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Oxygen availability drives changes in microbial diversity and biogeochemical cycling between the aerobic surface layer and the anaerobic core in nitrite-rich anoxic marine zones (AMZs), which constitute huge oxygen-depleted regions in the tropical oceans. The current paradigm is that primary production and nitrification within the oxic surface layer fuel anaerobic processes in the anoxic core of AMZs, where 30-50% of global marine nitrogen loss takes place. Here we demonstrate that oxygenic photosynthesis in the secondary chlorophyll maximum (SCM) releases significant amounts of O 2 to the otherwise anoxic environment. The SCM, commonly found within AMZs, was dominated by the picocyanobacteria Prochlorococcus spp. Free O 2 levels in this layer were, however, undetectable by conventional techniques, reflecting a tight coupling between O 2 production and consumption by aerobic processes under apparent anoxic conditions. Transcriptomic analysis of the microbial community in the seemingly anoxic SCM revealed the enhanced expression of genes for aerobic processes, such as nitrite oxidation. The rates of gross O 2 production and carbon fixation in the SCM were found to be similar to those reported for nitrite oxidation, as well as for anaerobic dissimilatory nitrate reduction and sulfate reduction, suggesting a significant effect of local oxygenic photosynthesis on Pacific AMZ biogeochemical cycling.Prochlorococcus | oxygen minimum zone | secondary chlorophyll maximum | metatranscriptomics | aerobic metabolism

Bacteria of the NC10 phylum link anaerobic methane oxidation to nitrite denitrification through a unique O 2 -producing intra-aerobic methanotrophy pathway. A niche for NC10 in the pelagic ocean has not been confirmed. We show that NC10 bacteria are present and transcriptionally active in oceanic oxygen minimum zones (OMZs) off northern Mexico and Costa Rica. NC10 16S rRNA genes were detected at all sites, peaking in abundance in the anoxic zone with elevated nitrite and methane concentrations. Phylogenetic analysis of particulate methane monooxygenase genes further confirmed the presence of NC10. rRNA and mRNA transcripts assignable to NC10 peaked within the OMZ and included genes of the putative nitrite-dependent intra-aerobic pathway, with high representation of transcripts containing the unique motif structure of the nitric oxide (NO) reductase of NC10 bacteria, hypothesized to participate in O 2 -producing NO dismutation. These findings confirm pelagic OMZs as a niche for NC10, suggesting a role for this group in OMZ nitrogen, methane and oxygen cycling.

The use of vanadium (III) has been proposed recently as a suitable alternative to cadmium for the reduction of NO 3 to NO 2 during spectrophotometric analysis. However, the methods proposed suffer from decreased sensitivity and additional steps for the measurements of nitrite and nitrate. We have developed an improved fast and sequential protocol that permits the determination of low concentrations of nitrite and nitrate in marine and freshwater samples using small volumes. NO 2 concentration is firstly determined using the common Griess reaction. The subsequent addition of a 2% VCl 3 solution in 6N HCl in the same sample and the reaction at 60ºC for 25 minutes results in an efficient reduction of the NO 3 to NO 2-(> 95%), which is also detected by the already added Griess reagents. The method has a detection limit <0.05 µM, a high precision (ranging from 0.2 to 11%) and accuracy (0.07 µM) for the determination of NO 3-+ NO 2 concentrations lower than 30 µM. Comparison of the proposed method with the established Cd column method using samples from a variety of environments (fresh water reservoir, sediment freeze lysable pore water, estuarine water samples and samples from an acid mine drainage impacted reservoir) showed good agreement between the two methods, with a difference between methods of 0.073 ± 0.099 µM. The analysis can be performed in large batches (~60 samples) using small sample volumes (≤1 mL) for the determination of both NO 3 and NO 2 in less than one hour.

We investigated methane oxidation in the oxygen minimum zone (OMZ) of the eastern tropical North Pacific (ETNP) off central Mexico. Methane concentrations in the anoxic core of the OMZ reached ~ 20 nmol L−1 at off shelf sites and 34 nmol L−1 at a shelf site. Rates of methane oxidation were determined in ship‐board incubations with 3H‐labeled methane at O2 concentrations 0–75 nmol L−1. In vertical profiles at off‐shelf stations, highest rates were found between the secondary nitrite maximum at ~ 130 m and the methane maximum at 300–400 m in the anoxic core. Methane oxidation was inhibited by addition of 1 μmol L−1 oxygen, which, together with the depth distribution, indicated an anaerobic pathway. A coupling to nitrite reduction was further indicated by the inhibitory effect of the nitric oxide scavenger 2‐phenyl‐4,4,5,5‐tetramethylimidazoline‐1‐oxyl‐3‐oxide (PTIO). Metatranscriptomes from the anoxic OMZ core supported the likely involvement of nitrite‐reducing bacteria of the NC10 clade in anaerobic methane oxidation, but also indicated a potential role for nitrate‐reducing euryarchaeotal methane oxidizers (ANME‐2d). Gammaproteobacteria of the Methanococcales were further detected in both 16S rRNA gene amplicons and metatranscriptomes, but the role of these presumed obligately aerobic methane oxidizers in the anoxic OMZ core is unclear. Given available estimates of water residence time, the measured rates and rate constants (up to ~ 1 yr−1) imply that anaerobic methane oxidation is a substantial methane sink in the ETNP OMZ and hence attenuates the emission of methane from this and possibly other OMZs.

Most commercially available optical oxygen sensors target the measuring range of 300 to 2 μmol L-1. However these are not suitable for investigating the nanomolar range which is relevant for many important environmental situations. We therefore developed a miniaturized phase fluorimeter based measurement system called the LUMOS (Luminescence Measuring Oxygen Sensor). It consists of a readout device and specialized “sensing chemistry” that relies on commercially available components. The sensor material is based on palladium(II)-5,10,15,20-tetrakis-(2,3,4,5,6-pentafluorphenyl)-porphyrin embedded in a Hyflon AD 60 polymer matrix and has a KSV of 6.25 x 10-3 ppmv-1. The applicable measurement range is from 1000 nM down to a detection limit of 0.5 nM. A second sensor material based on the platinum(II) analogue of the porphyrin is spectrally compatible with the readout device and has a measurement range of 20 μM down to 10 nM. The LUMOS device is a dedicated system optimized for a high signal to noise ratio, but in principle any phase flourimeter can be adapted to act as a readout device for the highly sensitive and robust sensing chemistry. Vise versa, the LUMOS fluorimeter can be used for read out of less sensitive optical oxygen sensors based on the same or similar indicator dyes, for example for monitoring oxygen at physiological conditions. The presented sensor system exhibits lower noise, higher resolution and higher sensitivity than the electrochemical STOX sensor previously used to measure nanomolar oxygen concentrations. Oxygen contamination in common sample containers has been investigated and microbial or enzymatic oxygen consumption at nanomolar concentrations is presented.

Nitrite is a pivotal component of the marine nitrogen cycle. The fate of nitrite determines the loss or retention of fixed nitrogen, an essential nutrient for all organisms. Loss occurs via anaerobic nitrite reduction to gases during denitrification and anammox, while retention occurs via nitrite oxidation to nitrate. Nitrite oxidation is usually represented in biogeochemical models by one kinetic parameter and one oxygen threshold, below which nitrite oxidation is set to zero. Here we find that the responses of nitrite oxidation to nitrite and oxygen concentrations vary along a redox gradient in a Pacific Ocean oxygen minimum zone, indicating niche differentiation of nitrite-oxidizing assemblages. Notably, we observe the full inhibition of nitrite oxidation by oxygen addition and nitrite oxidation coupled with nitrogen loss in the absence of oxygen consumption in samples collected from anoxic waters. Nitrite-oxidizing bacteria, including novel clades with high relative abundance in anoxic depths, were also detected in the same samples. Mechanisms corresponding to niche differentiation of nitrite-oxidizing bacteria across the redox gradient are considered. Implementing these mechanisms in biogeochemical models has a significant effect on the estimated fixed nitrogen budget.

The decomposition of macroalgal detritus (tubular and planar Ulva spp.) was studied in a microcosm under a daily light:dark cycle to simulate the decomposition on intertidal sediment. The consequences of bloom decay were evaluated in the bulk water phase and in the sediment. ), dissolved organic carbon (DOC) and inorganic carbon (DIC) were measured in the inflowing and outflowing seawater. Vertical microprofiles of O 2 , pH and H 2 S at the sediment -water interface, sediment contents of organic matter (OM), inorganic and organic carbon (C org ), total nitrogen (N) and inorganic nutrients were measured before and after addition of macroalgal detritus. Changes in the taxonomic composition of microphytobenthos were studied by optical microscopy and by the analysis of photosynthetic pigments. Macroalgal detritus vanished from the sediment surface in 6 d. Macroalgal decomposition shifted the microcosm net balance to higher releases of DOC, DIC and inorganic nutrients, suggesting rapid release from macroalgal biomass. Besides being released to the water column, a fraction of macroalgal carbon and of nitrogen was incorporated into the sediment as indicated by a transient increase in C org and N. Aerobic mineralization of macroalgal detritus reduced O 2 in the water column and the sediment. Microbenthos photosynthetic activity was initially suppressed but recovered from the third day as macroalgal detritus decomposed. Photosynthetic O 2 production by microbenthos largely determined the fraction of macroalgal detritus that was aerobically mineralised. Decomposition of macroalgal detritus favoured the dominance of cyanobacteria over diatoms in the microbenthos. KEY WORDS: Macroalgal blooms · Microbenthos · Microelectrodes · Macroalgal decomposition · Macroalgal detritusResale or republication not permitted without written consent of the publisher