Microbial degradation of dissolved organic matter (DOM) in planktonic ecosystems is carried out by diverse prokaryotic communities, whose growth rates and patterns of DOM utilization modulate carbon and nutrient biogeochemical cycles at local and global scales. Nine dilution experiments (September 2007 to June 2008) were conducted with surface water from the highly productive coastal upwelling system of the Ría de Vigo (NW Iberian Peninsula) to estimate bacterial growth rates of six relevant marine bacterial groups: Roseobacter, SAR11, Betaproteobacteria,Gammaproteobacteria, SAR86 and Bacteroidetes. Surprisingly, SAR11 dominated over the other bacterial groups in autumn, likely associated to the entry of nutrient-rich, DOC-poor Eastern North Atlantic Central Water (ENACW) into the embayment. Roseobacter and SAR11 showed significantly opposing growth characteristics. SAR11 consistently grows at low rates (range 0.19-0.71 day(-1) ), while Roseobacter has a high growth potential (range 0.70-1.64 day(-1) ). In contrast, Betaproteobacteria, Bacteroidetes, SAR86 and Gammaproteobacteria growth rates widely varied among experiments. Regardless of such temporal variability, mean SAR86 growth rate (range 0.1-1.4 day(-1) ) was significantly lower than that of Gammaproteobacteria (range 0.3-2.1 day(-1) ). Whereas the relative abundance of different bacterial groups showed strong correlations with several environmental variables, group-specific bacterial growth rates did not co-vary with ambient conditions. Our results suggest that different bacterial groups exhibit characteristic growth rates, and, consequently, distinct competitive abilities to succeed under contrasting environmental conditions.
 At least 15% of the 2 Pg y À1 of dissolved organic carbon (DOC) that accumulates in the surface layer of the open ocean has been exported from the ocean margins. The C: N: P stoichiometry of the production and microbial degradation of dissolved organic matter (DOM) in the coastal ocean conditions the quality of the exported substrates. In this work, DOC, dissolved organic nitrogen (DON) and phosphorus (DOP) bioavailability measurements from published bottle incubation experiments have been compiled and reanalyzed to examine the role of bioavailable DOM (BDOM) in the coastal ocean. DOM bioavailability decreased significantly (p < 0.001) in the sequence DOP > DON > DOC, with bioavailable DOC (BDOC) representing 22 AE 12% (mean AE SD) of the total DOC, bioavailable DON (BDON) 35 AE 13% of the total DON and bioavailable DOP (BDOP) 70 AE 18% of the total DOP. This suggests that the role of DOM on the recycled and export production of the coastal ocean is more relevant for the P than for the N and for the N than for the C biogeochemical cycles. First-order microbial degradation rate constants (k) of BDOM (normalized to 15 C) increased significantly (p < 0.05) in the same sequence, with k C being 0.066 AE 0.065 day À1 , k N 0.111 AE 0.096 and k P 0.154 AE 0.137 day À1 . Significant (p < 0.001) power relationships were found among k C , k N and k P (R 2 = 0.84-0.87). The C: N: P molar ratio of the DOM that resists microbial degradation was extremely depleted in N and P, 2835 (AE3383): 159 (AE187): 1, compared with the BDOM fraction, 197 (AE111): 25 (AE16): 1. The flushing time (t) of the coastal ocean in relation to the turnover time of BDOM (1/k), i.e., t Á k, dictates the fate -degradation versus accumulation-of the large scale export of BDOM, which could fuel parts of the oceanic new production and influence the N/P limitation of the open ocean.Citation: Lønborg, C., and X. A. Álvarez-Salgado (2012), Recycling versus export of bioavailable dissolved organic matter in the coastal ocean and efficiency of the continental shelf pump, Global Biogeochem. Cycles, 26, GB3018,
The bioavailability and bacterial degradation rates of dissolved organic matter (DOM) were determined over a seasonal cycle in Loch Creran (Scotland) by measuring the decrease in dissolved organic carbon (DOC), nitrogen (DON) and phosphorous (DOP) concentrations during long-term laboratory incubations (150 days) at a constant temperature of 14ºC. The experiments showed that bioavailable DOC (BDOC) accounted for 29 ± 11 % of DOC (average ± SD), bioavailable DON (BDON) for 52 ± 11% of DON and bioavailable DOP (BDOP) for 88 ± 8 % of DOP. The seasonal variations in DOM concentrations were mainly due to the bioavailable fraction. BDOP was degraded at a rate of 12 ± 4 % d-1 (average ± SD) while the degradation rates of BDOC and BDON were 7 ± 2 % d-1 and 9 ± 2 % d-1 respectively, indicating a preferential mineralization of DOP relative to DON and of DON relative to DOC. Positive correlations between concentration and degradation rate of DOM suggested that the higher the concentration the faster DOM would be degraded. On average, 77 ± 9 % of BDOP, 62 ± 14 % of BDON and 49 ± 19 % of BDOC were mineralized during the residence time of water in Loch Creran, showing that this coastal area exported C-rich DOM to the adjacent Firth of Lorne. Four additional degradation experiments testing the effect of varying temperature on bioavailability and degradation rates of DOM were also conducted throughout the seasonal cycle (summer, autumn, winter and spring). Apart from the standard incubations at 14ºC, additional studies at 8ºC and 18°C were also conducted. Bioavailability did not change with temperature, but degradation rates were stimulated by increased temperature, with a Q 10 of 2.6 ± 1.1 for DOC and 2.5 ± 0.7 for DON (average ± SD).
15The time course of colored dissolved organic matter (CDOM) absorption and 16 fluorescence were monitored during 50 to 70 days of laboratory incubations with water 17 collected in the coastal upwelling system of the Ría de Vigo (NW Iberian Peninsula) under 18 contrasting hydrographic conditions. CDOM fluorescence at peak-T (Ex/Em, 280/350 nm), 19 characteristic of protein-like materials, decayed at a 1 st order rate constant (k T ) of 0.28 ± 20 0.13 day -1 (average ± SD). k T covaried (R 2 = 0.86, p<0.0002) with the rate constant of the 21 bulk DOC (k DOC ), but the protein-like materials degraded 72 ± 23% faster than DOC. 22Therefore, this study confirms that the CDOM fluorescence at peak-T can be used as a 23 proxy to a DOM fraction significantly more labile than the bulk bioavailable DOC. In 24 parallel with the decay of DOC and protein-like fluorescence, an increase in CDOM 25 fluorescence at peak-M (Ex/Em, 320/410 nm) during the course of the incubations verified 26 the production of marine humic-like substances as a by product of the microbial 27 metabolism. CDOM fluorescence at peak-M built up at a production rate (k M ) of 0.06 ± 28 0.01 day -1 (average ± SD) in the Ría de Vigo. Furthermore, the slope of the linear 29 regression between k DOC and k M (R 2 = 0.64, p< 0.001) revealed that the formation of marine 30 humic-like substances occurred at about one fifth of the rate of net DOC consumption. 31 32 Keywords: DOC, bioavailable, refractory, rate constant, fluorescence spectroscopy 33 has also been identified as a by-product of in situ microbial degradation processes (Nieto-52 Cid et al. 2006; Yamashita and Tanoue 2008). These studies suggest that the protein-and 53 humic-like fluorescence could be used to study labile and refractory DOM in the marine 54 environment. However, quantitative relationships between these variables are still lacking. 55 The coastal upwelling area of the Ría de Vigo (NW Iberian Peninsula) produces and 56 processes large amounts of DOC (Álvarez-Salgado et al. 2001), and is therefore an 57 3 appropriate area to establish if a quantitative relationship between fluorescence 58 spectroscopy measurements and the bioavailability and rate constant of DOC exists. 59 Complementing the study by Lønborg et al. (2009b) on the kinetics and C: N: P molar 60 ratios of DOM degradation in the Ría de Vigo, we show here new insights on the dynamics 61 of the consumption of labile and the production of refractory DOM based on fluorescence 62 spectroscopy measurements during the course of the same experiments. 63 64 2. Material and methods 65 2.
Organic matter (OM) plays a major role in both terrestrial and oceanic biogeochemical cycles. The amount of carbon stored in these systems is far greater than that of carbon dioxide (CO2 ) in the atmosphere, and annual fluxes of CO2 from these pools to the atmosphere exceed those from fossil fuel combustion. Understanding the processes that determine the fate of detrital material is important for predicting the effects that climate change will have on feedbacks to the global carbon cycle. However, Earth System Models (ESMs) typically utilize very simple formulations of processes affecting the mineralization and storage of detrital OM. Recent changes in our view of the nature of this material and the factors controlling its transformation have yet to find their way into models. In this review, we highlight the current understanding of the role and cycling of detrital OM in terrestrial and marine systems and examine how this pool of material is represented in ESMs. We include a discussion of the different mineralization pathways available as organic matter moves from soils, through inland waters to coastal systems and ultimately into open ocean environments. We argue that there is strong commonality between aspects of OM transformation in both terrestrial and marine systems and that our respective scientific communities would benefit from closer collaboration.
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