Abstract. Oxygen-deficient zones (ODZs) are major sites of net natural nitrous oxide (N2O) production and emissions. In order to understand changes in the magnitude of N2O production in response to global change, knowledge on the individual contributions of the major microbial pathways (nitrification and denitrification) to N2O production and their regulation is needed. In the ODZ in the coastal area off Peru, the sensitivity of N2O production to oxygen and organic matter was investigated using 15N tracer experiments in combination with quantitative PCR (qPCR) and microarray analysis of total and active functional genes targeting archaeal amoA and nirS as marker genes for nitrification and denitrification, respectively. Denitrification was responsible for the highest N2O production with a mean of 8.7 nmol L−1 d−1 but up to 118±27.8 nmol L−1 d−1 just below the oxic–anoxic interface. The highest N2O production from ammonium oxidation (AO) of 0.16±0.003 nmol L−1 d−1 occurred in the upper oxycline at O2 concentrations of 10–30 µmol L−1 which coincided with the highest archaeal amoA transcripts/genes. Hybrid N2O formation (i.e., N2O with one N atom from NH4+ and the other from other substrates such as NO2-) was the dominant species, comprising 70 %–85 % of total produced N2O from NH4+, regardless of the ammonium oxidation rate or O2 concentrations. Oxygen responses of N2O production varied with substrate, but production and yields were generally highest below 10 µmol L−1 O2. Particulate organic matter additions increased N2O production by denitrification up to 5-fold, suggesting increased N2O production during times of high particulate organic matter export. High N2O yields of 2.1 % from AO were measured, but the overall contribution by AO to N2O production was still an order of magnitude lower than that of denitrification. Hence, these findings show that denitrification is the most important N2O production process in low-oxygen conditions fueled by organic carbon supply, which implies a positive feedback of the total oceanic N2O sources in response to increasing oceanic deoxygenation.

Human population growth has increased the demand for water and clean energy, leading to the massive construction of reservoirs. Reservoirs can emit greenhouse gases (GHG) affecting the atmospheric radiative budget. The radiative forcing due to CO 2 , CH 4 , and N 2 O emissions and the relative contribution of each GHG in terms of CO 2 equivalents to the total forcing is practically unknown. We determined simultaneously the CO 2 , CH 4 , and N 2 O fluxes in reservoirs from diverse watersheds and under variable human pressure to cover the vast idiosyncrasy of temperate Mediterranean reservoirs. We obtained that GHG fluxes ranged more than three orders of magnitude. The reservoirs were sources of CO 2 and N 2 O when the watershed lithology was mostly calcareous, and the crops and the urban areas dominated the landscape. By contrast, reservoirs were sinks of CO 2 and N 2 O when the watershed lithology was predominantly siliceous, and the landscape had more than 40% of forestal coverage. All reservoirs were sources of CH 4 , and emissions were determined mostly by reservoir mean depth and water temperature. The radiative forcing was substantially higher during the stratification than during the mixing. During the stratification the radiative forcings ranged from 125 mg CO 2 equivalents m −2 d −1 to 31 884 mg CO 2 equivalents m −2 d −1 and were dominated by the CH 4 emissions; whereas during the mixing the radiative forcings ranged from 29 mg CO 2 equivalents m −2 d −1 to 722 mg CO 2 equivalents m −2 d −1 and were dominated by CO 2 emissions. The N 2 O contribution to the radiative forcing was minor except in one reservoir with a landscape dominated by crops and urban areas. Future construction of reservoirs should consider that siliceous bedrocks, forestal landscapes, and deep canyons could minimize their radiative forcings.

<p><strong>Abstract.</strong> Oxygen deficient zones (ODZs) are major sites of net natural nitrous oxide (N<sub>2</sub>O) production and emissions. In order to understand changes in the magnitude of N<sub>2</sub>O production in response to global change, knowledge on the individual contributions of the major microbial pathways (nitrification and denitrification) to N<sub>2</sub>O production and their regulation is needed. In the ODZ in the coastal area off Peru, the sensitivity of N<sub>2</sub>O production to oxygen and organic matter was investigated using <sup>15</sup>N-tracer experiments in combination with qPCR and microarray analysis of total and active functional genes targeting archaeal <i>amoA</i> and <i>nirS</i> as marker genes for nitrification and denitrification, respectively. Denitrification was responsible for the highest N<sub>2</sub>O production with a mean of 8.7 nmol L<sup>−1</sup> d<sup>−1</sup> but up to 118 ± 27.8 nmol L<sup>−1</sup> d<sup>−1</sup> just below the oxic-anoxic interface. Highest N<sub>2</sub>O production from ammonium oxidation (AO) of 0.16 ± 0.003 nmol L<sup>−1</sup> d<sup>−1</sup> occurred in the upper oxycline at O<sub>2</sub> concentrations of 10–30 µmol L<sup>−1</sup> which coincided with highest archaeal <i>amoA</i> transcripts/genes. Oxygen responses of N<sub>2</sub>O production varied with substrate, but production and yields were generally highest below 10 µmol L<sup>−1</sup> O<sub>2</sub>. Particulate organic matter additions increased N<sub>2</sub>O production by denitrification up to 5-fold suggesting increased N<sub>2</sub>O production during times of high particulate organic matter export. High N<sub>2</sub>O yields of 2.1 % from AO were measured, but the overall contribution by AO to N<sub>2</sub>O production was still an order of magnitude lower than that of denitrification. Hence, these findings show that denitrification is the most important N<sub>2</sub>O production process in low oxygen conditions fueled by organic carbon supply, which implies a positive feedback of the total oceanic N<sub>2</sub>O sources in response to increasing oceanic deoxygenation.</p>

Marine invertebrates, as holobionts, contain symbiotic bacteria that coevolve and develop antimicrobial substances. These symbiotic bacteria are an underexplored source of new bioactive molecules to face the emerging antibiotic resistance in pathogens. Here, we explored the antimicrobial activity of bacteria retrieved from the microbiota of two sea anemones (Anemonia sulcata, Actinia equina) and two holothurians (Holothuria tubulosa, Holothuria forskali). We tested the antimicrobial activity of the isolated bacteria against pathogens with interest for human health, agriculture and aquaculture. We isolated 27 strains with antibacterial activity and 12 of these isolates also showed antifungal activity. We taxonomically identified these strains being Bacillus and Vibrio species the most representative producers of antimicrobial substances. Microbiome species composition of the two sea anemones was similar between them but differed substantially of seawater bacteria. In contrast, microbiome species composition of the two holothurian species was different between them and in comparison with the bacteria in holothurian feces and seawater. In all the holobiont microbiomes Bacteroidetes was the predominant phylum. For each microbiome, we determined diversity and the rank-abundance dominance using five fitted models (null, pre-emption, log-Normal, Zipf and Zipf-Mandelbrot). The models with less evenness (i.e. Zipf and Zipf-Mandelblot) showed the best fits in all the microbiomes. Finally, we tracked (using the V4 hypervariable region of 16S rRNA gene) the relative abundance of these 27 isolates with antibacterial activity in the total pool of sequences obtained for the microbiome of each holobiont. Coincidences, although with extremely low frequencies, were detected only in the microbiome of H. forskali. This fact suggests that these isolated bacteria belong to the long tail of rare symbiotic bacteria. Therefore, more and more sophisticated culture techniques are necessary to explore this apparently vast pool of rare symbiontic bacteria and to determine their biotechnological potentiality.

Waterbird aggregations and droughts affect nutrient and microbial dynamics in wetlands. We analysed the effects of high densities of flamingos on nutrients and microbial dynamics in a saline lake during a wet and a dry hydrological year, and explored the effects of guano on prokaryotic growth. Concentrations of dissolved organic carbon, total phosphorus and total nitrogen in the surface waters were 2–3 fold higher during the drought and were correlated with salinity. Flamingos stimulated prokaryotic heterotrophic production and triggered cascading effects on prokaryotic abundance, viruses and dissolved nitrogen. This stimulus of heterotrophic prokaryotes was associated with soluble phosphorus inputs from guano, and also from sediments. In the experiments, the specific growth rate and the carrying capacity were almost twice as high after guano addition than in the control treatments, and were coupled with soluble phosphorus assimilation. Flamingo guano was also rich in nitrogen. Dissolved N in lake water lagged behind the abundance of flamingos, but the causes of this lag are unclear. This study demonstrates that intense droughts could lead to increases in total nutrients in wetlands; however, microbial activity is likely constrained by the availability of soluble phosphorus, which appears to be more dependent on the abundance of waterbirds.