Large Stimulation of Recalcitrant Dissolved Organic Carbon Degradation by Increasing Ocean Temperatures

Christian Lønborg,Robert T Letscher,Xosé A Álvarez–Salgado,Dennis A Hansell

doi:10.3389/fmars.2017.00436

Abstract

More than 96% of organic carbon in the ocean is in the dissolved form, most of it with lifetimes of decades to millennia. Yet, we know very little about the temperature sensitivity of dissolved organic carbon (DOC) degradation in a warming ocean. Combining independent estimates from laboratory experiments, oceanographic cruises and a global ocean DOC cycling model, we assess the relationship between DOC decay constants and seawater temperatures. Our results show that the apparent activation energy of DOC decay (Ea) increases by 3-fold from the labile (lifetime of days) and semi-labile (lifetime of months) to the semi-refractory (lifetime of decades) DOC pools, with only minor differences between the world’s largest ocean basins. This translates into increasing temperature coefficients (Q10) from 1.7–1.8 to 4–8, showing that the generalised assumption of a constant Q10 of ~2 for biological rates is not universally applicable for the microbial degradation of DOC in the ocean. Therefore, rising ocean temperatures will preferentially impact the microbial degradation of the more recalcitrant and larger of the three studied pools. Assuming a uniform 1oC warming scenario throughout the ocean, our model predicts a global decrease of the DOC reservoir by 7 ± 1 Pg C. This represents a 15% reduction of the semi-labile + semi-refractory DOC pools.

Highlights

Dissolved organic carbon (DOC) in the ocean represents one of the largest reservoirs of organic matter on the Earth’s surface, containing a similar amount of carbon as atmospheric carbon dioxide (CO2) (Hedges, 2002)
The DOC pool comprises a myriad of compounds covering a widespread molecular size-reactivity continuum (Amon and Benner, 1996; Benner and Amon, 2015), it is operationally divided into a biologically labile fraction (DOCL) that is degraded over hours to days, a semi-labile fraction (DOCSL) with turnover times from weeks to months, a semirefractory fraction (DOCSR) that can be stored for decades, a refractory fraction (DOCR) with lifetimes of thousands of years, and an ultra-refractory fraction (DOCUR) resistant to removal for tens of thousands of years (Hansell, 2013)
The Arrhenius plot of the DOCSR decay constants, obtained from the DOC concentrations and CFC apparent water ages collected during the CLIVAR A16N cruise, clearly show two distinct data clusters for the bathy- and mesopelagic North Atlantic Ocean (Figure 1) with significantly different (t-test, p < 0.05) apparent energy of DOC decay (Ea) of 87 ± 9 and 146 ± 21 kJ mol−1, which translate into Q10 values of 3.8 ± 0.5 and 8 ± 3, respectively (Table 1)

Summary

Introduction

Dissolved organic carbon (DOC) in the ocean represents one of the largest reservoirs of organic matter on the Earth’s surface, containing a similar amount of carbon as atmospheric carbon dioxide (CO2) (Hedges, 2002). Bioavailability has traditionally been linked to the chemical composition and structure of DOC, but more recently it has been shown to vary depending on a range of other external factors (Raymond and Spencer, 2015) These factors include nutrient status, redox state, mineral-particle associations, terrestrial inputs, sunlight, biological production of recalcitrant compounds, changing microbial community composition, and the effect of priming or the extreme dilution of individual molecules (e.g., Amon and Benner, 1996; Thingstad et al, 1999; Del-Giorgio and Davies, 2003; Bianchi, 2011; Keil and Mayer, 2014; Arrieta et al, 2015; Letscher et al, 2015). Previous studies have proposed that changes in the degradation of ocean DOC over global scales could have an important role in regulating the long-term climate, with the rate and magnitude of degradation impacting atmospheric CO2 levels, which in turn influences the cooling and warming of the Earth surface (Peltier et al, 2007; Swanson-Hysell et al, 2010; Sexton et al, 2011)

Methods

Results

Discussion

Conclusion