Quantofix test strips

For chemical analysis, the water samples were filtered through 0.2 µm filters (Merck Millipore, Darmstadt, Germany) and analyzed with ICP-MS and ion chromatography, as reported previously [44 (link)]. Gas composition in the grown cultures was determined using a Kristall-5000.1 gas chromatograph (Chromatec, Yoshkar-Ola, Mari El, Russia) equipped with a katharometer detector. Volatile fatty acids were analyzed using a GC-2010 Plus gas chromatograph (Shimadzu, Kyoto, Japan). The concentration of nitrate ions was determined using an Expert-001 ionometer (Econix-Expert, Moscow, Russia), as described previously [45 (link)]. Nitrite was determined using Quantofix test strips (Macherey-Nagel, Düren, Germany). Sulfide was monitored with the colorimetrical method by Trüper and Schlegel [46 (link)].

Salmo salar parr were purchased from AquaGen S.A. (Melipeuco, Chile), and they were maintained in accordance with the research guidelines for the use of laboratory animals established by the Chilean National Commission for Scientific and Technological Research (CONICYT) and authorized by the University of Concepcion Institutional Animal Care. The recirculating fish system (Animal Care) consisted of sterilized freshwater and a flow rate of 5 m³ per hour (hr). Water temperature was measured twice per day (Figure 1). Dissolved oxygen was measured daily and kept above 9 mg/L. Ammonia, nitrite, and pH were measured twice per week and kept under 0.05 mg/L, 0.01 mg/L, and 8.0 ± 0.5, respectively. A 12 h light/12 h dark photoperiod was used to artificially reproduce the autumn–winter, in correspondence with the annual cycle of species [62 (link)]. Water temperature, pH, and dissolved oxygen were measured using a Multiparameter Water Quality Meter 9829 (Hanna Instruments®, Woonsocket, RI, USA). Ammonia and nitrate/nitrite were measured using QUANTOFIX® test strips (MACHEREY-NAGEL, Dueren, Germany).

To monitor the levels of sodium nitrate in the BBM during the microalgae culture, the nitrogen-nitrate concentration in the 1-L lab-scale culture was determined using a method with NitraVer X Nitrogen-Nitrate Reagent (Method 100 Hach Lange, Loveland, CO, USA) based on the chromotropic acid method. The samples with reagents were measured by portable colorimeter (DR 900, Hach Lange, Loveland, CO, USA) with a reading range of 0–30 mg·L⁻¹ NO₃-N. Because of this range, samples containing nitrate levels exceeding 30 mg·L⁻¹ were diluted by a factor of 5 or 10. At pilot scale, NO₃⁻ monitoring was carried out using Quantofix test strips (Macherey-Nagel, Düren, Germany, ref. 91313) with a reading range of 10–500 mg·L⁻¹ NO₃⁻.