Interactions between microalgae and bacteria are often obligatory for harmful algal blooms (HABs). Here, we investigated the specific bacterial communities associated with Alexandrium tamarense and Cochlodinium polykrikoides, which cause ecological and economic damage during their blooms. To this end, the bacterial metagenome was selectively isolated from the two dinoflagellates and subsequently used for 16S rRNA analysis via the Nanopore MinION and Illumina sequencing platforms. Although the full-length 16S rRNA reads from the MinION platform showed high correlation in higher taxonomic ranks to the partial-length 16S rRNA reads from the Illumina platform, there was less correlation at the genus and species levels. MinION reads that are similar in the V3-V4 hypervariable regions with Illumina reads are classified to different taxonomies due to the extra information encoded in the full-length 16S rRNA reads. This indicates that bias arising from the short length Illumina reads can be supplemented by MinION reads. Furthermore, integrated analysis of the Illumina and MinION data showed that A. tamarense was predominantly enriched in the Roseobacter clade and C. polykrikoides was enriched in Gammaproteobacteria and Alphaproteobacteria. These results suggest that the association of different bacterial communities with A. tamarense and C. polykrikoides may be required for HABs.

Geosmin and 2-methylisoborneol (2-MIB), responsible for earthy or musty smell, are a major concern for safe drinking water supplies. This study investigated the effects of environmental factors on odorous compound production and cell growth in cyanobacterial strains. sp. FACHB-1384, a 2-MIB producer, was sensitive to low temperature (<20°C). However, geosmin producers, sp. Chusori and sp. NIER, were sensitive to high light intensity (>100 μmol/m/sec), but not to low temperature. Geosmin concentrations increased under higher nitrate concentrations, being linearly proportional to cell density. A P-limited chemostat showed that P-stress decreased the geosmin productivity and extracellular geosmin amount per cell in sp. NIER. However, only 2-MIB productivity was reduced in sp. FACHB-1374 under P-limitation. The extracellular 2-MIB amount per cell remained constant at all dilution rates. In conclusion, high light intensity and P-stress can contribute to the lower incidence of geosmin, whereas 2-MIB reduction could be attainable at a lower temperature.

Periphyton comprises a broad range of autotrophic and heterotrophic organisms that grow on submerged surfaces in aquatic environments. To investigate the ecological roles of periphyton and their symbiotic bacterial assemblages related to the control of cyanobacterial blooms, mesocosm experiments were performed in a eutrophic lake that is usually infested with harmful cyanobacterial blooms. Our results showed that periphyton, together with their symbionts, reduced Chl-a concentrations (up to 94%), improved water clarity and effectively controlled cyanobacterial blooms in the treatment mesocosm. Planktonic bacterial compositions varied greatly in the pre-bloom/bloom/post-bloom periods in both mesocosms and were mainly influenced by total dissolved nitrogen (TDN) concentrations. The phylum Cyanobacteria was the major component in the water samples until bloom peak, but it was replaced by Actinobacteria in the post-bloom period. However, periphyton niches were dominated by Alphaproteobacteria throughout the experiments, Cyanobacteria proportion being lower. Overall, the results indicated that periphyton and their unique bacterial partners could effectively compete with cyanobacteria and improve water quality. Their underlying interaction mechanism was also suggested to explain how periphyton and their symbionts can reduce cyanobacterial blooms in eutrophic water.

Ettlia sp. YC001, a highly settleable and productive microalga, was shown to be effective in removing nutrients and capturing suspended solids from eutrophic pond water. The optimum conditions for the Ettlia sp. YC001 cultivation were investigated using water from a landscape pond. The pond water was supplemented with different N : P ratios by weight, and the biomass production and nutrient removal compared in batch cultures. The maximum removal rate of N and P was with an N : P ratio of 16 : 1. Plus, the turbidity dropped to near zero within 4 days. Meanwhile, chemostat cultivation showed that the biomass productivity and nutrient removal rate increased when increasing the dilution rate, where a dilution rate of 0.9 d -1 showed the highest N and P removal rate at 32.4 mg L -1 d -1 and 1.83 mg L -1 d -1 , respectively, and highest biomass and lipid productivity at 0.432 g L -1 d -1 and 67.8 mg L -1 d -1 , respectively. The turbidity was also reduced by 98% in the chemostat cultivation. Moreover, auto-flocculation and pH were closely connected to the turbidity removal. As a result, this study identified the optimal N : P ratio for small pond water treatment using an Ettlia sp. YC001, while also establishing the optimal conditions for nutrient removal, turbidity reduction, and biomass production.