To determine the effects of Saharan dust on the abundance, biomass, community structure, and metabolic activity of oceanic microbial plankton, we conducted eight bioassay experiments between ca. 30uN and 30uS in the central Atlantic Ocean. We found that, although bulk abundance and biomass tended to remain unchanged, different groups of phytoplankton and bacterioplankton responded differently to Saharan dust addition. The predominant type of metabolic response depended on the ecosystem's degree of oligotrophy and was modulated by competition for nutrients between phytoplankton and heterotrophic bacteria. The relative increase in bacterial production, which was the dominant response to dust addition in ultraoligotrophic environments, became larger with increasing oligotrophy. In contrast, primary production, which was stimulated only in the least oligotrophic waters, became less responsive to dust as the ecosystem's degree of oligotrophy increased. Given the divergent consequences of a predominantly bacterial vs. phytoplanktonic response, dust inputs can, depending on the ecosystem's degree of oligotrophy, stimulate or weaken biological CO 2 drawdown. Thus, the biogeochemical implications of changing dust fluxes might not be universal, but variable through both space and time.

Oxygen consumption during in vitro dark bottle incubations is the most common method to estimate planktonic respiration. This method is time consuming and labor instensive and, consequently, the database of planktonic respiration rates is scarce. Electron Transport System (ETS) activity measurement has gained acceptance as a routine technique to estimate respiration due to its high sensitivity. However, the in vitro ETS assay commonly used yields potential rates. Hence, the empirically determined ratio between in situ respiration and potential ETS activity is not constant, varying over two orders of magnitude, highly limiting the routine application of this technique to estimate microbial respiration. We hypothesized that in vivo ETS activity should be a reliable estimator of in situ respiration in marine plankton communities. The aim of this study was to develop a methodology to estimate in vivo ETS activity rates by using tetrazolium salt 2‐para (iodo‐phenyl)‐3(nitrophenyl)‐5(phenyl)tetrazolium chloride (INT) as electron acceptor. We established a procedure to apply the in vivo INT reduction method to natural marine microplankton samples. Optimum incubation times were found to be lower than 2–6 h, even for oligotrophic waters. The method was tested in natural waters, covering a very wide range of respiration rates and trophic conditions. A significant linear relationship was found between in situ respiration and in vivo INT reduction. Moreover, the ratio between oxygen consumption and INT reduction was constant across contrasting environments, which reveals the INT reduction method as a simple, quick, sensitive, and robust technique suitable to substantially improve the microbial plankton respiration database.

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.

Marine bacterioplankton are essential in global nutrient cycling and organic matter turnover. Time-series analyses, often at monthly sampling frequencies, have established the paramount role of abiotic and biotic variables in structuring bacterioplankton communities and productivities. However, fine-scale seasonal microbial activities, and underlying biological principles, are not fully understood. We report results from four consecutive years of high-frequency time-series sampling in the Baltic Proper. Pronounced temporal dynamics in most investigated microbial variables were observed, including bacterial heterotrophic production, plankton biomass, extracellular enzyme activities, substrate uptake rate constants of glucose, pyruvate, acetate, amino acids, and leucine, as well as nutrient limitation bioassays. Spring blooms consisting of diatoms and dinoflagellates were followed by elevated bacterial heterotrophic production and abundances. During summer, bacterial productivity estimates increased even further, coinciding with an initial cyanobacterial bloom in early July. However, bacterial abundances only increased following a second cyanobacterial bloom, peaking in August. Uptake rate constants for the different measured carbon compounds varied seasonally and inter-annually and were highly correlated to bacterial productivity estimates, temperature, and cyanobacterial abundances. Further, we detected nutrient limitation in response to environmental conditions in a multitude of microbial variables, such as elevated productivities in nutrient bioassays, changes in enzymatic activities, or substrate preferences. Variations among biotic variables often occurred on time scales of days to a few weeks, yet often spanning several sampling occasions. Such dynamics might not have been captured by sampling at monthly intervals, as compared to more predictable transitions in abiotic variables such as temperature or nutrient concentrations. Our study indicates that high resolution analyses of microbial biomass and productivity parameters can help out in the development of biogeochemical and food web models disentangling the microbial black box.

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.