Dataset: Ulva pHT monitoring

Ulva australis was collected from Blackmans Bay, Tasmania, Australia (approx. 42°59’56"S 147°19’8"E) in July 2015 (Austral winter). Three Ulva australis thalli were placed in chambers filled with sterile seawater and circulated with fresh seawater. The pHT of seawater pumped to each tank was maintained using an automated pH control system. Seawater was equilibrated using a membrane contactor where the appropriate mix of N2 and CO2 gas was achieved using three pairs of mass flow controllers (MFCs) set to pHTs of 8.05, 7.85, and 7.65. 

Each of the three MFCs were randomly assigned to four ambient NH4+ and four enriched NH4+growth chambers for a total of 24 chambers. The pHT within each culture chamber was measured every 1.5–3 hours throughout the week-long experiment, monitoring the effect of U. australis photosynthesis and respiration on seawater pHT. The seaweed biomass: seawater volume ratio affected the pHT of the culture chambers so the average pHT of each chamber was denoted by measurements of pHT during the dark cycle throughout the entire experiment which resulted in a continuous range of pHTs (7.56–7.85) representative of future seawater pH conditions.

The ambient NH4+ concentration (n = 12) served as a control for the nutrient treatment and consisted of natural, UV-sterilized, filtered seawater. The enriched concentration of NH4+ (n = 12) was achieved using an auto-dosing peristaltic pump programmed to deliver 12 mL of a 1000 μM NH4Cl solution to growth chambers every two hours. Based on NH4+dosing rate, the NH4+ concentration in the enriched treatment was 20 μM. However, discrete measurements of seawater NH4+ concentrations on days 0, 3, and 6 showed that the average NH4+ concentration was 0.4 ± 0.3 μM in the ambient treatment and 38.0 ± 18.6 μM the enriched treatment.

A syringe pump and two 12-port rotary valves were used to sample seawater directly from each growth chamber. For each spectrophotometric pH measurement, a reference spectrum was acquired after flushing 25 mL of seawater through a 1 cm flow-through quartz cuvette. A spectrum (400–800 nm) was acquired using an LED light source and a UV-Vis spectrometer (BluLoop and USB2000+, Ocean Optics, USA). A dye + seawater spectrum was then obtained after mixing 200 μL of 2 mM metacresol purple sodium salt dye (211761-10G, Sigma Aldrich, Australia) with an additional 25 mL of seawater within the syringe pump. The two spectra were used to calculate an absorbance spectrum. pHT was calculated using the quadratic fits of the absorbance spectra between 429–439 nm, 573–583 nm and a background signal averaged between 750–760 nm. When compared to calculations based on a single wavelength, the quadratic fit approach leads to a three-fold improvement in measurement precision. Each recorded pHT was the average of four replicate measurements, which took approximately three minutes to obtain. The temperature of each sample was recorded with a PT100 temperature sensor and a high-precision data logger. All instrument control, spectra manipulations, and pHT calculations were done using LabVIEW 2014 (National Instruments, USA).

A complete description of methods for implementation and monitoring of experimental treatments, and sampling methods to estimate response variables provided in the following publication:

Related Reference:

Reidenbach LB, Fernandez PA, Leal PP, Noisette F, McGraw CM, Revill AT, et al. (2017). Growth, ammonium metabolism, and photosynthetic properties of Ulva australis (Chlorophyta) under decreasing pH and ammonium enrichment. PLoS ONE 12(11): e0188389. https://doi.org/10.1371/journal.pone.0188389