Effect of rainfall in shaping microbial community during Microcystis bloom in Nakdong River, Korea.

Various environmental factors play a role in the formation and collapse of Microcystis blooms. This study investigates the impact of heavy rainfall on cyanobacterial abundance, microbial community composition, and functional dynamics in the Nakdong River, South Korea, during typical and exceptionally rainy years. The results reveal distinct responses to rainfall variations, particularly in cyanobacterial dominance and physicochemical characteristics. In 2020, characterized by unprecedented rainfall from mid-July to August, Microcystis blooms were interrupted significantly, exhibiting lower cell densities and decreased water temperature, compared to normal bloom patterns in 2019. Moreover, microbial community composition varied, with increases in Gammaproteobacteria and notably in genera of Limnohabitans and Fluviicola. These alterations in environmental conditions and bacterial community were similar to those of the post-bloom period in late September 2019. It shows that heavy rainfall during summer leads to changes in environmental factors, consequently causing shifts in bacterial communities akin to those observed during the autumn-specific post-bloom period in typical years. These changes also accompany shifts in bacterial functions, primarily involved in the degradation of organic matter such as amino acids, fatty acids, and terpenoids, which are assumed to have been released due to the significant collapse of cyanobacteria. Our results demonstrate that heavy rainfall in early summer induces changes in the environmental factors and subsequently microbial communities and their functions, similar to those of the post-bloom period in autumn, leading to the earlier breakdown of Microcystis blooms.

Sphingobium cyanobacteriorum sp. nov., isolated from fresh water.

A novel Gram-stain-negative, yellow-pigmented, short rod-shaped bacterial strain, HBC34T, was isolated from a freshwater sample collected from Daechung Reservoir, Republic of Korea. The results of 16S rRNA gene sequence analysis indicated that HBC34T was affiliated with the genus Sphingobium and shared the highest sequence similarity to the type strains of Sphingobium vermicomposti (98.01 %), Sphingobium psychrophilum (97.87 %) and Sphingobium rhizovicinum (97.59 %). The average nucleotide identity (ANI) and digital DNA-DNA hybridisation (dDDH) values between HBC34T and species of the genus Sphingobium with validly published names were below 84.01 and 28.1 %, respectively. These values were lower than the accepted species-delineation thresholds, supporting its recognition as representing a novel species of the genus Sphingobium. The major fatty acids (>10 % of the total fatty acids) were identified as summed feature 8 (C18 : 1ω7c and/or C18 : 1ω6c) and summed feature 3 (C16 : 1ω7c and/or C16 : 1ω6c). The main polar lipids were phosphatidylglycerol, phosphatidylcholine, phosphatidylethanolamine, two phospholipids and two unidentified polar lipids. The respiratory quinone was Q-10. The genomic DNA G+C content of HBC34T was 64.04 %. The polyphasic evidence supports the classification of HBC34T as the type strain of a novel species of the genus Sphingobium, for which the name Sphingobium cyanobacteriorum sp. nov is proposed. The type strain is HBC34T (= KCTC 8002T= LMG 33140T).

Response of particle-attached and free-living bacterial communities to Microcystis blooms.

The massive proliferation of Microcystis threatens freshwater ecosystems and degrades water quality globally. Understanding the mechanisms that contribute to Microcystis growth is crucial for managing Microcystis blooms. The lifestyles of bacteria can be classified generally into two groups: particle-attached (PA; > 3 µm) and free-living (FL; 0.2-3.0 µm). However, little is known about the response of PA and FL bacteria to Microcystis blooms. Using 16S rRNA gene high-throughput sequencing, we investigated the stability, assembly process, and co-occurrence patterns of PA and FL bacterial communities during distinct bloom stages. PA bacteria were phylogenetically different from their FL counterparts. Microcystis blooms substantially influenced bacterial communities. The time decay relationship model revealed that Microcystis blooms might increase the stability of both PA and FL bacterial communities. A contrasting community assembly mechanism was observed between the PA and FL bacterial communities. Throughout Microcystis blooms, homogeneous selection was the major assembly process that impacted the PA bacterial community, whereas drift explained much of the turnover of the FL bacterial community. Both PA and FL bacterial communities could be separated into modules related to different phases of Microcystis blooms. Microcystis blooms altered the assembly process of PA and FL bacterial communities. PA bacterial community appeared to be more responsive to Microcystis blooms than FL bacteria. Decomposition of Microcystis blooms may enhance cooperation among bacteria. Our findings highlight the importance of studying bacterial lifestyles to understand their functions in regulating Microcystis blooms. KEY POINTS: • Microcystis blooms alter the assembly process of PA and FL bacterial communities • Microcystis blooms increase the stability of both PA and FL bacterial communities • PA bacteria seem to be more responsive to Microcystis blooms than FL bacteria.

Genomic Insights into Paucibacter aquatile DH15, a Cyanobactericidal Bacterium, and Comparative Genomics of the Genus Paucibacter.

Microcystis blooms threaten ecosystem function and cause substantial economic losses. Microorganism-based methods, mainly using cyanobactericidal bacteria, are considered one of the most ecologically sound methods to control Microcystis blooms. This study focused on gaining genomic insights into Paucibacter aquatile DH15 that exhibited excellent cyanobactericidal effects against Microcystis. Additionally, a pan-genome analysis of the genus Paucibacter was conducted to enhance our understanding of the ecophysiological significance of this genus. Based on phylogenomic analyses, strain DH15 was classified as a member of the species Paucibacter aquatile. The genome analysis supported that strain DH15 can effectively destroy Microcystis, possibly due to the specific genes involved in the flagellar synthesis, cell wall degradation, and the production of cyanobactericidal compounds. The pan-genome analysis revealed the diversity and adaptability of the genus Paucibacter, highlighting its potential to absorb external genetic elements. Paucibacter species were anticipated to play a vital role in the ecosystem by potentially providing essential nutrients, such as vitamins B7, B12, and heme, to auxotrophic microbial groups. Overall, our findings contribute to understanding the molecular mechanisms underlying the action of cyanobactericidal bacteria against Microcystis and shed light on the ecological significance of the genus Paucibacter.

Cultivation of Chlorella sp. HS2 using wastewater from soy sauce factory.

Incorporation of wastewater from industrial sectors into the design of microalgal biorefineries has significant potential for advancing the practical application of this emerging industry. This study tested various food industrial wastewaters to assess their suitability for microalgal cultivation. Among these wastewaters, defective soy sauce (DSS) and soy sauce wastewater (SWW) were chosen but DSS exhibited the highest nutrient content with 13,500 ppm total nitrogen and 3051 ppm total phosphorus. After diluting DSS by a factor of 50, small-scale cultivation of microalgae was conducted to optimize culture conditions. SWW exhibited optimal growth at 25-30 °C and 300-500 µE m-2 s-1, while DSS showed optimal growth at 30-35 °C. Based on a 100-mL lab-scale and 3-L outdoor cultivation with an extended cultivation period, DSS outperformed SWW, exhibiting higher final biomass productivity. Additionally, nutrient-concentrated nature of DSS is advantageous for transportation at an industrial scale, leading us to select it as the most promising feedstock for microalgal cultivation. With further optimization, DSS has the potential to serve as an effective microalgal cultivation feedstock for large-scale biomass production.

Novosphingobium cyanobacteriorum sp. nov., isolated from a eutrophic reservoir during the Microcystis bloom period.

A novel Gram-stain-negative, aerobic and rod-shaped bacterial strain, HBC54T, was isolated from periphyton during a Microcystis bloom. Based on the results of the 16S rRNA gene sequence analysis, strain HBC54T was closely related to Novosphingobium aerophilum 4Y4T (98.36 %), Novosphingobium aromaticivorans DSM 12444T (98.08 %), Novosphingobium huizhouense c7T (97.94 %), Novosphingobium percolationis c1T (97.65 %), Novosphingobium subterraneum DSM 12447T (97.58 %), Novosphingobium olei TW-4T (97.58 %) and Novosphingobium flavum UCT-28T (97.37 %). The average nucleotide identity and digital DNA-DNA hybridization values between HBC54T and its related type stains were below 78.97 and 23.7 %, which are lower than the threshold values for species delineation. The major fatty acids (>10.0 %) were identified as C14 : 0 2-OH, summed feature 3 (C16 : 1 ω7c and/or C16 : 1 ω6c) and summed feature 8 (C18 : 1 ω7c and/or C18 : 1 ω6c) and the respiratory quinone was ubiquinone Q-10. The main polar lipids detected in the strain were phosphatidylethanolamine, phosphatidylglycerol, phosphatidylcholine, diphosphatidylglycerol and three unidentified phospholipids. The genomic DNA G+C content was 64.8 mol%. Strain HBC54T is considered to represent a novel species within the genus Novosphingobium, for which the name Novosphingobium cyanobacteriorum sp. nov. is proposed. The type strain is HBC54T (=KCTC 92033T=LMG 32427T).

Comparative Genome analysis of the Genus Curvibacter and the Description of Curvibacter microcysteis sp. nov. and Curvibacter cyanobacteriorum sp. nov., Isolated from Fresh Water during the Cyanobacterial Bloom Period.

The three Gram-negative, catalase- and oxidase-positive bacterial strains RS43T, HBC28, and HBC61T, were isolated from fresh water and subjected to a polyphasic study. Comparison of 16S rRNA gene sequence initially indicated that strains RS43T, HBC28, and HBC61T were closely related to species of genus Curvibacter and shared the highest sequence similarity of 98.14%, 98.21%, and 98.76%, respectively, with Curvibacter gracilis 7-1T. Phylogenetic analysis based on genome sequences placed all strains within the genus Curvibacter. The average nucleotide identity (ANI) and digital DNA-DNA hybridization (dDDH) values between the three strains and related type strains supported their recognition as two novel genospecies in the genus Curvibacter. Comparative genomic analysis revealed that the genus possessed an open pangenome. Based on KEGG BlastKOALA analyses, Curvibacter species have the potential to metabolize benzoate, phenylacetate, catechol, and salicylate, indicating their potential use in the elimination of these compounds from the water systems. The results of polyphasic characterization indicated that strain RS43T and HBC61T represent two novel species, for which the name Curvibacter microcysteis sp. nov. (type strain RS43T =KCTC 92793T=LMG 32714T) and Curvibacter cyanobacteriorum sp. nov. (type strain HBC61T =KCTC 92794T =LMG 32713T) are proposed.

Dynamic response of bacterial communities to Microcystis blooms: A three-year study.

Although nutrient availability is widely recognized as the driving force behind Microcystis blooms, identifying the microorganisms that play a pivotal role in their formation is a challenging task. Our understanding of the contribution of bacterial communities to the development of Microcystis blooms remains incomplete, despite the fact that the relationship between Microcystis and bacterial communities has been extensively investigated. Most studies have focused on their interaction for a single year rather than for multiple years. To determine key bacteria crucial for the formation of Microcystis blooms, we collected samples from three sites in the Daechung Reservoir (Chuso, Hoenam, and Janggye) over three years (2017, 2019, and 2020). Our results indicated that Microcystis bloom-associated bacterial communities were more conserved across stations than across years. Bacterial communities could be separated into modules corresponding to the different phases of Microcystis blooms. Dolichospermum and Aphanizomenon belonged to the same module, whereas the module of Microcystis was distinct. The microbial recurrent association network (MRAN) showed that amplicon sequence variants (ASVs) directly linked to Microcystis belonged to Pseudanabaena, Microscillaceae, Sutterellaceae, Flavobacterium, Candidatus Aquiluna, Bryobacter, and DSSD61. These ASVs were also identified as key indicators of the bloom stage, indicating that they were fundamental biological elements in the development of Microcystis blooms. Overall, our study highlights that, although bacterial communities change annually, they continue to share core ASVs that may be crucial for the formation and maintenance of Microcystis blooms.

Mucilaginibacter straminoryzae sp. nov., isolated from rice straw used for growing periphyton.

A rod-shaped, non-motile, Gram-negative bacterium, strain RS28T, was isolated from rice straw used as material for periphyton growth. Phylogenetic analysis of the 16S rRNA gene sequence revealed that strain RS28T was affiliated with the genus Mucilaginibacter and had the highest sequence similarity to Mucilaginibacter ginkgonis HMF7856T (96.47 %) and Mucilaginibacter polytrichastri DSM 26907T (96.12 %). Strain RS28T was found to grow at pH 5.5-8.0, 17-40 °C and in the presence of 0-1.5 % (w/v) NaCl. Strain RS28T contained summed feature 3 (comprising C16 : 1 ω7c and/or C16 : 1 ω6c), iso-C15 : 0 and iso-C17 : 0 3-OH as the major fatty acids (> 10.0 %). The major polar lipids were phosphatidylethanolamine, two unidentified phospholipids, two unidentified aminophospholipids, three unidentified aminolipids and one unidentified lipid. The respiratory quinone was menaquinone 7. The genomic DNA G+C content was 44.7 mol%. Strain RS28T possessed six putative secondary metabolite gene clusters involved in the synthesis of resorcinol, NRPS-like, terpene, lassopeptide, T3PKS and arylpolyene. On the basis of the phenotypic, chemotaxonomic, and phylogenetic characteristics, strain RS28T represents a novel species of the genus Mucilaginibacter, for which the name Mucilaginibacter straminoryzae sp. nov. is proposed. The type strain is RS28T (=KCTC 92039T=LMG 32424T).

How do freshwater microalgae and cyanobacteria respond to antibiotics?

Antibiotic pollution is an emerging environmental challenge. Residual antibiotics from various sources, including municipal and industrial wastewater, sewage discharges, and agricultural runoff, are continuously released into freshwater environments, turning them into reservoirs that contribute to the development and spread of antibiotic resistance. Thus, it is essential to understand the impacts of antibiotic residues on aquatic organisms, especially microalgae and cyanobacteria, due to their crucial roles as primary producers in the ecosystem. This review summarizes the effects of antibiotics on major biological processes in freshwater microalgae and cyanobacteria, including photosynthesis, oxidative stress, and the metabolism of macromolecules. Their adaptive mechanisms to antibiotics exposure, such as biodegradation, bioadsorption, and bioaccumulation, are also discussed. Moreover, this review highlights the important factors affecting the antibiotic removal pathways by these organisms, which will promote the use of microalgae-based technology for the removal of antibiotics. Finally, we offer some perspectives on the opportunities for further studies and applications.

Algicidal activity of a novel bacterium, Qipengyuania sp. 3-20A1M, against harmful Margalefidinium polykrikoides: Effects of its active compound.

Margalefidinium polykrikoides causes significant economic losses in the aquaculture industry by red tide formation. Algicidal bacteria have attracted research interests as a potential bloom control approach without secondary pollution. Qipengyuania sp. 3-20A1M, isolated from surface seawater, exerted an algicidal effect on M. polykrikoides. However, it exhibited a significantly lower algicidal activity toward other microalgae. It reduced photosynthetic efficiency of M. polykrikoides and induced lipid peroxidation and cell disruption. The growth inhibition of M. polykrikoides reached 64.9 % after 24 h of co-culturing, and expression of photosynthesis-related genes was suppressed. It killed M. polykrikoides indirectly by secreting algicidal compounds. The algicide was purified and identified as pyrrole-2-carboxylic acid. After 24 h of treatment with pyrrole-2-carboxylic acid (20 µg/mL), 60.8 % of the M. polykrikoides cells were destroyed. Overall, our results demonstrated the potential utility of Qipengyuania sp. 3-20A1M and its algicidal compound in controlling M. polykrikoides blooms in the marine ecosystem.

Mucilaginibacter aquariorum sp. nov., Isolated from Fresh Water.

A Gram-stain-negative, rod-shaped bacterial strain, JC4T, was isolated from a freshwater sample and determined the taxonomic position. Initial identification based on 16S rRNA gene sequences revealed that strain JC4T is affiliated to the genus Mucilaginibacter with a sequence similarity of 97.97% to Mucilaginibacter rigui WPCB133T. The average nucleotide identity and digital DNA-DNA hybridization values between strain JC4T and Mucilaginibacter species were estimated below 80.92% and 23.9%, respectively. Strain JC4T contained summed feature 3 (C16:1 ω7c and/or C16:1 ω6c) and iso-C15:0 as predominant cellular fatty acids. The dominant polar lipids were identified as phosphatidylethanolamine, one unidentified aminophospholipid, one unidentified phospholipid, and two unidentified lipids. The respiratory quinone was MK-7. The genomic DNA G+C content of strain JC4T was determined to be 42.44%. The above polyphasic evidences support that strain JC4T represents a novel species of the genus Mucilaginibacter, for which the name Mucilaginibacter aquariorum sp. nov. is proposed. The type strain is JC4T (= KCTC 92230T = LMG 32715T).

New Multidrug Efflux Systems in a Microcystin-Degrading Bacterium Blastomonas fulva and Its Genomic Feature.

A microcystin-degrading bacterial strain, Blastomonas fulva T2, was isolated from the culture of a microalgae Microcystis. The strain B. fulva T2 is Gram-stain-negative, non-motile, aerobic, non-spore-forming and phototrophic. The cells of B. fulva T2 are able to grow in ranges of temperature from 15 to 37 °C, with a pH of 6 to 8 and a salinity of 0 to 1% NaCl. Here, we sequenced the complete genome of B. fulva T2, aiming to better understand the evolutionary biology and the function of the genus Blastomonas at the molecular level. The complete genome of B. fulva T2 contained a circular chromosome (3,977,381 bp) with 64.3% GC content and a sizable plasmid (145.829 bp) with 60.7% GC content which comprises about 3.5% of the total genetic content. A total of 3842 coding genes, including 46 tRNAs and 6 rRNAs, were predicted in the genome. The genome contains genes for glycolysis, citric acid cycle, Entner-Doudoroff pathways, photoreaction center and bacteriochlorophylla synthesis. A 7.9 K gene cluster containing mlrA, mlrB, mlrC and mlrD1,2,3,4 of microcystin-degrading enzymes was identified. Notably, eight different efflux pumps categorized into RND, ABC and MFS types have been identified in the genome of strain T2. Our findings should provide new insights of the alternative reaction pathway as well as the enzymes which mediated the degradation of microcystin by bacteria, as well as the evolution, architectures, chemical mechanisms and physiological roles of the new bacterial multidrug efflux system.

The cyanobactericidal bacterium Paucibacter aquatile DH15 caused the decline of Microcystis and aquatic microbial community succession: A mesocosm study.

Microcystis blooms pose a major threat to the quality of drinking water. Cyanobactericidal bacteria have attracted much attention in the research community as a vehicle for controlling Microcystis blooms because of their ecological safety. Nonetheless, most studies on cyanobactericidal bacteria have been conducted on a laboratory scale but have not been scaled-up as field experiments. Thus, our understanding of the microbial response to cyanobactericidal bacteria in natural ecosystems remains elusive. Herein, we applied Paucibacter aquatile DH15 to control Microcystis blooms in a 1000 L mesocosm experiment and demonstrated its potential with the following results: (1) DH15 reduced Microcystis cell density by 90.7% within two days; (2) microcystins released by Microcystis death decreased to the control level in four days; (3) during the cyanobactericidal processes, the physicochemical parameters of water quality remained safe for other aquatic organisms; and (4) the cyanobactericidal processes promoted the growth of eukaryotic microalgae, replacing cyanobacteria. The cyanobactericidal processes accelerated turnover rates, decreased stability, and altered the functional profile of the microbial community. Network analysis demonstrated that this process resulted in more complex interactions between microbes. Overall, our findings suggest that strain DH15 could be considered a promising candidate for controlling Microcystis blooms in an eco-friendly manner.

Microcystis colony formation: Extracellular polymeric substance, associated microorganisms, and its application.

Microcystis sp., amongst the most prevalent bloom-forming cyanobacteria, is typically found as a colonial form with multiple microorganisms embedded in the mucilage known as extracellular polymeric substance. The colony-forming ability of Microcystis has been thoroughly investigated, as has the connection between Microcystis and other microorganisms, which is crucial for colony development. The following are the key subjects to comprehend Microcystis bloom in depth: 1) key issues related to the Microcystis bloom, 2) features and functions of extracellular polymeric substance, as well as diversity of associated microorganisms, and 3) applications of Microcystis-microorganisms interaction including bloom control, polluted water bioremediation, and bioactive compound production. Future research possibilities and recommendations regarding Microcystis-microorganism interactions and their significance in Microcystis colony formation are also explored. More information on such interactions, as well as the mechanism of Microcystis colony formation, can bring new insights into cyanobacterial bloom regulation and a better understanding of the aquatic ecosystem.

Hymenobacter cyanobacteriorum sp. nov., isolated from a freshwater reservoir during the cyanobacterial bloom period.

A Gram-negative, red-colored, and rod-shaped bacterial strain, DH14T, was isolated from a eutrophic reservoir. The 16S rRNA gene sequence analysis showed that strain DH14T was most closely related to Hymenobacter terrigena (98.3% similarity) and Hymenobacter terrae (98.1%). The average nucleotide identity (ANI) and digital DNA-DNA hybridization (dDDH) values between strain DH14T and its related type strains were below 82.9% and 27.2%, respectively. Strain DH14T contained iso-C15:0 (32.6%), anteiso-C15:0 (14.0%), and summed feature 3 (C16:1 ω6c and/or C16:1 ω7c) (25.8%) as major cellular fatty acids. The main polar lipids were phosphatidylethanolamine, two unidentified aminophospholipids, and one unidentified lipid. The respiratory quinone was menaquinone 7 (MK-7). The genomic DNA G + C content was 62.1%. These evidences support the classification of strain DH14T as a novel species in the genus Hymenobacter, for which the name Hymenobacter cyanobacteriorum sp. nov. is proposed. The type strain is DH14T (= KCTC 92040T = LMG 32425T).

An acceleration of carotenoid production and growth of Haematococcus lacustris induced by host-microbiota network interaction.

Haematococcus lacustris is a chlamydomonadalean with high biotechnological interest owing to its capacity to produce astaxanthin, a valuable secondary carotenoid with extraordinary antioxidation properties. However, its prolonged growth has limited its utility commercially. Thus, rapid growth to attain high densities of H. lacustris cells optimally producing astaxanthin is an essential biotechnological target to facilitate profitable commercialisation. Our study focused on characterising the bacterial communities associated with the alga's phycosphere by metagenomics. Subsequently, we altered the bacterial consortia in combined co-culture with key beneficial bacteria to optimise the growth of H. lacustris. The algal biomass increased by up to 2.1-fold in co-cultures, leading to a 1.6-fold increase in the astaxanthin yield. This study attempted to significantly improve the H. lacustris growth rate and biomass yield via Next-Generation Sequencing analysis and phycosphere bacterial augmentation, highlighting the possibility to overcome the hurdles associated with astaxanthin production by H. lacustris at a commercial scale.

Panacibacter microcysteis sp. nov., isolated from a eutrophic reservoir during the Microcystis bloom period.

A Gram-stain-negative, rod-shaped bacterial strain DH6T was isolated from fresh water of the Daechung Reservoir during the Microcystis bloom period. The strain grew at pH 6.0-8.5, at temperature 17-40 °C, and at 0-1% (w/v) NaCl concentration. Comparison of 16S rRNA gene sequence indicated that strain DH6T exhibits the highest similarity with Panacibacter ginsenosidivorans Gsoil 1550T (96.6%). The average nucleotide identity (ANI), digital DNA-DNA hybridization (dDDH), and average amino acid identity (AAI) values of strain DH6T compared to its related type strains were below 74.2%, 22.3%, and 74.8%, respectively. The predominant fatty acids (> 5.0%) were identified as iso-C17:0 3-OH, iso-C13:0, iso-C15:0, C17:0 2-OH, iso-C11:0, anteiso-C13:0, and iso-C15:1 G. The polar lipid profile contained phosphatidylethanolamine, four unidentified aminolipids, and three unidentified lipids. The respiratory quinone was menaquinone 7 (MK-7). The genomic DNA G + C content was 42.6%. Collectively, strain DH6T should be classified as a novel species within the genus Panacibacter, for which the name Panacibacter microcysteis sp. nov. is proposed. The type strain is DH6T (= KCTC 82471T = LMG 32426T).

Functional role of a novel algicidal compound produced by Pseudoruegeria sp. M32A2M on the harmful algae Alexandrium catenella.

A marine phytoplankton dinoflagellate, Alexandrium sp. is known to cause worldwide harmful algal blooms, resulting in paralytic shellfish poisoning. In this study, we isolated a novel compound secreted by the marine bacterium Pseudoruegeria sp. M32A2M, and showed that it displays algicidal activity against A. catenella (group I). The molecular structure of the compound was analyzed by using 1H nuclear magnetic resonance (NMR), 13C NMR, and gas chromatography-mass spectrometry, which revealed that the compound was a diketopiperazine, cyclo[Ala-Gly]. Cyclo[Ala-Gly] induced a rapid decrease in the active chlorophyll a content and maximal quantum yield of photosystem II, leading to membrane disintegration after 24 h of its treatment. It showed the highest algicidal effect against diketopiperazines and also showed specific algicidal activities against several dinoflagellate species, but not for diatom species. In particular, cyclo[Ala-Gly] caused the transcriptional downregulation of the photosynthesis-related membrane complex in A. catenella, but not in the diatom Chaetoceros simplex. Based on structural modeling, we elucidated that cyclo[Ala-Gly] has a structure similar to that of plastoquinone, which transfers electrons by binding to the photosystem II core proteins PsbA and PsbD. This suggests a novel role for cyclo[Ala-Gly] as a potential inhibitor of photosynthesis.

Caenimonas aquaedulcis sp. nov., Isolated from Freshwater of Daechung Reservoir during Microcystis Bloom.

A Gram-stain-negative, white-coloured, and rod-shaped bacterium, strain DR4-4T, was isolated from Daechung Reservoir, Republic of Korea, during Microcystis bloom. Strain DR4-4T was most closely related to Caenimonas terrae SGM1-15T and Caenimonas koreensis EMB320T with 98.1% 16S rRNA gene sequence similarities. The average nucleotide identity (ANI) and digital DNA-DNA hybridization (dDDH) values between strain DR4-4T and closely related type strains were below 79.46% and 22.30%, respectively. The genomic DNA G+C content was 67.5%. The major cellular fatty acids (≥10% of the total) were identified as C16:0, cyclo C17:0, summed feature 3 (C16:1ω7c and/or C16:1ω6c), and summed feature 8 (C18:1ω7c and/or C18:1ω6c). Strain DR4-4T possessed phosphatidylethanolamine, diphosphatidylglycerol, and phosphatidylglycerol as the main polar lipids and Q-8 as the respiratory quinone. The polyamine profile was composed of putrescine, cadaverine, and spermidine. The results of polyphasic characterization indicated that the isolated strain DR4-4T represents a novel species within the genus Caenimonas, for which the name Caenimonas aquaedulcis sp. nov. is proposed. The type strain is DR4-4T (=KCTC 82470T =JCM 34453T).