Low microbial respiration of leucine at ambient oceanic concentration in the mixed layer of the central Atlantic Ocean

Abstract

Bacterioplankton are the primary consumers of dissolved organic matter in the ocean, thus the quantification of bacterioplankton production (BP) is essential to our understanding of carbon cycling in the largest ecosystems on Earth. We compared BP, measured as the rate of 14 C-leucine or 3 H-leucine uptake at close to saturating concentration (20 nmol L 21 ), with ambient uptake measured from dilution bioassays. We hypothesized that saturation with leucine would lead to its respiration as a carbon source, thereby not truly representing ambient BP. Seawater was collected from the photic zone throughout the Atlantic Ocean. Respiration as a proportion of total consumption (uptake + respiration) of close to ambient (0.4 nmol L 21 ) and close to saturating (20 nmol L 21 ) 14 C-leucine concentrations were compared. Saturating 3 H-leucine additions overestimated ambient leucine uptake at low rates (200% 6 100% ambient) and underestimated uptake at high rates (90% + 20% ambient). The proportion of total leucine uptake that was respired was threefold higher for 20 nmol L 21 14 C-leucine additions than 0.4 nmol L 21 14 C-leucine additions (15% 6 8% and 5% 6 4%, respectively). Consequently, microbial efficiency of leucine assimilation-an indicator of bacterioplankton growth efficiency-was significantly higher and more stable at close to ambient 14 C-leucine additions than at saturating concentrations (95% 6 4% and 85% 6 8%, respectively). Thus, saturation of oligotrophic open Atlantic Ocean bacterioplankton with leucine, or other molecules indicative of microbial metabolism, leads to the measurement of a response to a nutrient addition, rather than an ambient measurement.

Elevated temperature increases carbon and nitrogen fluxes between phytoplankton and heterotrophic bacteria through physical attachment

“…The N transferred may have originated through leucine respiration by the bacterial cells and the subsequent production of ammonium (Kirchman et al, 1985). Although this process occurs in oligotrophic conditions (Alonso-Sáez et al, 2007;Del Giorgio et al, 2011), at saturating leucine concentrations, as used in this work, leucine respiration may increase up to threefold (Hill et al, 2013). Furthermore, the preference of picophytoplankton to incorporate ammonium rather than other nitrogen compounds such as nitrate (Le Bouteiller, 1986;Harrison et al, 1996;Rees et al, 1999), and the described inhibition of nitrate uptake by ammonium (L'Helguen et al, 2008), suggest that autotrophs may, more likely, have incorporated this newly remineralized ammonium as a N source.…”

Marine Photochemistry of Organic Matter

“…In addition to interference effects, the added radiolabel can be partially respired. Hill et al (2013) found that 3 H-leucine additions overestimated ambient leucine uptake at low uptake rates and underestimated uptake at high uptake rates, which, in turn, would affect BGE estimates. Clearly, these questions, as well as those involving the complex interactions between biological systems and abiotic photochemical reactions discussed above, need to be addressed in future studies designed to evaluate or model the impact of photochemistry on aquatic food web dynamics (Reader and Miller, 2014).…”

Sample Dilution and Bacterial Community Composition Influence Empirical Leucine-to-Carbon Conversion Factors in Surface Waters of the World's Oceans

“…Considering the widely demonstrated variability in eCFs, the use of constant theoretical CFs may produce erroneous estimates of prokaryotic heterotrophic production (PHP) by relying only on estimates of substrate incorporation rates rather than also on the fate of the incorporated compounds (i.e., fraction of substrate that is not assimilated into biomass) (6,28). Several studies have shown that the use of theoretical leucine-to-carbon CFs may overestimate both temporal and spatial variability in prokaryotic biomass production (4,5).…”