JOURNAL OF THE WORLD AQUACULTURE SOCIETY Vol. 43, No. 4 August, 2012 Quantitative and Qualitative Aspects of Bacterial Communities Associated with Cultures of Chlorella minutissima Pavlos Makridis 1,2 Institute of Aquaculture, Hellenic Center for Marine Research, P.O. Box 2214, 71003 Iraklio, Crete, Greece Tania Ferreira Universidade do Algarve, Campus de Gambelas, Faro, Portugal Fotini Kokou University of Crete, Department of Biology, Greece Costas S. Tsigenopoulos Institute of Marine Biology and Genetics, Hellenic Center for Marine Research, P.O. Box 2214, 71003 Iraklio, Crete, Greece Pascal Divanach Institute of Aquaculture, Hellenic Center for Marine Research, P.O. Box 2214, 71003 Iraklio, Crete, Greece Abstract Microbiological conditions in 200 L cultures of microalgae Chlorella minutissima in polyethylene sleeves were examined. The influence of addition of antibiotic (nitrofurantoin) was studied. Samples were taken 2, 5, 9, and 14 d after inoculation and were spread on solid Zobell medium and on thiosulfate citrate bile salts sucrose agar for the enumeration of total numbers of bacteria and numbers of presumptive Vibrio bacteria, respectively. Microalgae grew well in the cultures and high numbers of culturable bacteria (about 10 7 bacteria/mL) were present in all microalgae cultures. Addition of antibiotic in 200-L cultures of C. minutissima did not result in a significant decrease of total counts of bacteria per unit volume (P> 0.05), while the numbers of presumptive Vibrio were higher in cultures added antibiotic compared with the control treatment (P< 0.05). The numbers of presumptive Vibrio per unit volume and the percentage of fast-growing bacteria were higher in microalgae supernatant than in noncentrifuged samples. Representative colonies were taken from all samples and in total 649 bacterial strains were isolated during this study. A range of phylotypes was identified by amplification and sequencing of the 16S rDNA gene and phylogenetic trees were constructed. Several members of the Roseobacter clade were dominant among the culturable isolates. In the Mediterranean region, microalgae used in marine hatcheries for the rearing of marine fish larvae and bivalves are usually produced on-site in several types of systems such as sleeves, columns, and photobioreactors 1 Corresponding author. 2 Present address: Biology Department, University of Patras, Rio 26500, Patras, Greece. (Muller-Fuega 2004). Up-scaling from stock cultures takes place in several steps in flasks or other containers before the final produc- tion stage. As these nonaxenic microalgae cul- tures are applied in green-water technique, it is important to describe the bacterial micro- biota associated with the microalgae cultures. In marine hatcheries, microalgae cultures are usually harvested and added to the larval tanks © Copyright by the World Aquaculture Society 2012 571