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The filamentous Trichodesmium and Richelia/Calothrix appear to be crucial N2‐fixing cyanobacteria throughout the warm oligotrophic ocean, particularly in western boundary currents. The Kuroshio, the hotspot of N2 fixation in the Pacific Ocean, intrudes into the East China Sea (ECS) and southern Yellow Sea (SYS), which may profoundly influence composition and distribution of filamentous diazotrophs. We provide high spatial resolution dataset of filamentous diazotrophs in the ECS and SYS in July and August 2013. Results showed that Trichodesmium colonies and Richelia/Calothrix were strictly restricted to warm, saline, nitrogen‐limited waters of the ECS influenced by Kuroshio and were not detected in the SYS. The density of Trichodesmium in the ECS and SYS was 8.48 × 106 trichomes/m2, and colonial trichomes accounted for 40%. In addition to free filaments, Richelia/Calothrix were found to be symbiotic with diatoms Hemiaulus, Rhizosolenia/Guinardia, Chaetoceros, and Bacteriastrum with a total density at 2.13 × 106 heterocysts/m2 in the ECS. The densities of filamentous diazotrophs were significantly higher in high‐salinity region of the ECS (mainly controlled by Kuroshio) than in low‐salinity region of the ECS and in the SYS. The densities of Trichodesmium and Richelia/Calothrix were significantly and positively correlated with temperature, salinity, and mixed‐layer depth but were negatively correlated with NO3, turbidity, and chlorophyll‐a. These results suggested that composition and distribution of filamentous diazotrophs in the ECS and SYS were largely shaped by Kuroshio and associated physicochemical properties. We hypothesize that active N2 fixation in Kuroshio may be substantially supported by Richelia/Calothrix in addition to Trichodesmium. Plain Language Summary: The filamentous diazotrophs Trichodesmium and Richelia/Calothrix are abundantly distributed and they frequently form blooms in western boundary currents. Their N2‐fixing activity effectively alleviated nitrogen limitation to primary production, which are crucially involved in nitrogen budget and biological pump. The Kuroshio, a western boundary current, is recognized as the hotspot of N2 fixation in the Pacific and strongly intrudes into China's marginal seas. The incursion of Kuroshio may profoundly influence composition and distribution of filamentous diazotrophs. We first provide high spatial resolution dataset of filamentous diazotrophs throughout the East China Sea and southern Yellow Sea. Richelia/Calothrix were found to be symbiosis with diatoms Hemiaulus, Rhizosolenia, Guinardia, Chaetoceros, and Bacteriastrum. Trichodesmium colonies and Richelia/Calothrix were strictly restricted to Kuroshio‐affected waters of the East China Sea, which were characterized by high temperature, limited nitrogen, and available iron, phosphate, and silicate. Our results suggested that the Kuroshio mainstream is the hotspot of Trichodesmium and Richelia/Calothrix, thus probably contributing to active N2 fixation. We highlight the significance of Kuroshio and associated physicochemical conditions in shaping composition and distribution of filamentous diazotrophs. Our study will be useful for understanding diazotrophic diversity and nitrogen and carbon biogeochemical processes in Kuroshio and adjacent marginal seas. Key Points: We provide baseline data on Richelia/Calothrix in the East China Sea and southern Yellow SeaTrichodesmium colonies and Richelia/Calothrix were restricted to warm, saline, N‐limited waters in the southeastern East China SeaAbundant Trichodesmium and Richelia/Calothrix were found in the Kuroshio mainstream, thus probably contributing to active N2 fixation [ABSTRACT FROM AUTHOR]

The dinoflagellate Prorocentrum donghaiense is a dominant harmful algal bloom (HAB) species on the East China Sea (ECS) coast. The co-occurrence of Karlodinium veneficum with P. donghaiense is often observed and can later develop into dense blooms. However, the role of K. veneficum in P. donghaiense population dynamics is unknown. In the current study, three K. veneficum (GM1, GM2, and GM3) strains were isolated from the ECS with one (GM1) from a mixed, dense bloom of P. donghaiense and other HAB species. All three isolates had identical ITS sequences that were concordant with the species designation. Unique karlotoxin congeners were isolated from one strain (GM2). The sterol compositions of P. donghaiense and K. veneficum were consistent with sensitivity to karlotoxin in the former and insensitivity in the latter. Additional experimentation showed that: (1) in monocultures, higher growth rate of P. donghaiense than K. veneficum is observed in nutrient-enriched and nutrient-depleted media. In co-cultures, the growth of P. donghaiense is inhibited; (2) feeding on P. donghaiense by K. veneficum is clearly demonstrated by fluorescent dye tracking; and (3) the isolated karlotoxin is lethal to P. donghaiense in a concentration-dependent manner. From these studies we propose that K. veneficum may play a negative role in P. donghaiense bloom maintenance and that P. donghaiense may in turn be a bloom initiator as a prey item for K. veneficum . [ABSTRACT FROM AUTHOR]

The dinoflagellate Karlodinium veneficum is a harmful algal bloom species with a worldwide distribution. This small athecate dinoflagellate makes a family of polyketide toxins that are hemolytic, cytotoxic and ichthyotoxic. The first chemical structure for karlotoxins from East China Sea (ECS) is reported here. The two new karlotoxins, namely 4,5-dihydro-KmTx 2 (compound 1 ) and 4,5-dihydro-dechloro-KmTx 2 (compound 2 ), were isolated and purified from monoalgal cultures of K. veneficum strain GM2. Their structures were determined by spectroscopic analysis, including tandem mass spectrometry as well as 1D and 2D NMR experiments. These new karlotoxin congeners feature a saturated polyol arm different from previously reported for KmTx 2 that appears to increase hemolytic activity. [ABSTRACT FROM AUTHOR]

We analyzed the composition and distribution of phytoplankton in relation to physicochemical factors in the Changjiang (Yangtze River) Estuary and adjacent East China Sea shelf in June and August 2009. Diatoms and dinoflagellates dominated the community, particularly in eutrophic inshore waters controlled by the Changjiang Diluted Water (CDW), coastal current, and upwelling. However, high densities of cyanobacteria and cryptophytes were observed in the oligotrophic offshore waters influenced by the Taiwan Warm Current (TWC) and Kuroshio. In June, the northeastward CDW plume combined with the Yellow Sea Coastal Current induced algal bloom in the northern part of the CE. In August, the enhanced CDW formed two narrow, low-salinity tongues that extended eastward and southward (associated with the upwelling and coastal current), resulting in phytoplankton blooms off the CE and in the Zhejiang coastal waters, respectively. Phytoplankton abundance in August was considerably higher than in June, with increased solar radiation, CDW, and upwelling. The maximum abundance occurred on the surface in inshore turbid waters and on the subsurface (5–30 m) in offshore clear waters with increased stratification. Based on multidimensional scaling and cluster analysis, we found appreciable spatio-temporal variations in algal community composition. Different ecological groups corresponded with hydrographic distributions. Canonical correspondence analysis showed that nutrients, salinity, temperature, and suspended particulate matter were the main variables associated with community distribution. We suggest that the variations in summer phytoplankton community are highly correlated with the significant monthly and spatial variability in physicochemical properties, which are primarily controlled by the CDW and TWC. [ABSTRACT FROM AUTHOR]

Abstract: Karlodinium micrum (Strain NMBjah047) was isolated from the water samples of East China Sea (ECS). The hemolytic, ichthyotoxic, and cytotoxic activities of the algae was characterized. Embryotoxicity of both intra and extracellular extracts were also tested on a local sea urchin species. The algal intracellular hemolytic toxicity averaged about 87.5% at different algal growth phases. However, extracellular hemolytic activity depended on the population growth phase. The toxicity increased with the increase in the population size, reaching the highest hemolytic activity during the stationary phase, and maintained a relatively high activity even when the population declined. Time and density dependent ichthyotoxicity to Lateolabrax maculates juveniles was also detected. The LD50 in 24 h was 1.1 × 105 cells/mL. Inhibition of the fertilized egg hatching was also observed and estimated the IC50 in 40 h with 3.5 × 104 cells/mL. Extracellular extracts of K. micrum dense culture also showed significant cytotoxic activity on HUVEC (IC50 = 70.8 μg/mL). A dose dependent acute toxicity to embryos of sea urchin was also determined. The algal intracellular and extracellular extracts delayed or even restricted the embryological development of the sea urchin, illustrating the potential toxicity of K. micrum not only to vertebrates, but also to marine invertebrates. The hemolytic compounds in the ECS strain were extracted and analyzed. At least two fractions had significant hemolytic activities. A lipid-like compound, named Digalactosyldiacylglycerol (DGDG), was suggested to be responsible for the hemolytic activity in one of these fractions. From the results of the present studies, this strain of K. micrum isolated from the East China Sea might be considered a toxic strain with hemolytic activity, ichthyotoxicity, cytotoxicity and embryotoxicity. [Copyright &y& Elsevier]