Outdoor Arizona testbed pond cultivation: large scale

Three identical raceway ponds were utilized for the summer cultivation of Picochlorum celeri. Each pond surface area was ~ 4.2 m² with a nominal volume of 820 L at a depth of 20 cm (Commercial Algae Professionals, http://www.commercialalgae.com). The ponds were equipped with a YSI 5200A-DC (YSI Inc., Yellow Springs, OH, USA) water quality monitoring system simultaneously measuring pH, pond water temperature (°C), dissolved oxygen saturation (%), and dissolved salt concentrations (g L⁻¹). Ponds were also equipped with a stainless-steel paddle wheel and a CO₂ sparge line connected to a Sweetwater ceramic diffuser (Model# DYPFP4, Pentair Aquatic Ecosystems) for pH control linked to the YSI online pH probe. Each pond was monitored using a combination of the online equipment described above and daily manual grab samples for tracking biomass productivity (optical density, dry weight (DW) and ash-free dry weight (AFDW), nutrients, and optical microscopy as described in¹⁷. Weather data is recorded on site at the AzCATI testbed using a HOBO RX3000 Weather Station, equipped with air temperature, relative humidity, PAR, global light energy, rain and wind speed and direction sensors (Onset Computer Corporation, USA). PAR insolation (mol photons m⁻² d⁻¹) received by the ponds each day over the duration of the outdoor campaigns was calculated by integrating the measured PAR over the entire day.

Inoculum for the outdoor cultivation was produced in the same media as described for outdoor ponds. Once generated, the inoculum was transferred to outdoor ponds as described in McGowen et al.¹⁷. Ponds were operated with 24-h paddlewheel mixing at 7.2 rpm creating an average flow rate of 9.3 cm s⁻¹ with on-demand CO₂ sparging at 5 L min⁻¹ based on a pH set point of 7.0. Although carbon uptake efficiency is an important metric in terms of algal cultivation, the cultivation is setup to ensure CO₂ was not a limiting reagent and was supplied in excess. While the flow rate of CO₂ to the system was controlled through a pH setpoint, actual volume delivered was not quantified. The ponds were operated in a semi-continuous, drain and fill, mode of operation whereby three (3) times a week a portion of the culture volume was removed on Monday, Wednesday and Friday mornings. Fresh media was added back to the remaining culture, the culture was allowed to mix, and the cultivation was allowed to continue. For the 2019 cultivation trial, the average volume percent removed was ~ 76% for August and September, and for the 2020 cultivation trial, the average volume percent removed was 80% for June through September, dropping to 75% for October.