Abstract. The high availability of electron donors occurring in coastal upwelling ecosystems with marked oxyclines favours chemoautotrophy, in turn leading to high N 2 O and CH 4 cycling associated with aerobic NH + 4 (AAO) and CH 4 oxidation (AMO). This is the case of the highly productive coastal upwelling area off central Chile (36 • S), where we evaluated the importance of total chemolithoautotrophic vs. photoautotrophic production, the specific contributions of AAO and AMO to chemosynthesis and their role in gas cycling. Chemolithoautotrophy was studied at a time-series station during monthly (2007)(2008)(2009)) and seasonal cruises (January 2008, September 2008, January 2009) and was assessed in terms of the natural C isotopic ratio of particulate organic carbon (δ 13 POC), total and specific (associated with AAO and AMO) dark carbon assimilation (CA), and N 2 O and CH 4 cycling experiments. At the oxycline, δ 13 POC averaged −22.2‰; this was significantly lighter compared to the surface (−19.7‰) and bottom layers (−20.7‰). Total integrated dark CA in the whole water column fluctuated between 19.4 and 2.924 mg C m −2 d −1 , was higher during active upwelling, and contributed 0.7 to 49.7% of the total integrated autotrophic CA (photo plus chemoautotrophy), which ranged from 135 to 7.626 mg C m −2 d −1 , and averaged 20.3% for the whole sampling period. Dark CA was reduced by 27 to 48% after adding a specific AAO inhibitor (ATU) and by 24 to 76% with GC7, a specific archaea inhibitor. This indicates that AAO and AMO microbes (most of them archaea) were performing dark CA through the oxidation of NH

Research (CR)2, Santiago, ChileCyanobacteria from Subsection V (Stigonematales) are important components of microbial mats in non-acidic terrestrial hot springs. Despite their diazotrophic nature (N 2 fixers), their impact on the nitrogen cycle in such extreme ecosystems remains unknown. Here, we surveyed the identity and activity of diazotrophic cyanobacteria in the neutral hot spring of Porcelana (Northern Patagonia, Chile) during 2009 and 2011-2013. We used 16S rRNA and the nifH gene to analyze the distribution and diversity of diazotrophic cyanobacteria. Our results demonstrate the dominance of the heterocystous genus Mastigocladus (Stigonematales) along the entire temperature gradient of the hot spring (69-38°C). In situ nitrogenase activity (acetylene reduction), nitrogen fixation rates (cellular uptake of 15 N 2 ) and nifH transcription levels in the microbial mats showed that nitrogen fixation and nifH mRNA expression were light-dependent. Nitrogen fixation activities were detected at temperatures ranging from 58°C to 46°C, with maximum daily rates of 600 nmol C 2 H 4 cm − 2 per day and 94.1 nmol N cm − 2 per day. These activity patterns strongly suggest a heterocystous cyanobacterial origin and reveal a correlation between nitrogenase activity and nifH gene expression during diurnal cycles in thermal microbial mats. N and C fixation in the mats contributed~3 g N m − 2 per year and 27 g C m − 2 per year, suggesting that these vital demands are fully met by the diazotrophic and photoautotrophic capacities of the cyanobacteria in the Porcelana hot spring.

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Abstract. Coastal upwelling ecosystems with marked oxyclines (redoxclines) present high availability of electron donors that favour chemoautotrophy, leading in turn to high N2O and CH4 cycling associated with aerobic NH4+ (AAO) and CH4 oxidation (AMO). This is the case of the highly productive coastal upwelling area off Central Chile (36° S), where we evaluated the importance of total chemolithoautotrophic vs. photoautotrophic production, the specific contributions of AAO and AMO to chemosynthesis and their role in gas cycling. Chemoautotrophy (involving bacteria and archaea) was studied at a time-series station during monthly (2002–2009) and seasonal cruises (January 2008, September 2008, January 2009) and was assessed in terms of dark carbon assimilation (CA), N2O and CH4 cycling, and the natural C isotopic ratio of particulate organic carbon (δ13POC). Total Integrated dark CA fluctuated between 19.4 and 2.924 mg C m−2 d−1. It was higher during active upwelling and represented on average 27% of the integrated photoautotrophic production (from 135 to 7.626 mg C m−2d−1). At the oxycline, δ13POC averaged -22.209‰ this was significantly lighter compared to the surface (-19.674‰) and bottom layers (-20.716‰). This pattern, along with low NH4+ content and high accumulations of N2O, NO2- and NO3- within the oxycline indicates that chemolithoautotrophs and specifically AA oxydisers were active. Dark CA was reduced from 27 to 48% after addition of a specific AAO inhibitor (ATU) and from 24 to 76% with GC7, a specific archaea inhibitor, indicating that AAO and maybe AMO microbes (most of them archaea) were performing dark CA through oxidation of NH4+ and CH4. AAO produced N2O at rates from 8.88 to 43 nM d−1 and a fraction of it was effluxed into the atmosphere (up to 42.85 μmol m−2 d−1). AMO on the other hand consumed CH4 at rates between 0.41 and 26.8 nM d−1 therefore preventing its efflux to the atmosphere (up to 18.69 μmol m−2 d−1). These findings show that chemically driven chemoautotrophy (with NH4+ and CH4 acting as electron donors) could be more important than previously thought in upwelling ecosystems and open new questions concerning its future relevance.