Microbial environments in marine larviculture: Impacts of algal growth rates on the bacterial load in six microalgae

The impacts of varying algal growth rates on the bacterial load in the cultures were studied for six marine microalgae: Skeletonema costatum , Chaetoceros mulleri (Bacillariophyceae), Nannochloropsis oculata (Eustigmatophyceae), Isochrysis galbana, Pavlova lutheri (Haptophyceae), and Tetraselmis sp. (Prasinophyceae). Samples were taken in batch cultures during early exponential phase and in stationary phase when the growth rate was below 5% of maximum growth rate (mu (max)). Except for C. mulleri and N. oculata, the microalgae were thereafter grown semi-continuously at dilution rates of 20% and 60% of mu (max) and samples taken during a period of 10 days. Higher bacterial levels were associated with slow-growing microalgae on a per volume basis. Per algal cell, variations in the bacterial load were more related to species. Relatively high bacterial densities (5.7-16.4 CFU algal cell(-1)) with large proportions of opportunistic and haemolytic species were associated with the Bacillariophyceae. For the other species, the bacterial density was 0.2-4.3 CFU algal cell(-1). Bacterial levels were in general lower and not affected to the same extent by algal growth conditions in Tetraselmis sp. and P. lutheri. No or very low levels of presumptive Vibrio spp. were observed, except in semi-continuous cultures of P. lutheri. Cultures of Tetraselmis sp. had low levels of opportunistic bacteria at all growth conditions.