosynthetic quotients in aquatic ecosystems

Abstract

Accurate measures of primary production in aquatic ecosystems are necessary to quantify energy availability to higher trophic levels and biological effects on global CO 2 concentrations, among other reasons. However, we commonly measure primary production using O 2 because it is easier than measuring changes in CO 2 and the subsequent modeling of the full carbonate system. Although we measure O 2 we are often more interested in the rate of fixed C, and thus use the photosynthetic quotient (the ratio of O 2 release to CO 2 fixed, photosynthetic quotient [PQ]) to convert O 2 -based metabolism to CO 2 . This study summarizes the mismatch between our current knowledge and the application of PQ, highlights knowledge gaps, and emphasizes the need to use literature-based sensitivity analyses rather than uninformed fixed PQ values.

Structural complexity and benthic metabolism: resolving the links between carbon cycling and biodiversity in restored seagrass meadows

“…Deviations from the theoretical 1 : 1 relationship between F O 2 and F DIC (Fig. 3d) are thus ubiquitous in the literature (e.g., Barron et al, 2006;Turk et al, 2015;Trentman et al, 2023). In fact, the slope we observed is identical to what Pinardi et al (2009) observed in sediments vegetated with the freshwater macrophyte Vallisneria spiralis using sediment cores.…”

“…Converting these fluxes into carbon currency often relies on assuming a constant stoichiometric 1 : 1 ratio between oxygen and dissolved inorganic carbon (O 2 : DIC) fluxes which may significantly under-or overestimate actual metabolism (e.g., Barron et al, 2006;Duarte et al, 2010;Turk et al, 2015). For marine sediments, this ratio has been estimated to range between 0.8-1.2 on annual timescales (Glud, 2008, and references therein), but the variability is poorly constrained and likely higher in seagrass systems and on shorter timescales (Turk et al, 2015;Trentman et al, 2023). The discrepancy from a 1 : 1 ratio between benthic oxygen and DIC fluxes can stem from a wide range of processes, including anaerobic sediment processes, nitrate assimilation, photorespiration, and differences in solubility and air-sea gas exchange rates between O 2 and CO 2 (Weiss, 1970;Trentman et al, 2023).…”

“…For marine sediments, this ratio has been estimated to range between 0.8-1.2 on annual timescales (Glud, 2008, and references therein), but the variability is poorly constrained and likely higher in seagrass systems and on shorter timescales (Turk et al, 2015;Trentman et al, 2023). The discrepancy from a 1 : 1 ratio between benthic oxygen and DIC fluxes can stem from a wide range of processes, including anaerobic sediment processes, nitrate assimilation, photorespiration, and differences in solubility and air-sea gas exchange rates between O 2 and CO 2 (Weiss, 1970;Trentman et al, 2023). In seagrasses, storage in tissues and transport of oxygen to roots and subsequent radial oxygen loss (ROL) can also contribute to deviations from the theoretical 1 : 1 relation (Borum et al, 2007;Ribaudo et al, 2011;Berg et al, 2019).…”

Seasonality Drives Carbon Emissions Along a Stream Network

“…We assumed that NEP was spatially constant throughout the stream network. We assumed a respiratory quotient of 1 mol C to 1 mol O to convert these oxygen‐based estimates to carbon equivalents, although we acknowledge that this quotient likely varies (Trentman et al., 2023).…”

Infrared gas analysis as a method of measuring seagrass photosynthetic rate in the face of desiccation stress

“…Non-specific oxygen consuming process (e.g., microbial respiration, geochemical reactions, gas ebullition) can interfere with measurements. Furthermore, the ratio of oxygen produced to carbon fixed (i.e., photosynthetic quotient) varies significantly between species and physiological conditions (e.g., 0.1 -4.2) introducing large error and uncertainty (Trentman et al, 2023).…”