(668c) Use of Unionized Ammonia to Control Zooplankton Grazers in Cultures of Scenedesmus

Algae have been targeted as a potential biomass source for scalable biofuel and bioproducts development. Unfortunately, the presence of zooplankton grazers has remained a problem in open pond systems, reducing yields and causing repeated culture crashes. At Old Dominion University's open pond, zooplankton grazers were controlled by maintaining a high pH (>9.5 pH) and total nitrogen concentration of 30 mg/L using urea. These values correspond to unionized ammonia values previously reported as toxic to the zooplankton, specifically rotifers, but not the algae in studies designed specifically for each organism. To understand this phenomenon further, particularly in the relevant combined (algae/zooplankton) system, 2.5 L cultures of zooplankton contaminated Scenedesmus were inoculated in bench scale photobioreactors at various pH's (8.0, 10.0 and 12.0) and unionized ammonia concentrations (0.0, 0.1, and 1.0 mM). These treatments were tested in a factorial experimental design. Trials lasting five days each were broken down by each ammonia treatment. Each trial was tested across each pH plus a control with only culturing media to ensure the algae cultures were growing normally (4 treatments per trial in duplicate). The algal and zooplankton populations were monitored by cell counts (hemocytometer) and total suspended solids (TSS) measurements (1.5 µm filter). Unionized ammonia was supplied in the form of ammonium chloride and the pH was daily adjusted using hydrochloric acid and sodium hydroxide additions. After the repeated crashes of the 12.0 pH cultures across all ammonia treatments, this pH treatment was removed from the experiment. To improve control over pH, phosphate and carbonate buffer systems were employed in the second set of trials, but the unbuffered systems were still tested for comparison. This resulted in 4 pH treatments plus a control in duplicate for each trial. Total ammonia was measured in filtered (1.5 μm) liquid samples collected each day after TSS and frozen at -20 ^oC to quantify losses due to uptake by the algae and/or stripping to the atmosphere. Preliminary results indicate that 1.0 mM NH₃ concentration reduced and maintained a reduced zooplankton population for at least several days while slightly reducing the growth of the algae population before ammonia concentrations were reduced to non-inhibitory levels.