Freshwater environments are rich ecosystems that support diverse microbial communities, including members of the phylum Actinomycetota critical for nutrient cycling, organic matter decomposition, and water quality maintenance. Actinomycetota known to produce numerous bioactive secondary metabolites are valuable in biotechnology, medicine, and agriculture. Despite their significance, the diversity and distribution of Actinomycetota in freshwater habitats, especially in the Republic of Korea, are underexplored. This study aimed to report the isolation and characterization of 22 previously unrecorded bacterial species of Actinomycetota from various freshwater environments in Korea. Using standard dilution plating techniques on six different culture media, 22 bacterial strains were isolated, incubated, and characterized based on colony and cellular morphologies, Gram staining, and biochemical properties. Genomic DNA was extracted and the 16S rRNA gene was sequenced to determine species identity using the EzBioCloud service with a cutoff of 98.7% sequence similarity for classification as unreported species. These strains were phylogenetically diverse, belonging to two classes, ten orders, and eighteen genera. This study enhances our understanding of bacterial diversity in freshwater ecosystems and underscores the importance of exploring microbial diversity in underexplored habitats, potentially leading to discovery of novel bioactive compounds. Findings of this study contribute valuable insights into ecological roles and biotechnological potential of Actinomycetota in freshwater environments.

Diatoms, which are reported to have over 18,000 species worldwide and approximately 2,400 species in Korea, can be found in various environments including freshwater, seawater, and wetlands. They are particularly valuable for understanding global environmental changes throughout history due to their ability to maintain their shape for extended periods of time. Instead of collecting floating diatoms using nets, low-layer substrates such as gravel and leaves, as well as sedimentary surface layers, were gathered in order to identify attached diatom species. This is because attached diatoms demonstrate higher species diversity compared to floating diatoms. In this study, seven previously unrecorded diatoms were discovered in various domestic freshwater environments. Two species were found in reservoirs (Eunotia yanomami, Gomphonella pseudookunoi), two in parasitic cones (Eunotia carverenensis, Luticola minor), two in rivers (Cavinula maculata and Prestauroneis integra), and one (Surirella brebissonii var. kuetzingii) in a lagoon. The shapes, structures, and morphological characteristics of each diatom were identified using electron microscopy.

Zooplankton biomass is essential for understanding the quantitative structure of lake food webs and for the functional assessment of biotic interactions. In this study, we aimed to propose a biomass (dry weight) estimation method using the body length of cyclopoid copepods. These copepods play an important role as omnivores in lake zooplankton communities and contribute significantly to biomass. We validated several previously proposed estimation equations against direct measurements and compared the suitability of prosomal length versus total length of copepods to suggest a more appropriate estimation equation. After comparing the regression analysis results of various candidate equations with the actual values measured on a microbalance-using the coefficient of variation, mean absolute error, and coefficient of determination-it was determined that the Total Length-DW exponential regression equation [W=0.7775×e2.0183L; W (μg), L (mm)] could be used to calculate biomass with higher accuracy. However, considering practical issues such as the morphological similarity between species and genera of copepods and the limitations of classifying copepodid stages, we derived a general regression equation for the pooled copepod community rather than a species-specific regression equation.

In ecosystems within limited resources, interspecific competition is inevitable, often leading to the competitive exclusion of inferior species. This study aims to provide foundational information for the conservation and restoration management of Microphysogobio rapidus by evaluating species’ ecological response to biological factors within its habitat. To understand this relationship, we collected food web organisms from site where M. rapidus coexist with Microphysogobio yaluensis, a specie ecologically similar to M. rapidus, and evaluated the trophic levels (TL), isotopic niche space (INS), and the overlap of INS among fishes within the habitat using stable isotope analysis. Our analysis revealed that the M. rapidus exhibited a higher TL than M. yaluensis, with TL of 2.6 and 2.4, respectively. M. yaluensis exhibited a broad INS, significantly influencing the feeding characteristics of most fish. Conversely, M. rapidus showed a narrow INS and asymmetric feeding relationships with other species, in habitats with high competition levels. This feeding characteristics of M. rapidus indicate that the increase in competitors sharing the similar resources lead to a decrease in available resources and, consequently, is expected to result in a decrease in their density.

Captive breeding and reintroduction are crucial strategies for conserving endangered species populations. However, fish raised in predator-free environments, show a lack of recognition of predationrelated stimuli such as chemical and visual signals. It is critical to recognize chemical signals from injured conspecifics, also known as alarm signals, and the order or shape of predators to indicate the spread of predation risk in the habitat. We conducted a laboratory experiment to determine and adjust the optimal exposure period to induce appropriate anti-predator behavior response to different types of stimuli (Chemical, Visual and Chemical+Visual) for the endangered species Microphysogobio rapidus. Our results demonstrate that predator avoidance behavior varies depending on the types of stimuli and the duration of predation risk exposure. First, the results showed captive-breed M. rapidus show lack of response against conspecific alarm signal (Chemical cue) before the predation risk exposure period and tend to increase response over predation risk exposure time. Second, response to predator (visual cue) tend to peak at 48 hours cumulative exposure, but show dramatic decrease after 72 hours cumulative exposure. Finally, response to the mixed cue (Chemical+visual) tend to peak prior to the predation risk exposure period and show reduced response during subsequent exposure periods. This experiment confirms the lack of responsiveness to conspecific alarm signals in captive-bred M. rapidus and the need for an optimal nature behavior enhancement program prior to release of endangered species. Furthermore, responsiveness to predator visual signal peak at 48 hours cumulative exposure, suggest an optimal predation risk exposure period of up to 48 hours. Key words: predator cognition, captive breeding, chemical signal, visual signal, endangered

To determine the cause of the population decline in Gobiobotia naktongensis, substrate preference and burying behaviour were investigated in this study. In general, the species was shown to prefer a substrate size of 1 mm or less, depending on the flow. In addition, the burying depth varied according to the size of the fish and increased with a decrease in water temperature. Our findings showed that the main cause of the population reduction was the physical changes in the substrate structure due to the dams or barrages construction. Notably, the accumulation of silt and mud in the substrate upon the formation of an upstream lentic water region for structural construction and bed armouring caused by scouring and reduced downstream inflow of fine sediment were deterministic in the fish habitat changes, causing problems in burying. As sand substrate structure is critical for the survival and inhabitation of psammophilous species, efficient strategies should be developed with proper habitat management to reduce the anthropogenic damage

큰징거미새우(Macrobrachium rosenbergii)는 양식 시설 내에서 체색 불량 및 갑각 약화와 같은 문제로 경제적 손실을 겪고 있다. 이 종은 동물성 원료 기반의 사료로 양식되지만, 야생에서는 식물체 비중이 높은 detritus를 주로 섭취한다. 새우가 야생에서 섭취한 식물체는 기본 영양소 뿐만 아니라 체색의 재료인 카로티노이드의 공급원이기도 하다. 개나리(Forsythia koreana)는 우리나라에 널리 분포하는 꽃나무로 잎에는 황색 당근에 버금가는 양의 카로티노이드가 함유 되어 있다. 본 연구에서는 큰징거미새우에게 개나리 잎을 공급하여 체색 및 건강도에 미치는 효과를 조사하였다. 실험사료는 「배합사료 100%(대조구), CON」, 「배합사료 80%+개나리 잎 분말 20%, FP」, 「배합사료 80%+가공하지 않은 개나리 잎 20%, FL」의 세 가지였으며, 각 조건 의 사료를 평균 체중 1.1 ± 0.2 g의 어린 새우들에게 10주간 공급하였다. 실험 결과, 체색의 경 우, CON의 새우들은 일관적으로 투명한 상아색을 띠었으나, FP와 FL 새우의 경우 사육일의 경과에 따라 푸른색을 거쳐 암갈색으로 변화하였다. 생존과 성장은 CON과 FP 또는 FL 사이에 유의한 차이가 없었다. 간췌장을 조직학적으로 비교한 결과, hepatopancreatic tubule의 구성 세포 중 B cell의 vacuole 크기가 CON에 비해 FP과 FL에서 훨씬 컸다. B cell의 vacuole은 영 양소의 흡수 및 소화의 역할을 하며, 개나리 잎의 공급이 큰징거미새우의 건강에 긍정적으로 작용했을 가능성을 시사한다. 이상의 결과는 큰징거미새우의 양식에 개나리 잎을 활용하면 성장 저해 없이 체색의 개선 및 건강도의 향상을 기대할 수 있음을 보여준다.

남강댐 하류역에 위치한 사천만, 진주만, 강진만에서 가화천 방류에 따른 담수 배제의 분배 특성을 평가하기 위하여 입자추적 수치실험을 실험하였다. 가화천을 통해 3개의 방류조건(무방류, 강우시 방류, 홍수시 방류)에서 입자 1000개를 투여하여 노량수도, 대방수 도, 창선해협으로 빠져나가는 입자의 수를 비교하였다. 가화천을 통한 방류량이 늘어날수록 노량수도로의 입자 분배율이 증가하고, 대방 수도로의 분배율이 감소하는 것을 확인할 수 있었다. 즉, 평소에는 가화천 하류에 위치한 물질의 약 95%가 대방수도를 통해 빠져나가다 가, 강우에 의해 남강댐의 방류량이 증가하면서 노량수도로의 분배율이 증가하며, 홍수시에는 노량수도로의 입자 분배율이 45.5% 까지 증가하는 것을 확인할 수 있었다.

Freshwater jellyfish, a type of jellyfish exclusively found in freshwater, has a limited number of species but is found globally. However, their ecology and causes of occurrence are largely unknown. Therefore, understanding the distribution of polyps, which produce the larvae of freshwater jellyfish, can provide important data for comprehending their ecology. This study aims to explore the COI gene of freshwater jellyfish using environmental DNA from the microbial film in the Miho River system. Among the 12 survey points in the Miho River watershed, genetic material of freshwater jellyfish was detected in 8 points, mainly located upstream near reservoirs. These genetic materials were identified as genes of the well-known freshwater jellyfish species, Craspedacusta sowerbii. Notably, the C. sowerbii genes found in the Miho River watershed survey points were closely related to a species previously discovered in Italy. Consequently, utilizing environmental DNA to explore the genetic traces of freshwater jellyfish enables rapid screening of areas with a high likelihood of freshwater jellyfish occurrence. This approach is deemed to provide crucial information for understanding the distribution and ecology of freshwater jellyfish in Korea.

The aquarium pet trade is a source of potentially invasive crayfish species, which can be subsequently intentionally or unintentionally introduced into new environments. There were 34 species of freshwater crayfish imported into Korea for ornamental purposes. Starting with 1 species in 2008, it shows a trend of continuous increase every year with the maximum of 25 species in 2020. The number of freshwater crayfish imported into Korea for ornamental purposes was 1,172,159, with an annual average of 78,144 being imported. The population also recorded a record high in 2017 with a 38% increase in population imports compared to the previous year. Among the 34 pet crayfish imported into Korea, four species classified as high-risk and managed in the US and Europe were American crayfish (Procambarus clarkii), Cherax quadricarinatus, Cherax cainii, and Cherax destructor. In addition to American crayfish (P. clarkii), 3 types of high-risk invasive crayfish are designated as legally managed species by conducting an ecological risk assessment, raising awareness among importers, retailers and consumers through awareness-raising education on freshwater crayfish, and measures for route management such as species identification and improvement of labeling methods are needed.

Microphysogobio rapidus is designated as endangered species class I by Ministry of Environment, and its distribution and population have been gradually declining, and it is now limited to the Nam River and some tributary streams of the Nakdong River Watershed. For the restoration of this highly endangered species, it is important to identify the causes of the decline and establish appropriate restoration plans. However, due to lack of basic data and ecological research, most steps are stagnant. Therefore, in this study, we identified the differences in the physical, biological, and sociological habitats between current and past distributed sites through field surveys and literature reviews. As a result of the field survey, there were differences in conductivity between the current and past distributed sites, and fish communities were also showed differences. The literature data also showed that the physico-chemical values of the past distributed sites were generally unfavorable, which generated negative consequences on biological factors. In particular, the effects of urbanization were found to be a major factor affecting the habitat of M. rapidus. Habitat stabilization is crucial for the recovery of this endangered species. However, in the past distributed sites, disturbances such as stream development and weir construction have altered streams physico-chemically and result in changes of M. rapidus. Therefore, a comprehensive plan that considers both stream connectivity and water quality is needed to manage and restore the habitat of M. rapidus.

Hymenobacter 속(genus)은 Bacteroidota 문(phylum), Hymenobacteraceae 과(family)의 대 표 속(type genus)이다. 이 속에 속하는 세균들은 붉은색 색소를 함유하는 그람 음성 간균으로서, 자연 계의 다양한 환경에서 분리되고 있다. 본 연구에서 붉은색 색소를 함유하는 그람 음성 간균이 경남 창 원시 소재 창원대학교 교내의 연못에서 분리되었고, 이 세균은 균주 B2로 명명되었다. 균주 B2를 계통 분석 및 생화학적으로 분석한 결과, Hymenobacter 속에 속하는 것으로 밝혀졌다. 이 세균의 16S rRNA 유전자 염기서열을 genbank의 BLAST로 분석해 본 결과, 다른 어떠한 세균과도 16S rRNA 유전 자 염기서열의 상동성이 새로운 미생물로 인정되는 기준인 98.7%보다 낮은 것으로 나타났다. 균주 B2 의 지방산을 분석해 본 결과, 주된 지방산은 summed feature 3(C16:1 ω7c and/or C16:1 ω6c, 22.8%), iso-C15:0(16.2%), anteiso-C15:0(12.9%), C16:1ω5c(12.4%) 및 summed feature 4 (iso-C17:1 I/anteiso-C17:1)(9.5%)인 것으로 밝혀졌는데, 결과적으로 균주 B2의 지방산 함량은 다른 Hymenobacter 종들의 지방산 함량과 뚜렷한 차이가 있는 것을 알 수 있었다. 이 세균의 16S rRNA 유전자 염기서열 은 genbank에 accession number OQ318247로 등록되었다.

The medium-large cladoceran species Simocephalus spp. predominantly occur in habitats with developed aquatic vegetation. Accordingly, due to Simocephalus’ high contribution to zooplankton community biomass in the lake’s littoral zone and wetland habitats, estimating their biomass is important to understand the matter cycling based on biological interactions within the aquatic food web. In this study, we reviewed the length-weight regression equations used previously to estimate Simocephalus biomass, directly measured S. serrulatus’ body specification (length, width and area) and their biomass (dry weight) using instruments such as a microscopic digital camera and a microscale, and performed regression analysis between each other. When S. serrulatus biomass was estimated using the equation (Kawabata and Urabe, 1998) presented in 『Biomonitoring Survey and Assessment Manual』, Korea, errors between estimates and measures were relatively large compared to the S. serrulatus species-specific biomass estimate equation developed by Lemke and Benke (2003). In addition, both equations showed not only increasing trends in error (estimate-measure) with increasing S. serrulatus’ body length, but also in error variance among similar-sized individuals. The results of regression analysis with dry weight by body specifications indicated that the most appropriate equation for estimating the biomass of S. serrulatus was derived from the width-dry weight exponential regression equation (R2=0.9555). The review and development study of such species-specific biomass estimation equations for zooplankton can be used as a tool to understand their role and function in aquatic ecosystem food webs.

Cyanobacterial resting cells, such as akinetes, are important seed cells for cyanobacteria’s early development and bloom. Due to their importance, various methods have been attempted to isolate resting cells present in the sediment. Ludox is a solution mainly used for cell separation in marine sediments, but finding an accurate method for use in freshwater is difficult. This study compared the two most commonly used Ludox methods (direct sediment treatment and sediment distilled water suspension treatment). Furthermore, we proposed a highly efficient method for isolating cyanobacterial resting cells and eDNA amplification from freshwater sediments. Most of the resting cells found in the sediment were akinete to the Nostocale and were similar to those of Dolichospermum, Cylindrospermum, and Aphanizomenon. Twenty times more akinetes were found in the conical tube column using the sediment that had no treatment than in the sample treated by suspending the sediment in distilled water. Akinete separated through Ludox were mainly spread over the upper and lower layers in the column rather than concentrated at a specific depth in the column layer. The mibC, Geo, and 16S rDNA genes were successfully amplified using the sediment directly in the sample. However, the amplification products of all genes were not found in the sample in which the sediment was suspended in distilled water. Therefore, 5 g to 10 g of sediment is used without pretreatment when isolating cyanobacterial resting cells from freshwater sediment. Cell isolation and gene amplification efficiency are high when four times the volume of Ludox is added. The Ludox treatment method presented in this study isolates cyanobacterial resting cells in freshwater sediment, and the same efficiency may not appear in other biotas. Therefore, to apply Ludox to the separation of other biotas, it is necessary to conduct a pre-experiment to determine the sediment pretreatment method and the water layer where the target organism exists.

Freshwater and brackish blue-green algae were collected at 43 freshwater and brackish sites (including lakes, ponds, swamps, streams, and rivers and estuaries) throughout South Korea from March 2017 to October 2018, and were identified using light microscopy. A total 223 taxa in freshwater and 230 taxa in brackish waters in 2017 and 274 taxa in fresh and brackish waters in 2018 were identified and among them, 20 taxa were unreported taxa of blue-green algae in Korea; The new recorded taxa were Aphanocapsa marina, Calothrix fusca f. durabilis, Calothrix littoralis, Calothrix parva, Chamaesiphon minimus, Chroococcidiopsis cubana, Chroococcidiopsis fissurarum, Coelosphaerium aerugineum, Dolichospermum mendotae, Eucapsis alpine, Gomphosphaeria cordiformis, Gomphosphaeria natans, Merismopedia danubiana, Lynbya aestuarii var. gaditana, Tolypothrix tenuis, Pseudocapsa maritima, Pseudocapsa sphaerica, Pseudophormidium tenue, Trichodesmus sp. and Woronichinia elorantae.

Targeting Microcystin (MC), which is most abundantly detected in the North-Han River water area, we analyzed the relationship between the MC biosynthesis gene (mcyA gene), cyanobacteria cell density, and MC concentration, derived an RNA-MC conversion formula, and derived the cyanobacteria. The concentration of MC present in cells was predicted. In the North-Han River waters, the mcyA gene was found mainly at downstream sites of the North-Han River after Muk-Hyeon Stream junction, and higher copy numbers were found on average than other sites. In the Uiam Lake waters upstream of the North-Han River, the mcyA gene copy number increased at the Kong-Ji Stream point, and after September, the mcyA gene copy number decreased throughout the North-Han River waters. The expression of the mcyA gene was concentrated in the short period of summer due to the spatio-temporal difference between upstream and downstream water bodies. The mcyA gene expression level was not only highly correlated with MC concentration, but also correlated with the cell density of Microcystis aeruginosa and Dolichospermum circinale, which are known to biosynthesize MC. Six conversion formulas derived based on the RNA-MC relationship showed statistical significance (p<0.05) and exhibited high correlation coefficients (r) of 0.9 or higher. The expression level of MC biosynthesis gene present in eRNA determines the synthesis of cyanotoxin substances in water, quickly quantifies gene activity, and can be fully utilized for early warning of MC development.

A total of 31 bacterial strains were isolated from the Geum River basin in the Republic of Korea during our investigation of indigenous prokaryotic species. The isolated bacterial strains had high 16S rRNA gene sequence similarity (>98.7%) with those of validly published bacterial species, which have not been reported in Republic of Korea. The 31 bacterial strains were phylogenetically diverse and assigned to 4 phyla, 8 classes, 18 orders, 21 families, and 27 genera. At the genus level, the unreported species were affiliated with Kineococcus, Pedococcus, Rhodoluna, Salinibacterium, Rhodoluna, Arthrobacter, Williamsia, Nakamurella, Nocardioides of the class Actinobacteria, Patulibacter of the class Thermoleophilia, Pontibacter, Hymenobacter of the class Cytophagia, Flavobacterium of the class Flavobacteriia, Geomicrobium of the class Bacilli, Brevundimonas, Gellertiella, Rhizobium, Paracoccus, Taonella, Sphingomonas of the class Alphaproteobacteria, Burkholderia, Polaromonas, Hydrogenophaga, Chitinilyticum, Azospira, Zoogloea of the class Betaproteobacteria, and Pseudomonas of the class Gammaproteobacteria. The unreported bacterial species were further characterized by examining their morphological, cultural, physiological, and biochemical properties. The detailed descriptions of the 31 bacterial strains were provided.

Flavobacterium 속(genus)은 Bacteriodetes 문(phylum), Flavobacteriaceae 과(family)의 대표 속(type genus)으로서, 이 속의 세균들은 황색 색소를 함유하는 그람 음성 간균이다. 이 속 세균들은 자 연계의 다양한 환경에서 분리되고 있다. 황색색소를 함유하는 그람음성 간균이 경남 창원시 소재 창원대 학교 교내의 연못에서 분리되었고, 이 세균은 균주 B1으로 명명되었다. 균주 B1을 생리학적, 생화학적 및 계통분석학적으로 분석한 결과, Flavobacterium 속에 속하는 것으로 결론지어졌다. 이 세균의 16S rRNA 유전자 염기서열을 BLAST로 분석해 본 결과, 다른 어떠한 세균과도 16S rRNA 유전자 염기서 열의 상동성이 99.0%를 넘지 않았다. 균주 B1의 주된 지방산은 iso-C15:0(19.6%), summed feature 3(C16:1 ω7c and/or C16:1 ω6c, 16.1%), iso-C17:0 3OH(10.2%), iso-C15:0 3OH(8.4%) 및 iso-C15:1 G(6.6%)인 것으로 밝혀졌는데, 이는 다른 Flavobacterium 종들의 지방산 함량과 확연한 차이가 있는 것을 알 수 있었다. 이 세균의 16S rDNA 염기서열은 genbank에 accession number OP060681로 등록 되었다.