From ideal to toxic: controlling the nitrogen environment for optimal Rhodomonas salina production

Abstract

The microalga Rhodomonas salina is a widely used species for rearing live feed organisms in the aquaculture feed market. A species specific media is an essential step towards enhancing productivity and decreasing production costs for microalgae cultivation. However, relevant aspects of media composition such as nitrogen source and elemental ratio have not yet been characterized for this alga. This study aimed to optimize the following three aspects of culture media: 1) optimal ratio between nitrogen and phosphorus (N:P ratio); 2) preferred source of nitrogen; and 3) tolerance of R. salina towards free ammonia. To investigate this, we conducted a series of controlled laboratory experiments in shake flasks. Our experiments revealed a 45% increase in growth rate when an N:P ratio of 15:1 was used compared to the standard ratio of 25:1. Ammonium and nitrate were equally well accepted as a nitrogen source, however, a mix of ammonium and nitrate resulted in significant growth reduction. Free ammonia did not affect growth of the alga at the tested concentrations of up to 5 mg ammonia-nitrogen L− 1. We conclude that for optimal R. salina cultivation, an N:P ratio of 15:1 is strongly preferred, as it leads to a significant increase in growth rate. Further, media with a single source of nitrogen promote faster growth over media with mixed sources, and ammonium may safely be used as a nitrogen source, since R. salina tolerates certain levels of free ammonia. Overall, this work provides insights into the optimal cultivation conditions for R. salina, allowing for more efficient and reliable production of this relevant species.