Microbial conditions and antimicrobial activity in cultures of two microalgae species, Tetraselmis chuii and Chlorella minutissima, and effect on bacterial load of enriched Artemia metanauplii

The microbial conditions and antimicrobial activity were determined in batch cultures of two microalgae species, Tetraselmis chuii and Chlorella minutissima. The number of bacteria associated with the microalgae cultures showed an exponential growth 2, 10, and 16 days after inoculation, and they were higher in T. chuii in all three sampling points compared with C. minutissima. No presumptive Vibrio strains were observed in any of the samples, as measured by the growth on TCBS agar. A total of 17 and 30 bacterial strains were isolated from C. minutissima and T. chuii, respectively. A high percentage of Gram-positive strains was detected among the bacterial strains isolated, as Gram-positive strains constituted 82% (14 / 17) and 73% (22 / 30) of the total numbers of isolates in C. minutissima and T. chuii, respectively. The isolated bacteria were screened in vitro for inhibition against two pathogenic strains, and nine of the 34 strains tested (26%) showed inhibition in vitro against either Photobacterium damselae, susp. piscicida or Vibrio anguillarum. Incubation of enriched Artemia in cultures of the two microalgae for 30 min resulted in a significant decrease of the bacterial load in Artemia (P < 0.05), and a significant decrease of the level of presumptive Vibrio in Artemia homogenates (P < 0.05). The results of this study demonstrate a simple and practical approach to decrease the microbial load and at the same time reduce the percentage of Vibrio among the bacteria associated with enriched Artemia.