Vario macro cube elemental analyzer

Soil samples were extracted with 2M potassium chloride solution for the detection of NH₄⁺-N and NO₃^–-N (Maharjan and Venterea, 2013 ). The soil organic matter (SOM) and total nitrogen (TN) were determined by an Element Vario MACRO cube elemental analyzer (Elementar, Germany). The total phosphorus (TP) and available phosphorus (AP) were determined by the molybdenum blue colorimetric method with a spectrophotometer. The total potassium (TK) and available potassium (AK) were determined by a BWB-XP flame photometer (BWB Technologies, UK). Soil mineral N was extracted with 2 M KCl solution in a 1:5 soil-solution ratio, and the NH₄⁺-N and NO₃^–-N concentrations were determined by a Skalar San++ continuous flow analyzer (Skalar, Netherlands). The cellulose and lignin contents were determined as described in a previous study (Chen et al., 2019 (link)).

The elemental composition (C, N, H, and S) of the biomass of thraustochytrid isolates was analyzed for the exponential (24 h) and stationary (96 h) growth phases. The time points 24 h and 96 h were chosen as exponential and stationary phases, respectively, based on the growth curves of the isolates (Figure S4). The isolates were cultivated in 50 mL of sterilized M4 medium inoculated with respective seed cultures (5% v/v) for 7 days at 28 ℃ and 170 rpm. Seed cultures (5% v/v) were prepared as described in Section 4.1. Samples were collected from the culture broth and a subsample of about 8 mL was used for cell harvesting by centrifugation (20 min, 8000 rpm, 20 ℃). The resulting cell pellet was washed several times with 5 mL of sterile deionized water and pre-frozen at −80 ℃ and then lyophilized in a freeze dryer (CHRIST ALPHA 1-2 LD Plus, Germany) for 48 h. The freeze-dried cells were weighed to obtain their dry cell weight. The elemental composition of the freeze-dried cells (2 mg) was determined on a vario MACRO cube elemental analyzer (Elementar Analysensysteme GmbH, Germany) following the manufacturer’s instructions.