Abstract
Marine prokaryotes play a key role in cycling of organic matter and nutrients in the ocean. Using a unique dataset (>14,500 samples), we applied a space-for-time substitution analysis to assess the temperature dependence of prokaryotic heterotrophic production (PHP) in epi- (0–200 m), meso- (201–1,000m) and bathypelagic waters (1,001–4,000 m) of the global ocean. Here, we show that the temperature dependence of PHP is fundamentally different between these major oceanic depth layers, with an estimated ecosystem-level activation energy (Ea) of 36 ± 7 kJ mol–1 for the epipelagic, 72 ± 15 kJ mol–1 for the mesopelagic and 274 ± 65 kJ mol–1 for the bathypelagic realm. We suggest that the increasing temperature dependence with depth is related to the parallel vertical gradient in the proportion of recalcitrant organic compounds. These Ea predict an increased PHP of about 5, 12 and 55% in the epi-, meso- and bathypelagic ocean, respectively, in response to a water temperature increase by 1oC. Hence, there is indication that a major thus far underestimated feedback mechanism exists between future bathypelagic ocean warming and heterotrophic prokaryotic activity.
Highlights
Marine bacteria and archaea, here collectively termed prokaryotes, are central components of marine food webs and play a key role in controlling ocean biogeochemistry
In order to avoid artifacts due to selection of different carbon conversion factors (CF), all prokaryotic heterotrophic production (PHP) data were converted using a factor of 1.95 kg C mol−1 leucine incorporated (Calvo-Díaz and Morán, 2009; Alonso-Sáez et al, 2010), 1.63 × 1018 cells mol−1 thymidine incorporated (Carlson et al, 1996) and assuming a carbon conversion factor of 12 fg C cell−1 (Fukuda et al, 1998)
The values we use in this study are averages of the most commonly used CFs; had we used lower or higher factors it would not have influenced the slope of the observed relationships between PHP and temperature, but only the origin intercept, i.e., the data position in the plots
Summary
Here collectively termed prokaryotes, are central components of marine food webs and play a key role in controlling ocean biogeochemistry. The dark ocean constitutes the largest habitat of the biosphere, comprising about 95% of the global ocean volume. This part of the ocean is subdivided into the mesopelagic zone (201–1000 m depth) with water mass renewal times of decades and the bathypelagic (1001– 4000 m depth) and abyssal zones (>4000 m depth) with water mass renewal times of centuries to millennia (Matsumoto, 2007). Microbial growth in the dark ocean is mainly supported by the dissolved semi-labile, suspended and sinking particulate organic matter exported from the epipelagic zone and most of this organic matter is degraded in the mesopelagic zone (Arístegui et al, 2009; Herndl and Reinthaler, 2013). Considering that the dark ocean contains around 75% of all pelagic prokaryotic biomass and a large fraction of the global ocean’s removal of organic matter occurs below 200 m depth (del Giorgio and Duarte, 2002; Arístegui et al, 2009), minor changes in prokaryotic activity due to climate change could have a large impact on ocean functioning, atmospheric CO2 concentrations and the Earth climate system
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