Harmful algal blooms (HABs) have occurred frequently in coastal waters of China, imposing negative effects on the marine ecological environment. A dataset of HABs and terrestrial runoff was collected and analyzed in this study, and factors responsible for HABs were further explored. Frequency and expansion of HABs peaked between 2001 and 2007, and although they have declined slightly since then, they have remained quite high. Frequency and accumulative area of HABs peaked in 2004–2005, and most occurred from April to August during these years. HABs occurred frequently in the Changjiang (Yangtze River) estuary, and Prorocentrum donghaiense, Noctiluca scientillans, Karenia mikimotoi, and Skeletonema costatum were the main algal species. The increases of eutrophication, the abnormal sea surface temperature caused by climate and ocean currents, and the species invasion caused by the discharge of ballast water may be important factors for the long-term outbreak of HABs in the Chinese coastal waters. These findings provide a better understanding of HABs in China, which will be helpful to further prevention and control.

Alexandriumpacificum is a typical toxic bloom-forming dinoflagellate, causing serious damage to aquatic ecosystems and human health. Many bacteria have been isolated, having algicidal effects on harmful algal species, while few algicidal bacteria have been found to be able to lyse A. pacificum. Herein, an algicidal bacterium, Shewanella Y1, with algicidal activity to the toxic dinoflagellate A. pacificum, was isolated from Jiaozhou Bay, China, and the physiological responses to oxidative stress in A. pacificum were further investigated to elucidate the mechanism involved in Shewanella Y1. Y1 exhibited a significant algicidal effect (86.64 ± 5.04% at 24 h) and algicidal activity in an indirect manner. The significant declines of the maximal photosynthetic efficiency (F_v/F_m), initial slope of the light limited region (alpha), and maximum relative photosynthetic electron transfer rate (rETRmax) indicated that the Y1 filtrate inhibited photosynthetic activities of A. pacificum. Impaired photosynthesis induced the overproduction of reactive oxygen species (ROS) and caused strong oxidative damage in A. pacificum, ultimately inducing cell death. These findings provide a better understanding of the biological basis of complex algicidal bacterium-harmful algae interactions, providing a potential source of bacterial agent to control harmful algal blooms.

Synechococcus is a dominant genus of the coastal phytoplankton with an effective contribution to primary productivity. Here, the phylogenetic and phenogenetic composition of Synechococcus in the coastal Yellow Sea was addressed by sequencing marker gene methods. Meanwhile, its co-occurrence pattern with bacterial and eukaryotic microbes was further investigated based on the construction of networks. The result revealed that Synechococcus abundance ranged from 9.8 × 10² cells mL⁻¹ to 1.6 × 10⁵ cells mL⁻¹, which was significantly correlated to sampling depth and nutrient contents of nitrite, ammonia, and dissolved silicon. A total of eight Synechococcus phylogenetic lineages were detected, of which clade III was dominant in most of the samples. Meanwhile, clade I increased along the water column and even reached a maximum value of 76.13% at 20 m of station B. Phenogenetically, Synechococcus PT3 was always the predominant pigment type across the whole study zone. Only salinity was significantly correlated to the phenogenetic constitution. The networks revealed that Synechococcus co-occurred with 159 prokaryotes, as well as 102 eukaryotes including such possible grazers as Gymnodinium clades and Alveolata. Potential function prediction further showed that microbes co-occurring with Synechococcus were associated with diverse element cycles, but the exact mechanism needed further experimentation to verify. This research promotes exploring regularity in the genomic composition and niche position of Synechococcus in the coastal ecosystem and is significant to further discuss its potential participation in materials circulation and bottom-up effects in microbial food webs.