Human activities have caused an imbalance in the input nitrogen and phosphorus (N/P) in the biosphere. The imbalance of N/P is one of the characteristics of water eutrophication, which is the fundamental factor responsible for the blooms. The effects of the N/P imbalance on diatom and phycospheric bacteria in blooms are poorly understood. In this study, the N/P molar ratio in real water (14:1) and the predicted N/P molar ratio in future water (65:1) were simulated to analyze the response of Cyclotella sp. and phycospheric bacteria to the N/P imbalance. The results showed that the N/P imbalance inhibited the growth of Cyclotella sp., but prolonged diatom bloom duration. The resistance of Cyclotella sp. to the N/P imbalance is related to phycospheric bacteria, and there are dynamic regulatory mechanisms within the phycospheric bacteria community to resist the N/P imbalance: (1) the increase of HNA bacterial density, the decrease of LNA bacterial density, (2) the increase of phycospheric bacterial diversity and eutrophic bacteria abundance, and the change of denitrifying bacteria abundance, (3) the activity of nitrogen and phosphorus metabolism of HNA bacteria enhanced, while that of LNA bacteria decreased. And the gene hosts of nitrogen and phosphorus metabolism were most enriched in Proteobacteria, indicating that Proteobacteria played an important role in maintaining the stability of phycospheric bacteria and was the dominant phylum resistant to the N/P imbalance. This study clarified that the algal-bacteria system was resistant to the N/P imbalance and implied that the N/P imbalance had little effect on the occurrence of diatom bloom events due to the presence of phycospheric bacteria.

How microzooplanktonic ciliate adaptative strategies differ across diatom bloom and non-diatom bloom areas in the Arctic Ocean remains poorly documented. To address this gap, two different situations were categorized in the Arctic Ocean at summer 2023: diatom bloom stations (DBS) (genus Thalassiosira, chain-like) and non-diatom bloom stations (nDBS). Total abundance of ciliate at 3 m and 25 m in DBS was 2.8 and 1.8 folds higher than in nDBS, respectively. Aloricate ciliates were singled out in both DBS and nDBS, whilst their average abundance and biomass of large size-fraction (>50 μm) in former were 4.5-5.6 folds higher than in latter. Regarding tintinnids, high abundance of Ptychocylis acuta (Bering Strait species) mainly occurred at DBS, coupled with distribution of co-occurring Pacific-origin species Salpingella sp.1, collectively suggested a strong intrusion of Pacific Inflow during summer 2023. Additionally, presence of high abundance of Acanthostomella norvegica and genus Parafavella in nDBS might indicate the trajectory of the Transpolar Drift. Alternatively, tintinnids can serve as credible bioindicators for either monitoring currents or evaluating microzooplankton Borealization. Average abundance of total ciliate within 15-135 μm body-size spectrum in DBS was higher than nDBS. Moreover, spearman\'s rank correlation between biotic and abiotic analysis revealed that temperature and dissolved oxygen at DBS determined tintinnid species richness and ciliate total abundance, respectively. The results clearly demonstrate that remarkable divergences in large size-fraction of ciliate abundance between DBS and nDBS validate their irreplaceable role in controlling phytoplankton outbreak and associated biological processes in polar seas.

Diatom-bacteria interactions and the associated bloom dynamics have not been fully understood in the coastal oceans. Here, we focus on the polyunsaturated aldehydes (PUAs) produced by diatoms in the post-bloom phase and look into their roles in microbial phosphorus (P) recycling outside of a P-limited estuary. The phytoplankton community in the bloom was dominated by PUAs-producing diatoms (Skeletonema costatum, Thalassiosira spp., and Pesudonitzschia delicates) with elevated concentrations of biogenic particulate PUAs. In addition, there were micromolar levels of particle-adsorbed PUAs hotspots with distinct compositions in and out of the bloom determined by a combining large-volume filtration and on-site derivation method. Field experiments were conducted to further assess the responses of particle-attached bacteria (PAB) to different PUAs amendments. We found no differences in the alkaline phosphatase (APase) activity and the abundance of PAB between inside and outside the bloom at a low PUAs dosage (<30 μM). However, for a high PUAs dosage (300 μM), APase activity and PAB growth were reduced significantly outside the bloom but no influences within the bloom. Our findings indicate that the hotspot-level oxylipins may play essential roles in bacterial P-remineralization in P-limited coastal areas. PAB can adapt to the high level of PUAs released by diatoms (or their resulting detritus) and potentially maintain a high rate of organic P recycling during the late stages of diatom blooms. Consequently, the interaction between oxylipin-rich diatoms and bacteria may affect phytoplankton blooms and carbon sequestration in the coastal oceans.

The transfer of dissolved silicate (DSi) from land to coastal environments is a crucial part of global biogeochemical cycling. However, the retrieval of coastal DSi distribution is challenging due to the spatiotemporal non-stationarity and nonlinearity of modeling processes and the low resolution of in situ sampling. To explore the coastal DSi changes in a higher spatiotemporal resolution, this study developed a spatiotemporally weighted intelligent method based on a geographically and temporally neural network weighted regression (GTNNWR) model, a Data-Interpolating Empirical Orthogonal Functions (DINEOF) model, and satellite observations. For the first time, the complete surface DSi concentrations of 2182 days at the 500-meter and 1-day resolution in the coastal sea of Zhejiang Province, China, were obtained (Testing R2 = 78.5 %) by using 2901 in situ records with concurrent remote sensing reflectance. The long-term and large-scale distributions of DSi reflected the changes in coastal DSi under the influences of rivers, ocean currents, and biological effects across multiple spatiotemporal scales. Benefiting from the high-resolution modeling, this study found that the surface DSi concentration had at least 2 declines during a diatom bloom process, which can provide crucial signals for the timely monitoring and early warning of diatom blooms and guide the management of eutrophication. It was also indicated that the correlation coefficient between the monthly DSi concentration and the Yangtze River Diluted Water velocities reached -0.462**, quantitatively revealing the significant influence of the terrestrial input. In addition, the daily-scale DSi fluctuations resulting from typhoon transits were finely characterized, which greatly reduces the monitoring cost compared with the field sampling. Therefore, this study developed an effective data-driven-based method to help explore the fine-scale dynamic changes of surface DSi in coastal seas.

Diatom blooms can significantly affect the succession of microbial communities, yet little is known about the assembly processes and interactions of microbial communities during autumn bloom events. In this study, we investigated the ecological effects of an autumn diatom bloom on prokaryotic communities (PCCs) and microeukaryotic communities (MECs), focusing on their assembly processes and interactions. The PCCs were largely dominated by Alphaproteobacteria, Gammaproteobacteria, Cyanobacteria, and Flavobacteria, while the MECs primarily included Diatomea, Dinoflagellata, and Chlorophyta. The succession of both PCCs and MECs was mainly driven by this diatom bloom and environmental factors, such as nitrate and silicate. Null modeling revealed that homogeneous selection had a more pronounced impact on the structure of PCCs compared with that of MECs. In particular, drift and dispersal limitation cannot be neglected in the assembly processes of MECs. Co-occurrence network analyses showed that Litorimicrobium, Cercozoa, Marine Group I (MGI), Cryptomonadales, Myrionecta, and Micromonas may affect the bloom process. In summary, these results elucidated the complex, robust interactions and obviously distinct assembly mechanisms of PCCs and MECs during a diatom bloom and extend our current comprehension of the ecological mechanisms and microbial interactions involved in an autumn diatom bloom process.

The selenium cycle in the marine environment is sensitive to biological activity, but knowledge of dissolved Se species dynamics during coastal algal blooms is limited. Selenium species dynamics during diatom blooms in the Changjiang Estuary were investigated in a survey of dissolved inorganic and organic Se. Dissolved inorganic Se (Se(IV) + Se(VI)) was the predominant species in river-dominanated areas, while dissolved organic selenide (DOSe) was predominant in ocean-dominanated areas. Relationships between DOSe and chromophoric dissolved organic matter involved both humic- and protein-like components, suggesting distinct sources of DOSe in river- and ocean-dominance areas, respectively. A three-endmember-mixing model was used to describe biological processes in ocean-dominanated surface waters. In diatom-bloom areas, the co-occurrence of depletion of Se(IV) and Se(VI) (of ~90% and 30%, respectively) and a 44% increase in production of DOSe indicates that phytoplankton act as vectors for Se species transformation. A Se(IV)*P indicator was developed to quantify limiting concentrations of Se(IV) in water relative to that of phosphorus. Negative Se(IV)*P concentrations indicate that Se(IV) is limited due to biological utilization of dissolved inorganic phosphorus by diatoms, resulting in secondary uptake of Se(VI) in the Changjiang Estuary.

The interaction and assembly processes of microeukaryotic community compositions (MECs) are rarely elucidated in environment with strong disturbance such as harmful algal blooms. To fill this gap, we analyzed changes of MECs induced by a diatom bloom using 18S rRNA gene amplicon sequencing. The MECs were mainly dominated by Cercozoa (average relative abundance, 49.2%), Diatom (25.5%) and Dinoflagellata (15.6%). MECs changed significantly (ANOSIM P < 0.01) in four-bloom stages. Environmental factors including pH, DO, nitrate and phosphate, together with bacterial communities could significantly influence the variation of MECs. Co-occurrence network analysis revealed a complex interaction between microeukaryotic and bacterial communities. Most OTUs in modules of the co-occurrence network were specific to one particular bloom stage. Phylogenetic based β-nearest taxon distance analyses revealed that stochastic processes mainly dominated microeukaryotic community assembly in the initial and after-bloom stage. However, microeukaryotic community assembly in middle and late stage of the bloom were driven by deterministic processes. In conclusion, both stochastic and deterministic processes play important roles in distinct bloom stages. These findings may expand current understandings of assembly mechanisms and microbial interactions underlying microeukaryotic dynamics in eutrophic aquatic ecosystems where harmful algal blooms occurred frequently.

Harmful algal blooms (HABs) are serious ecological disasters in coastal areas, significantly influencing biogeochemical cycles driven by bacteria. The shifts in microbial communities during HABs have been widely investigated, but the assembly mechanisms of microbial communities during HABs are poorly understood. Here, using 16S rRNA gene amplicon sequencing, we analyzed the microbial communities during an early-spring diatom bloom, in order to investigate the dynamics of microbial assembly processes. Rhodobacteraceae, Flavobacteriaceae, and Microbacteriaceae were the main bacterial families during the bloom. The 30 most abundant operational taxonomic units (OTUs) segregated into 4 clusters according to specific bloom stages, exhibiting clear successional patterns during the bloom process. The succession of microbial communities correlated with changes in the dynamics of algal species. Based on the β-nearest taxon distance, we constructed a simulation model, which demonstrated that the assembly of microbial communities shifted from strong heterogenous selection in the early stage of the bloom to stochasticity in the middle stage and then to strong homogeneous selection in the late and after-bloom stages. These successions were driven mainly by chlorophyll a contents, which were affected mainly by Skeletonema costatum Moreover, functional prediction of microbial communities showed that microbial metabolic functions were significantly related to nitrogen metabolism. In summary, our results clearly suggested a dominant role of determinacy in microbial community assembly in HABs and will facilitate deeper understanding of the ecological processes shaping microbial communities during the algal bloom process.IMPORTANCE Harmful algal blooms (HABs) significantly influence biogeochemical cycles driven by bacteria. The shifts in microbial communities during HABs have been studied intensively, but the assembly mechanisms of microbial communities during HABs are poorly understood, with limited investigation of the balance of deterministic and stochastic processes in shaping microbial communities in HABs. In this study, the dynamics and assembly of microbial communities in an early-spring diatom bloom process were investigated. Our data both confirm previously observed general microbial successional patterns and show new detailed mechanisms for microbial assembly in HABs. These results will facilitate deeper understanding of the ecological processes shaping microbial communities in HABs. In addition, predictions of metabolic potential in this study will facilitate understanding of the influence of HABs on nitrogen metabolism in marine environments.

To understand the variations of picoplankton (Prochlorococcus, Synechococcus, picoeukaryotes, and heterotrophic bacteria) abundances during diatom bloom, the distribution of picoplankton in the Taiwan Bank, South China Sea was investigated using flow cytometry during a Thalassiosira diporocyclus bloom in March 2016. The results indicated an abrupt abundance decrease for Prochlorococcus, Synechococcus, and picoeukaryotes within the bloom area while the abundance of heterotrophic bacteria showed no significant difference between the bloom and non-bloom areas. We found two sub-groups of heterotrophic bacteria: high- and low-nucleic acid content (HNA and LNA) bacteria with HNA dominated in the bloom area whereas LNA dominated in the non-bloom area. Among the picoplankton components, HNA represented the highest (61.1%) carbon biomass in the bloom area while picoeukaryotes represented the highest (37.6%) in the non-bloom area. Our findings implied that heterotrophic bacteria, especially HNA, played an essential role during the diatom bloom.