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Under increasing anthropogenic impacts, the hydrology, weathering, turbidity, and biogeochemistry of the Amazon and Pará river catchments are changing, potentially affecting the concentration and speciation of nitrogen (N) entering the estuary, as well as the subsequent primary productivity. Primary production and several N‐cycling rates have been studied in the northern river plume, and new studies focusing on specific processes in the river mouth emerged. Yet, a general overview is still lacking. For the first time, we provide an overview of primary production along with nitrification, ammonium (NH4+), nitrate (NO3−), and amino acid uptake rates in the Amazon and Pará river mouths which exhibit different degrees of catchment land use. Our results indicate that the Amazon supplies tenfold more NH4+ and NO3−, and its nitrification rates are four times higher than in the Pará River. The stable isotope composition of ambient water and NO3− confirmed that nitrification is a dominant pathway, with fluxes reaching 8.5 × 108 mol N d−1 in the Amazon. Nutrients are transported to regions with reduced turbidity and little nitrification, facilitating NH4+ (0.2–2.4 × 108 mol N d−1) and carbon uptake (4.6 and 14.0 × 108 mol C d−1 in the Pará and the Amazon, respectively) by phytoplankton. The lower N‐cycling rates measured in the Pará relative to the Amazon River mouth likely originate from differences in river discharge and watershed characteristics. Plain Language Summary: Riverine nitrogen can support intense production at the base of the food web in coastal oceans. Such productive ecosystems often show flourishing life and act as a strong atmospheric carbon sink. Since the extent of this nitrogen source is greatly influenced by the status of the river catchment area, it is crucial to have a good understanding of the amount and reactivity of riverine nitrogen at a given study site. Here, we used nitrogen isotope tracers to measure nitrogen assimilation and nitrification rates in the Amazon and Pará river mouths. We found that nitrification was happening at high rates in the Amazon River Mouth. In both estuaries, nitrate production is greater than consumption, allowing nitrate export beyond the turbid waters of the river mouths. Because the Amazon and Pará drainage basins are facing drastic changes, this nitrogen budget gives a baseline for nitrogen cycle pathways and export that will help to monitor the effect of these changes on estuarine nitrogen processes. Key Points: Nitrogen uptake and nitrification measurements reveal intense nitrification of autochthonous ammonium in the Amazon and Pará estuariesIn the turbid parts, nitrate produced via nitrification is not entirely consumed by primary production, leading to its exportRates of nitrogen cycle processes are 10 times higher in the Amazon River mouth, likely due to higher riverine nitrogen discharged [ABSTRACT FROM AUTHOR]

Focuses on crystallization and preliminary X-ray diffraction studies of the glutathione S-transferase from Plasmodium falciparum. Experimental observation of the analysis; Method used to study the analysis; Description about crystallization processes.