The Sensitivity of Subsurface Microbes to Ocean Warming Accentuates Future Declines in Particulate Carbon Export

Abstract

Under future warming Earth System Models (ESMs) project a decrease in the magnitude of downward particulate organic carbon (POC) export, suggesting the potential for carbon storage in the deep ocean will be reduced. Projections of POC export can also be quantified using an alternative physiologically-based approach, the Metabolic Theory of Ecology (MTE). MTE employs an activation energy (E a) describing organismal metabolic sensitivity to temperature change, but does not consider changes in ocean chemistry or physics. Many ESMs incorporate temperature dependent functions, where rates (e.g., respiration) scale with temperature. Temperature sensitivity describes how temperature dependence varies across metabolic rates or species. ESMs acknowledge temperature sensitivity between rates (e.g., between heterotrophic and autotropic processes), but due to a lack of empirical data cannot parameterize for variation within rates, such as differences within species or biogeochemical provinces. Here we investigate how varying temperature sensitivity affects heterotrophic microbial respiration and hence future POC export. Using satellite-derived data and ESM temperature projections we applied microbial MTE, with varying temperature sensitivity, to estimates of global POC export. In line with observations from polar regions and the deep ocean we imposed an elevated temperature sensitivity (E a = 1.0 eV) to cooler regions; firstly to the Southern Ocean (south of 40 • S) and secondly where temperature at 100 m depth <13 • C. Elsewhere in both these scenarios E a was set to 0.7 eV (moderate sensitivity/classic MTE). Imposing high temperature sensitivity in cool regions resulted in projected declines in export of 17 ± 1% (< 40 • S) and 23 ± 1% (< 13 • C) by 2100 relative to the present day. Hence varying microbial temperature sensitivity resulted in at least 2-fold greater declines in POC export than suggested by classic MTE derived in this study (12 ± 1%, E a = 0.7 eV globally) or ESMs (1-12%). The sparse observational data currently available suggests metabolic temperature sensitivity of organisms likely differs depending on the oceanic province they reside in. We advocate temperature sensitivity to be incorporated in biogeochemical models to improve projections of future carbon export, which could be currently underestimating the change in future POC export.

Reconciling the Size‐Dependence of Marine Particle Sinking Speed

“…Our results suggest a comparatively weak size‐dependence of sinking speed, which suggests this climate feedback may be weaker than previously assumed. A better parameterization of the size‐sinking relationships should also allow better isolation in Earth System Models of remineralization relationships that are expected to vary with warming due to faster rates of heterotrophic metabolism (Cael & Follows, 2016; Cavan & Boyd, 2018; Cavan et al., 2019), especially given the large vertical gradients in temperature in the upper ocean. Improved parameterization of particle flux thereby provides a means to better predict changes in the ocean carbon cycle with climate change.…”

Effects of Ocean Acidification on Marine Photosynthetic Organisms Under the Concurrent Influences of Warming, UV Radiation, and Deoxygenation

“…In laboratory experiments, increasing temperature below optimal values can increase phytoplankton growth (Fu et al, 2014;Summers et al, 2016;O'Donnell et al, 2017;Jiang et al, 2018). However, on a global scale, warming may decrease primary productivity due to nutrient limitation driven by enhanced stratification of the upper mixing layer (UML); it may also enhance fixed carbon remineralization by heterotrophic microorganisms and so reduce the strength of the ocean carbon sink (Danovaro et al, 2016;Cavan et al, 2019).…”

Microbial Competition in the Subpolar Southern Ocean: An Fe–C Co-limitation Experiment

“…Increasing temperature is also expected to increase bacterial respiration rates (Vázquez-Domínguez et al, 2007). Thus, phytoplankton standing stocks are likely to decline and the proportion of primary production respired in near-surface waters by heterotrophic bacteria will increase (Deppeler and Davidson, 2017;Cavan and Boyd, 2018;Cavan et al, 2019). The study of Cavan and Boyd (2018), which have predicted an increase in POC-normalized respiration, estimates that the biological pump efficiency (POC export scaled to primary production) would decrease by 17 ± 7% (SE) by 2100 for the subantarctic site SOTS.…”