Analysis of bacterial community and bacterial nutritional enzyme activity associated with the digestive tract of wild Chilean octopus (Octopus mimus Gould, 1852)

Information on the bacterial community associated with octopus is very scarce, unlike fish and other molluscs. This study revealed the bacterial community associated with digestive tract of wild Chilean octopus Octopus mimus using a culture‐dependent method and 16S rDNA clone library. Moreover, we analysed the bacterial nutritional enzyme activity of culturable bacteria. A culture‐dependent method showed that the composition of the culturable bacterial community was substantially different between female and male octopus. The predominant species in female octopus were Vibrionaceae and Streptococcaceae, whereas only Vibrionaceae was dominated in male octopus. Bacterial nutritional enzyme activities of culturable bacteria from male octopus were much higher than female octopus. The 16S rDNA clone library analysis showed that the bacterial community of male octopus exhibited a higher diversity than that of female octopus. The genus Mycoplasma was the predominant bacteria in the digestive tract of all octopus samples. The results obtained in this study raise the possibility that each octopus has different food consumption due to different bacterial community and nutritional enzyme activity, although Mycoplasma sp. is one of the predominant bacteria in the digestive tract. Moreover, our results are useful for the future of microbiological investigation associated with the octopus and for probiotics in the octopus aquaculture.     

Modulation of gut microbiota associated with abalone Haliotis gigantea by dietary administration of host-derived Pediococcus sp. Ab1

In the present study, we evaluated how dietary administration of host-derived Pediococcus sp. Ab1 has an effect on the abalone gut microbiota using a culture-dependent method and 16S rRNA gene library analysis. The culturable lactic acid bacteria number in the probiotic sample was 10⁵ higher than that in the non-probiotic sample, and we speculate that this significant increase was due to colonization of Ab1 into abalone gut. The result of a culture-dependent method showed that the proportion of Vibrio halioticoli clade, which is known to be a beneficial resident bacterium to abalone, was much higher in the probiotic sample than in the non-probiotic sample. 16S rRNA gene clone sequences revealed that gut microbiota in the probiotic sample was obviously diverse compared to the non-probiotic sample, probably due to improvement of the gut environment by Ab1 colonization. In addition, some beneficial bacteria–like sequences such as V. halioticoli were only found in the probiotic sample. These results suggest that the dietary administration of Ab1 to abalone gut has a great effect on modulation of not only culturable but also unculturable gut microbiota. Our results are useful for future investigations into understanding the effect of probiotics on gut microbiota.     

Isolation and use of beneficial microbiota from the digestive tract of lions‐paw scallop Nodipecten subnodosus and winged pearl oyster Pteria sterna in oyster aquaculture

We isolated microbiota from the digestive tract of Nodipecten subnodosus and Pteria sterna and determined in vitro their haemolytic activity, antagonism against Vibrio spp., bacterial hydrophobicity, production of extracellular enzymes and molecular identification. Five bacterial strains were selected: RL5 and C3 (Lactobacillus spp.) and PB1‐1, PB1‐5 and PB1‐6 (Bacillus spp.). The RL5 and C3 isolates showed antimicrobial activity against Vibrio spp. and the PB1‐1, PB1‐5 and PB1‐6 isolates showed enzymatic activity for amylase, protease, lipase and cellulose; the C3 and PB1‐5 isolates were highly hydrophobic. The selected strains of bacteria were tested in vivo as probiotics, together with a treatment of ampicillin and a control without bacteria on juvenile Kumamoto oysters Crassostrea sikamea. The strains were provided individually and as mixes of isolates. Survival, growth and biochemical composition of the juveniles were determined as in vivo indicators. Juveniles grew significantly larger and faster when treated with a specific mix of isolates (MIX‐B), compared with the control. The protein, lipid and carbohydrate concentrations were also significantly higher in oysters exposed to probiotic treatments, compared with the control and the antibiotic treatment. The selected microbiota showed probiotic proprieties for cultivating C. sikamea juveniles.     

Growth of Red Abalone,Haliotis rufescens,Fed Cold‐stored Benthic Diatoms

We measured the proximal composition of four benthic diatoms that were stored for 24 mo in the dark at low temperature (4 C by refrigeration) and examined their potential as feed for abalone, Haliotis rufescens, postlarvae. The proximal composition of the four diatoms was modified by species‐specific responses as a function of time in storage. The cultures of all stored diatoms contained low or undetectable concentrations of Vibrio‐like bacteria (<0.01 VLB/mL). As feed for abalone postlarvae, the four diatoms promoted growth under all experimental conditions. Greater shell lengths were measured on Day 14 when Navicula sp. and Navicula incerta were used as feed. Postlarvae that were fed N. incerta and Navicula sp. had higher growth rates. In contrast, lower growth rates were observed on Day 7 with fresh and stored Nitzschia thermalis as food. Survival was high in postlarvae that were fed the four stored diatoms (100%). This report demonstrates that cultures of benthic diatoms that have been stored by refrigeration for 24 mo can be used to feed abalone postlarvae and have an effect on improving growth and survival.     

Bacterial community of pond's water,sediment and in the guts of tilapia (Oreochromis niloticus) juveniles characterized by fluorescent in situ hybridization technique

Information about bacterial community structure and functioning in fish farming ponds remains scarce, mainly due to methodological difficulties in counting and identifying uncultured bacteria. The main objective of this study was to evaluate the degree of similarity between the bacterial community of the digestive tract of tilapia (Oreochromis niloticus) juveniles and that of the test pond's water and sediment, using the Fluorescent In Situ Hybridization (FISH) technique. Samples of water, sediment and gut content of 30 tilapia juveniles from a single nursery ground were collected in January 2010. Potentially probiotic and pathogenic bacteria of the species Bacillus, Lactobacillus brevis, Lactobacillus collinoides, Lactobacillus coryniformis, Lactobacillus farciminis, Vibrio and Pseudomonas fluorescens were found in different samples using specific fluorescent probes. The similarity between bacterial community environments and gastrointestinal tracts was determined using the Morisita‐Horn index. The fish guts presented higher abundances of Pseudomonas fluorescens, Bacillus, Lactobacillus brevis and Lactobacillus collinoides. The bacterial community composition of tilapia gastrointestinal tract was more similar to the water than the sediment of the pond. The results of this study showed that the FISH technique can be easily used for monitoring of probiotics and pathogen detection in aquaculture systems.     

Histological evidence of accumulation of iron in postlarvae of red abalone,Haliotis rufescens

The effect of iron on abalone postlarvae (Haliotis rufescens) was investigated in a controlled‐culturing system. Three iron concentrations (0.15, 1.5 and 15 mg L^?1 of Fe) and a control (no iron added) were used to culture H. rufescens postlarvae while being fed the diatom Navicula inserta over 10 days. Results indicate that H. rufescens postlarvae accumulate iron granules in the stomach, digestive gland and mantle, but not in the gills or other tissues. The number and diameter of iron granules in tissues increased with increasing iron concentration in the culturing environment. The iron accumulation is assumed to have been acquired in the digestive system through the iron‐enriched diatom feed and in the mantle through subcutaneous iron transfer. The lack of iron granules in the gills suggests that iron is not absorbed through the respiratory system, as is the case for many filter feeding bivalves. Exposure to the highest iron concentration (15 mg L^?1) resulted in tissue abnormalities where granules accumulated, and may have significantly affected the health of H. rufescens postlarvae. These findings provide valuable information for the regulation of appropriate iron levels within aquaculture settings and highlights the importance of monitoring iron levels within abalone larval culturing environments.     

Application of prospective probionts at early stages of Atlantic cod (Gadus morhua L.) rearing

This work aimed at validating the use of two prospective probionts (Arthrobacter sp. and Enterococcus sp.) at early stages of cod (Gadus morhua L.) rearing. Ova at late post‐fertilized stage and larvae during their first 4 weeks of life were bathed with both probionts, isolated previously from the cod‐rearing environment. This treatment was compared with groups fed rotifers supplemented with a commercial probiotic (Remus^®) and those untreated. Microbiological analyses (total viable counts, presumptive Vibrio and lactic acid bacteria) were performed in rearing systems and larval survival, growth and development were assessed. Larval development was evaluated by proteolytic activity of larval lysates and immunological analysis of important proteins: apolipoprotein A‐I, haemoglobin, C‐reactive protein, C3 and cod serum proteins. Bacterial bathing led to a significantly higher larval weight, length and culturable microbial load in larval gastrointestinal (GI) tract when compared with the control and Remus groups. Development occurred earlier in bathed larvae. However, their survival was negatively affected compared with the control group, but was significantly higher than for the Remus group. The non‐pathogenicity of both probionts was demonstrated by intraperitoneal injection of 13 g cod juveniles. The results suggest that Arthrobacter and Enterococcus probionts affected the larval GI microbiota and contributed to growth, development and digestion, either directly or indirectly.     

PCR‐DGGE analysis of the autochthonous gut microbiota of grouper Epinephelus coioides following probiotic Bacillus clausii administration

The autochthonous microbiota in the foregut, midgut and hindgut of juvenile grouper Epinephelus coioides following the dietary administration of probiotic Bacillus clausii for 60 days were assessed using polymerase chain reaction‐denaturing gradient gel electrophoresis (PCR‐DGGE). A complex and generally similar bacterial composition along the digestive tract of E. coioides was detected in the DGGE profiles, while several bacteria showed regional specialization. Similarity dendrogram revealed that the bacterial composition of the foregut was more similar to the midgut than the hindgut. Samples collected from the probiotic group and the control group showed generally similar DGGE patterns, while no significant difference in the total number of bands and Shannon index were observed between the probiotic group and the control group, suggested that probiotic B. clausii exerted no significant effect on the gut microbiota of E. coioides. However, various potentially beneficial bacteria, such as Enterococcus sp.‐like and Bacillus pumilus‐like bacterium were selectively stimulated by probiotic B. clausii, while some potential harmful species, like Staphylococcus sp.‐like and Vibrio ponticus‐like bacterium were depressed. These indicated that the gut microbiota was modified to some degree by probiotic B. clausii. Sequences analysis showed that the autochthonous gut bacteria of E. coioides could be classified into four groups, i.e. Proteobacteria, Firmicutes, Actinobacteria and unclassified bacteria.     

The use of probiotics during the nursery rearing of the pink shrimp Farfantepenaeus brasiliensis (Latreille, 1817) in a zero exchange system

The present work evaluated the use of probiotics during the nursery rearing of the pink shrimp Farfantepenaeus brasiliensis, in a zero exchange aerobic heterotrophic culture system during 30 days. Three replicate tanks were randomly assigned to the following treatments: (1) Bacillus spp. mixture (Sanolife Pro‐‐W^®), (2) Bacillus sp., Enterococcus sp., Lactobacillus spp. mixture (Biomin Start‐grow^®), (3) Bacillus cereus var. toyoi and (4) control treatment (without probiotic addition). Bacteriological analysis monitored the abundance of presumptive Vibrio spp. in the water of experimental tanks. For the immunological analysis, shrimp haemolymph was collected to determine the granular haemocyte count and total protein concentration. Results showed that mean final weight and specific growth rate of shrimp in the probiotic treatments were significantly higher. Furthermore, shrimp reared in the probiotic treatments showed higher levels of total protein and granular haemocyte. The bacteriological analysis showed that the concentration of Vibrio spp. measured in probiotic treatment tanks was lower than that recorded in the control tanks.     

Continuous and pulse‐feeding application of multispecies probiotic bacteria in whiteleg shrimp,Litopenaeus vannamei

Probiotic bacteria are known to support the gut health of shrimp and thereby improve performance and production efficiency. Among other factors, the timing of probiotic application is of importance. Thus, this experiment (12‐week feeding trial followed by intramuscular Vibrio challenge) aimed to compare the effects of a multispecies probiotic feed supplement (AquaStar® Growout, 3 g/kg feed) in a continuous application with three different alternating application protocols on growth performance and immune parameters in whiteleg shrimp, Litopenaeus vannamei. Juvenile shrimp were stocked in a recirculating aquaculture system at a density of 15 shrimp/100 L and were fed an appropriate commercial diet throughout the whole trial. Tanks were randomly assigned to the treatments, and shrimp were fed one of the following five experimental treatments: T1: no probiotics (control), T2: probiotics continuously, T3: alternating 1 week probiotics, 1 week control, T4: alternating 2 weeks probiotics, 2 weeks control, or T5: alternating 2 weeks probiotics, 1 week control. While any AquaStar® Growout application significantly improved growth performance, only continuous application delayed mortality after the Vibrio challenge. Results suggest that the beneficial effects were most pronounced when AquaStar® Growout was fed either continuously or according to the T5 treatment.     

Characterization of two bacteriophages for specific treatment of biofilm formed by a Vibrio sp. isolated from an abalone farm

Vibrio harveyi and related vibrios are potential pathogens causing luminous vibriosis in marine aquaculture systems. In this study, two lytic phages P4A and P4F isolated using Vibrio strains B4A and B4F as indicator bacteria, respectively, were isolated from seawater of an abalone farm. Vibrio strain B4F belongs to the Harveyi clade of the genus Vibrio and was found to cause mortality of abalones in laboratory microcosms. Both phages were able to lyse Vibrio strain B4F. Electron microscopy revealed that phage P4A had an icosahedral head while P4F possessed an elongated hexagonal head. Both phages belong to the family Siphoviridae with long non‐contractile tails. Restriction endonuclease analysis indicated that both phages were double‐stranded DNA viruses and the genome sizes of P4A and P4F were estimated to be about 49 and 44 kb respectively. One‐step growth curves revealed that these two phages exhibited distinct latent periods, exponential periods and burst sizes by infecting the same Vibrio strain B4F. Both phages were able to significantly reduce Vibrio population density in biofilm formed by Vibrio strain B4F on the surface of polyethylene film. It is suggested that these two phages may be promising candidates as biocontrol agents of infections caused by Vibrio strains belonging to the Haveyi clade in marine aquaculture systems.     

Autochthonous probiotic Lactobacillus sp. in the diet of bullfrog tadpoles Lithobates catesbeianus improves weight gain,feed conversion and gut microbiota

Dietary supplementation with probiotics in animal production is an alternative to antibiotics. In frog culture, studies involving native strains of probiotic bacteria and their effects on the performance and intestinal histology of farmed animals are scarce. Therefore, this study aimed to evaluate a diet supplemented with Lactobacillus sp. in tadpoles of Lithobates catesbeianus. This randomized test was performed with two dietary treatments: non‐supplemented control diet and diet supplemented with Lactobacillus sp., with nine replications. The growth performance of Lactobacillus sp., including its bacterial enzymatic activity and stability in feed, as well as colonization and histology of the intestinal tract, was evaluated after 42 days of experimentation. Animals fed with a supplemented diet showed higher weight gain and concentration of lactic acid bacteria in the gut and lower feed conversion. No significant difference was observed in survival, total heterotrophic bacterial count or histological change in the gut between the two treatments. The Lactobacillus sp. strain was able to colonize the intestinal tract and feed and remain at a high concentration of 10⁷ and 10⁶ CFU g^?1, respectively. It produced several enzymes, which might have contributed to the greater weight gain and lower feed conversion in the supplemented animals, thus demonstrating its probiotic potential for use as a dietary supplement in bullfrog tadpoles.     

Probiotic bacteria effects on growth and bacterial composition of black tiger shrimp (Penaeus monodon)

The effects of Bacillus probiotic forms on growth performance and microbiota were examined in black tiger shrimp (Penaeus monodon). Shrimp were fed for 120 days with one of three diets; a control diet alone or supplemented with either probiotics consisting of live‐sprayed Bacillus (LS) or freeze‐dried Bacillus (FD). Culturable bacteria counts in the hepatopancreas, intestine and culture water of the three treatments were relatively constant during the experiment. Bacillus numbers in the hepatopancreas and intestines of probiotic‐treated groups increased significantly (P < 0.05) after 30 treatment days and remained relatively constant afterwards while no appreciable changes occurred in the control. Shrimp supplemented with LS and FD contained significantly lower concentrations of Vibrio (P < 0.05) in the hepatopancreas, intestine and culture water, compared to those in the control group. Shrimp treated with LS and FD probiotic bacteria showed a significant improvement (P < 0.05) in specific growth rate (SGR) and feed efficiency (FE), compared to those of control group. Probiotic bacteria have the potential to enhance shrimp culture by reducing Vibrio and improving growth performance.     

健康和患病凡纳滨对虾幼虾消化道菌群结构的比较

凡纳滨对虾养殖过程中,早期阶段是病害易感阶段,而消化道菌群结构与对虾健康关系密切。因此,探讨幼虾的消化道菌群尤其是弧菌类细菌与对虾发病的关系对病害有效防治具有重要意义。本实验采用Illumina测序研究了凡纳滨对虾健康和患病幼虾的消化道细菌群落结构,并基于纯培养和16S r DNA,rec A和pyr H基因序列比对,分析了幼虾消化道中弧菌的主要种类组成。结果发现,健康幼虾消化道中α-变形菌纲和厚壁菌门丰度较高,而患病幼虾中γ-变形菌纲、拟杆菌门、放线菌门和β-变形菌纲较高,其中放线菌门丰度差异显著。基于科水平的响应比分析,发现患病幼虾消化道中动性球菌科和噬菌弧菌科的丰度显著降低,而弧菌科的丰度显著升高。相似度分析发现,驱动群落变异的OTU主要来源于弧菌属、海洋杆状菌属、冷杆菌属、假交替单胞菌属以及未分类至属的红杆菌科和微杆菌科。健康幼虾消化道弧菌组成以锡那罗亚州弧菌为主,而患病幼虾消化道弧菌组成以坎氏弧菌为主。尽管健康和患病幼虾消化道内细菌群落结构整体差异不显著,但一些重要类群丰度变化显著,其特征与已知的生态功能一致。     

A study on bacteria associated with the intestinal tract of farmed yellow seahorse,Hippocampus kuda (Bleeker, 1852): characterization and extracellular enzymes

The microenvironment of bacteria associated with the gastrointestinal tract of an animal influences the host in many ways, including the metabolism of several nutrients. Isolation, molecular characterization and enzymatic activities of culturable bacteria associated with the gastrointestinal tract of hatchery‐reared oceanic yellow seahorse, Hippocampus kuda, were investigated in the present study. 16S rRNA sequencing of bacterial isolates yielded 10 different bacterial genera belonging to two major groups: proteobacteria (62.9%) and firmicutes (37.03%). The predominant microbial flora belonged to the genera Vibrio spp., Enterovibrio sp. and Bacillus spp. Burkholderia cenocepacia, hitherto unreported from the gastrointestinal tract of marine fish, was retrieved from the seahorse gut, albeit in small numbers. Enzymatic studies indicated that the bacterial isolates possess the ability to degrade lipids, cellulose, xylan, starch and proteins. Lipase activity was exhibited by a majority of the isolates (70.4%), indicating that the bacterial flora associated with the intestinal tract of seahorses is capable of digesting foods rich in lipids. The possible implications for the role of these bacteria in the digestion and physiological processes are discussed. To our knowledge, this is the first time that the characterization and enzymatic activities of bacteria associated with the intestinal tract of farmed seahorses have been reported.     

Pathogenic threats and probiotic use in larviculture of the scallop,Pecten maximus

During a series of experiments, bacteriological elements in scallop larval rearing were investigated: larvae susceptibility to pathogens as a function of their age, and the use of probiotic bacteria during larviculture. Younger larvae (d5 PF) were highly more susceptible to pathogenic‐challenge than their older siblings, which were challenged at an older age (d15 PF). A challenge with 10⁴ CFU mL^?1 of V. pectenicida killed 100% of d5 PF larvae 7 days following challenge, yet killed only 9% of d15 PF larvae 9 days following challenge. Use of the probiotics Phaeobacter gallaeciensis, Alteromonas macleodii 0444 and Neptunomonas sp. 0536, provided for larger larvae, a high yield of competent larvae and, perhaps more importantly, protection against pathogen‐challenge similar to levels achieved from antibiotic use. When challenged with V. pectenicida, d29 survivals were 20.3%, 85.1% and 75.0% respectively for control (no probiotic), antibiotic treated, and ‘probiotic mix’ administered larvae. Use of potential probiotic Pseudoalteromonas sp. D41 appeared to hinder scallop larvae. Future use of probiotics in scallop larval rearing would benefit from combined use of P. gallaeciensis, A. macleodii 0444 and Neptunomonas sp. 0536.     

Temporal fluctuation in the abundance of alginate‐degrading bacteria in the gut of abalone Haliotis gigantea over 1 year

In this study, we identified and enumerated alginate‐degrading bacteria in the gut of abalone over 1‐year period. From a total of 360 colonies growing on agar medium enriched with alginate, 251 isolates (70%) had the ability to degrade alginate. In addition, a high number of viable alginate‐degrading bacteria were detected throughout the survey period. Alginate‐degrading bacteria were more abundant in the cold season relative to the summer season (10⁷ vs. 10⁴ CFU g^?1, respectively). Strong positive correlation was also observed between the number of alginate‐degrading bacteria and feed intake (R = 0.854; P < 0.01). The identified alginate‐degrading bacteria comprised of 35 species grouped into 11 genera including Algibacter, Formosa, Polarybacter, Tamlana, Tenacibaculum (CFB group), Roseobacter, Ruegeria, Silicibacter (α‐proteobacteria), Agarivorans, Shewanella and Vibrio (γ‐proteobacteria) respectively. More than 80% of the isolated alginate‐degrading bacteria belonged to the genus Vibrio, showing high homology to Vibrio cyclotorophicus, Vibrio splendidus, Vibrio halioticoli and Vibrio neonatus. Based on the results, it was suggested that algal‐polysaccharide (alginate) degrading bacteria (mainly Vibrio) commonly exist in the gut of abalone and may play an important role in the degradation and digestion of the host's feed.     

添加外源益生菌对大菱鲆育苗生物饵料菌群结构的影响

为评价在大菱鲆育苗生产中添加外源益生菌对生物饵料轮虫和卤虫微生物菌群结构的影响,运用基于illumina HiSeq平台的高通量测序技术对添加益生菌和按照生产流程正常强化的轮虫、卤虫进行了菌群结构的分析和对比。添加外源益生菌的实验组轮虫和卤虫中的菌群物种多样性均明显高于对照组。在轮虫强化过程中,不同时期的对照组样品中菌群结构差异较明显,优势细菌种类变化较大。而添加外源益生菌后的各时期实验组轮虫菌群结构很相似,优势菌群的种类更为丰富,Lactococcus sp.、Pseudoalteromonas sp.和Alteromonas sp.等一直是各实验组中的优势细菌。在卤虫强化过程中,各对照组样品的菌群结构高度相似,优势细菌Cobetia sp.的相对丰度高达54%~65.2%。而在添加益生菌后,各实验组中的菌群结构仍高度相似,但Cobetia sp.的比例下降至4.3%~25.3%,最优势的细菌为Pseudoalteromonas sp.和Alteromonas sp.等几种,菌群结构中的物种均匀度更好。研究表明,在轮虫和卤虫强化过程中添加外源益生菌,能够改变生物饵料的菌群结构,使生物饵料中的细菌种类均匀度更好,并使菌群结构趋于稳定。     

Colonization and probiotic effects of lactic acid bacteria in the gut of the abalone Haliotis gigantea

Strains of lactic acid bacteria (LAB) were isolated from several different sources and evaluated in vitro for potential probiotic effects in abalones. Two isolates (Lactobacillus sp. strain a3 and Enterococcus sp. strain s6) were highly resistant to bile salt and/or gastric juice and inhibited the growth of three abalone pathogens (Listonella anguillarum, Vibrio harveyi, and V. carchariae). Each of the LAB isolates was used to supplement diet of the abalone Haliotis gigantea for a period of 3 weeks. One group of animals received Lactobacillus sp. strain a3 added to commercial dry feed, one group received Enterococcus sp. strain s6 added to the feed, and a control group received only standard commercial feed. Culturable LAB counts of gut homogenates indicate the a3 colonized in the gut of abalones. Digestive enzyme activities and the concentrations of a number of volatile short-chain fatty acids (VSCFA) were elevated in the gut of abalones receiving feed supplemented with the two LAB strains. These results indicate that dietary supplementation can enable LAB colonization or persistence in the gut of abalone species and can potentially enhance probiotic effects.     

Quantitative and Qualitative Aspects of Bacterial Communities Associated with Cultures of Chlorella minutissima

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 10⁷ 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.