Isolation of marine algicidal bacteria from surface seawater and sediment samples associated with harmful algal blooms in Korea

Unique microbial module regulates the harmful algal bloom (Cochlodinium polykrikoides) and shifts the microbial community along the Southern Coast of Korea

Harmful algal blooms (HABs) of Cochlodinium (aka Margalefidinium) polykrikoides cause huge economic and ecological damages and thus are considered environmental problems. Previous studies uncovered that the formation and collapse of phytoplankton blooms could be closely related to their associated microbes although their roles in C. polykrikoides bloom have not been elucidated yet. To explore the potential interactions between C. polykrikoides and other microbes (archaea, bacteria, and phytoplankton), we collected water samples in the free-living (FL) (0.22 to 3 μm), nanoparticle-associated (NP) (3 to 20 μm), and microparticle-associated (MP) (>20 μm) fractions when C. polykrikoides blooms occurred from July to August in 2016, 2017, and 2018 in the South Sea of Korea. The microbial composition of the C. polykrikoides-associated microbial cluster (Module I) significantly differed from those of other modules associated with Alexandrium, Chaetoceros or Chattonella. Over half of the interspecies interactions in Module I occurred within the module. That is, specific microbial clusters were associated with the C. polykrikoides bloom. Structural equation modeling (SEM) further confirmed the stronger effects of Module I on C. polykrikoides blooms compared to environmental factors. Among the operational taxonomic units (OTUs) directly correlated with C. polykrikoides, Marine Group I was presumed to supply vitamin B₁₂, the essential element for C. polykrikoides growth, while the potential fish pathogens (Micrococcaceae and Piscirickettsiaceae) could contribute to the massive fish death together with C. polykrikoides itself. In addition, the zoospores of Syndiniales, a parasitoid to dinoflagellates, might be related to the sudden collapse of C. polykrikoides blooms. These microbial groups also contributed to significant alterations of the local microbial community structures. Collectively, network analysis and SEM revealed that the C. polykrikoides bloom is concomitant with distinct microbial communities, contributing to the rise and fall of the bloom, and finally determining the local microbial community structures.

Isolation of an algicidal bacterium and its effects against the harmful-algal- bloom dinoflagellate Prorocentrum donghaiense (Dinophyceae)

The relationship between algicidal bacteria and harmful-algal-bloom-forming dinoflagellates is understudied and their action modes are largely uncharacterized. In this study, an algicidal bacterium (FDHY-03) was isolated from a bloom of Prorocentrum donghaiense and the characteristics of its action against P. donghaiense was investigated at physiological, molecular, biochemical and cytological levels. 16S rDNA sequence analysis placed this strain in the genus of Alteromonas in the subclass of γ-proteobacteria. Algicidal activity was detected in the bacterial filtrate, suggesting a secreted algicidal principle from this bacterium. Strain FDHY-03 showed algicidal activity on a broad range of HAB-forming species, but the greatest effect was found on P. donghaiense, which showed 91.7% mortality in 24 h of challenge. Scanning electron microscopic analysis indicated that the megacytic growth zone of P. donghaiense cells was the major target of the algicidal action of FDHY-03. When treated with FDHY-03 culture filtrate, P. donghaiense cell wall polysaccharides decreased steadily, suggesting that the algicidal activity occurred through the digestion of cell wall polysaccharides. To verify this proposition, the expression profile of beta-glucosidase gene in FDHY-03 cultures with or without P. donghaiense cell addition was investigated using reverse-transcription quantitative PCR. The gene expression level increased in the presence of P. donghaiense cells, indicative of beta-glucosidase induction by P. donghaiense and the enzyme's role in this dinoflagellate's demise. This study has isolated a new bacterial strain with a strong algicidal capability, documented its action mode and biochemical mechanism, providing a potential source of bacterial agent to control P. donghaiense blooms.

Stakelama algicida sp. nov., novel algicidal species of the family Sphingomonadaceae isolated from seawater

We conducted a taxonomic study of two algicidal bacteria, designated strains Yeonmyeong 1-13^T and Yeonmyeong 1-11, isolated from seawater off Geoje Island in the South Sea, Republic of Korea. The two novel strains were yellow-pigmented, halotolerant, Gram-stain-negative, strictly aerobic, non-spore-forming, rod-shaped bacteria. Both strains were able to grow at 5-39 °C, pH 5.0-10.0 and 0-11 % (w/v) NaCl concentration. Based on the 16S rRNA gene sequence analysis, strains Yeonmyeong 1-13^T and Yeonmyeong 1-11 belonged to the genus Stakelama and are closely related to Stakelama pacifica JLT832^T (98.37% and 98.22 % sequence similarity, respectively). The pairwise sequence similarity between strains Yeonmyeong 1-13^T and Yeonmyeong 1-11 was observed to be 99.50 %. In both strains, the only respiratory quinone was ubiquinone-10; the major polar lipids were phosphatidylethanolamine, phosphatidylglycerol, diphosphatidylglycerol, phosphatidylcholine and sphingoglycolipid; the major fatty acids were C18 : 1ω7c, C16 : 0 and C14 : 0 2-OH. DNA G+C content values of strains Yeonmyeong 1-13^T and Yeonmyeong 1-11 were 65.1% and 64.9 mol%, respectively. The DNA-DNA relatedness between Yeonmyeong 1-13^T and S. pacifica DSM 25059^T was 28.7 %, which falls below the threshold value of 70 % for the strain to be considered as novel. The morphological, physiological, chemotaxonomic and phylogenetic analyses clearly distinguished strain Yeonmyeong 1-13^T from its closest phylogenetic neighbours. Thus, strains Yeonmyeong 1-13^T and Yeonmyeong 1-11 represent a novel species of the genus Stakelama, for which the name Stakelama algicida sp. nov. is proposed. The type strain is Yeonmyeong 1-13^T (=KEMB 9005-324^T=JCM 31498^T).