Contaminations in mass cultivation of cyanobacteria: Highly resilient Colpoda steinii leads to rapid crash of Synechocystis sp. cultures and is inhibited by partially anoxic conditions

Contaminations in microalgae cultures are one of the major drawbacks when cultivating in an industrial scale. What is often experienced as culture crash has been rarely investigated in detail. Here, we show that the ciliated protozoa Colpoda steinii is capable to clear a dense culture of the cyanobacterium Synechocystis sp. within 2-3 days. In a 200 L tubular photobioreactor, this ciliate frequently appeared and led to great losses. The ciliate was isolated and characterized in the laboratory. Its grazing rate exceeded the growth rate of three tested Synechocystis strains (Synechocystis PCC6803, Synechocystis CCALA192 and Synechocystis minuscula SAG 258.80) by far. Due to its devastating effect on Synechocystis sp., its ability to form cysts and its resilience, Colpoda steinii was experienced as serious threat for large scale cultures of Synechocystis sp. The most common means against contaminations are high salinities and high pH values. Though, cultivating with salinities up to 20‰ and pH values up to 10 did not inhibit Colpoda steinii. Other strategies like high ammonia concentration, carbon dioxide (CO2) asphyxiation and anoxic conditions during the dark period were tested against the ciliate as well. While high ammonia concentration and CO2 asphyxiation did no inhibit Colpoda steinii, partially anoxic conditions inhibited this ciliate effectively and appears to be a promising cultivation method for Synechocystis sp. under non-sterile conditions.