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Global climate change intensifies the water cycle and makes freshest waters become fresher and vice‐versa. But how this change impacts phytoplankton in coastal, particularly harmful algal blooms (HABs), remains poorly understood. Here, we monitored a coastal bay for a decade and found a significant correlation between salinity decline and the increase of Karenia mikimotoi blooms. To examine the physiological linkage between salinity decreases and K. mikimotoi blooms, we compare chemical, physiological and multi‐omic profiles of this species in laboratory cultures under high (33) and low (25) salinities. Under low salinity, photosynthetic efficiency and capacity as well as growth rate and cellular protein content were significantly higher than that under high salinity. More strikingly, the omics data show that low salinity activated the glyoxylate shunt to bypass the decarboxylation reaction in the tricarboxylic acid cycle, hence redirecting carbon from CO2 release to biosynthesis. Furthermore, the enhanced glyoxylate cycle could promote hydrogen peroxide metabolism, consistent with the detected decrease in reactive oxygen species. These findings suggest that salinity declines can reprogram metabolism to enhance cell proliferation, thus promoting bloom formation in HAB species like K. mikimotoi, which has important ecological implications for future climate‐driven salinity declines in the coastal ocean with respect to HAB outbreaks. [ABSTRACT FROM AUTHOR]

Simple Summary: Recently, the occurrence of harmful algal blooms (HABs) in coastal areas has increased rapidly, negatively impacting fishing resources, public health, and marine ecosystems. Certain microorganisms play a significant role in the termination of HABs. To exploit these algicidal bacteria effectively, it is imperative to amplify their algicidal ratio and devise efficient large-scale cultivation methods; however, research in these areas is underdeveloped. We previously identified Pseudoalteromonas sp. strain FDHY-MZ2, which exhibited significant algicidal activity against Karenia mikimotoi, a species globally acknowledged for forming HAB. To improve the algicidal efficacy of FDHY-MZ2, cultures were progressed from shaking flask conditions to small-scale (5 L) and pilot-scale fermentation (50 L). The optimal fermentation medium and conditions at a pilot-scale level were established, resulting in a substantial improvement in algicidal properties. This enhancement might be due to a rise in algal H2O2 production accompanied by increased cell membrane disintegration, pronounced damage to cell chlorophyll and algal photosynthesis, and severe protein degradation. These findings provide new insights into the previously unknown potential of systematically applied microbial agents for the treatment of HABs. The role of microorganisms in effectively terminating harmful algal blooms (HABs) is crucial for maintaining environmental stability. Recent studies have placed increased emphasis on bio-agents capable of inhibiting HABs. The bacterium Pseudoalteromonas sp. strain FDHY-MZ2 has exhibited impressive algicidal abilities against Karenia mikimotoi, a notorious global HAB-forming species. To augment this capability, cultures were progressively scaled from shake flask conditions to small-scale (5 L) and pilot-scale (50 L) fermentation. By employing a specifically tailored culture medium (2216E basal medium with 1.5% soluble starch and 0.5% peptone), under precise conditions (66 h, 20 °C, 450 rpm, 30 L/min ventilation, 3% seeding, and constant starch flow), a notable increase in algicidal bacterial biomass was observed; the bacterial dosage required to entirely wipe out K. mikimotoi within a day decreased from 1% to 0.025%. Compared to an unoptimized shake flask group, the optimized fermentation culture caused significant reductions in algal chlorophyll and protein levels (21.85% and 78.3%, respectively). Co-culturing induced increases in algal malondialdehyde and H2O2 by 5.98 and 5.38 times, respectively, leading to further disruption of algal photosynthesis. This study underscores the unexplored potential of systematically utilized microbial agents in mitigating HABs, providing a pathway for their wider application. [ABSTRACT FROM AUTHOR]

Algicidal bacteria exhibit promising potential against harmful algal blooms (HABs); however, their application has been limited due to their limited algicidal activity. This study demonstrates the enhanced algicidal activity of Alteromonas sp. FDHY-CJ bacteria against harmful Skeletonema costatum using a 5 L fermenter. Utilizing this refined framework increased the OD 600 value and algal cell mortality by 6.50 and 2.88 times, respectively, compared to non-optimized culture cultivated in a flask using marine broth 2216E medium. The mechanism of action involves significant inhibition of algal photosynthetic efficiency with concurrent degradation of photosynthetic pigments. Relative to the non-optimized group, the optimized bacterial treatment led to a significant increase in H 2 O 2 and MDA (malondialdehyde) by 19.54 and 4.22-fold, respectively, and resulted in membrane damage. The culture optimization procedure yielded effectual algicidal substances capable of considerably reducing the severity of S. costatum HABs through cell membrane disruption. [ABSTRACT FROM AUTHOR]

Highlights • A bacterium (FDHY-03) was isolated in a bloom of Prorocentrum donghaiense. • FDHY-03 strain showed high algicidal rate against P. donghaiense. • FDHY-03 lysed P. donghaiense cells from megacytic growth zone. • FDHY-03 lysed P. donghaiense cells by digesting cell wall polysaccharides. Abstract The relationship between algicidal bacteria and harmful-algal-bloom-forming dinoflagellates is understudied and their action modes are largely uncharacterized. In this study, an algicidal bacterium (FDHY-03) was isolated from a bloom of Prorocentrum donghaiense and the characteristics of its action against P. donghaiense was investigated at physiological, molecular, biochemical and cytological levels. 16S rDNA sequence analysis placed this strain in the genus of Alteromonas in the subclass of γ-proteobacteria. Algicidal activity was detected in the bacterial filtrate, suggesting a secreted algicidal principle from this bacterium. Strain FDHY-03 showed algicidal activity on a broad range of HAB-forming species, but the greatest effect was found on P. donghaiense , which showed 91.7% mortality in 24 h of challenge. Scanning electron microscopic analysis indicated that the megacytic growth zone of P. donghaiense cells was the major target of the algicidal action of FDHY-03. When treated with FDHY-03 culture filtrate, P. donghaiense cell wall polysaccharides decreased steadily, suggesting that the algicidal activity occurred through the digestion of cell wall polysaccharides. To verify this proposition, the expression profile of beta-glucosidase gene in FDHY-03 cultures with or without P. donghaiense cell addition was investigated using reverse-transcription quantitative PCR. The gene expression level increased in the presence of P. donghaiense cells, indicative of beta-glucosidase induction by P. donghaiense and the enzyme's role in this dinoflagellate's demise. This study has isolated a new bacterial strain with a strong algicidal capability, documented its action mode and biochemical mechanism, providing a potential source of bacterial agent to control P. donghaiense blooms. [ABSTRACT FROM AUTHOR]

Summary: Despite numerous laboratory studies on physiologies of harmful algal bloom (HAB) species, physiologies of these algae during a natural bloom are understudied. Here, we investigated a bloom of the raphidophyte Heterosigma akashiwo in the East China Sea in 2014 using metabarcode (18S rDNA) and metatranscriptome sequencing. Based on 18S rDNA analyses, the phytoplankton community shifted from high diversity in the pre‐bloom stage to H. akashiwo predominance during the bloom. A sharp decrease in ambient dissolved inorganic phosphate and strong up‐regulation of phosphate and dissolved organic phosphorus (DOP) uptake genes, including the rarely documented (ppGpp)ase, in H. akashiwo from pre‐bloom to bloom was indicative of rapid phosphorus uptake and efficient utilization of DOP that might be a driver of the H. akashiwo bloom. Furthermore, observed up‐regulated expression of mixotrophy‐related genes suggests potential contribution of mixotrophy to the bloom. Accelerating photosynthetic carbon fixation was also implied by the up‐regulation of carbonic anhydrase genes during the bloom. Notably, we also observed a strong morning‐to‐afternoon shift in the expression of many genes. Our findings provide insights into metabolic processes likely important for H. akashiwo bloom formation, and suggest the need to consider timing of sampling in field studies on this alga. [ABSTRACT FROM AUTHOR]

Gene structure and expression regulation of form II RuBisCO (rbcII) in dinoflagellates are still poorly understood. Here we isolated this gene (Pdrbc) and investigated its diel expression pattern in a harmful algal bloom forming dinoflagellate Prorocentrum donghaiense. We obtained cDNA sequences with triple tandem repeats of the coding unit (CU); the 5′ region has the sequence of a typical dinoflagellate plastid gene, encoding an N-terminus with two transmembrane regions separated by a plastid transit peptide. The CUs (1,455 bp except 1464 bp in last CU) are connected through a 63 bp spacer. Phylogenetic analysis showed that rbcII CUs within species formed monophyletic clusters, indicative of intraspecific gene duplication or purifying evolution. Using quantitative PCR (qPCR) we estimated 117±40 CUs of Pdrbc in the P. donghaiense genome. Although it is commonly believed that most dinoflagellate genes lack transcriptional regulation, our RT-qPCR analysis on synchronized cultures revealed remarkable diel rhythm of Pdrbc expression, showing significant correlations of transcript abundance with the timing of the dark-to-light transition and cell cycle G2M-phase. When the cultures were shifted to continuous light, Pdrbc expression remained significantly correlated with the G2M-phase. Under continuous darkness the cell cycle was arrested at the G1 phase, and the rhythm of Pdrbc transcription disappeared. Our results suggest that dinoflagellate rbcII 1) undergoes duplication or sequence purification within species, 2) is organized in tandem arrays in most species probably to facilitate efficient translation and import of the encoded enzyme, and 3) is regulated transcriptionally in a cell cycle-dependent fashion at least in some dinoflagellates. [ABSTRACT FROM AUTHOR]

• Transcriptomic analysis revealed 8802 unigenes (3.56%) in K. mikimotoi that responded to nitrogen deficiency. • Under N deficiency, K. mikimotoi shifted to assimilate organic N resource, and cellular N utilization changed to be economical to save N resource. • N limitation increased cellular N recycling via protein degradation and turnover. • K. mikimotoi induced substrate production for triacylglycerol biosynthesis by N starvation. The availability of ambient N nutrient is often correlated with the occurrences of harmful algal bloom formed by certain dinoflagellates, making it important to understand how these species might be responding to such conditions. Here, transcriptome sequencing of Karenia mikimotoi was conducted to understand the underlying molecular mechanisms by which this dinoflagellate copes with nitrogen (N) deficiency. Transcriptomic analysis revealed 8802 unigenes (3.56%) that were differentially expressed with ≥ 2-fold change. Under N-depleted conditions, genes involved in glycolysis, fatty acid metabolism, and the tricarboxylic acid (TCA) cycle as well as lipid accumulation were significantly upregulated. The elevated expression of enzymes used in protein degradation and turnover suggests possible metabolic reconfiguration towards accelerated N recycling. Moreover, a significant increase in urea transporter was observed, indicating increased assimilation of organic nitrogen resources as an alternative in N-depleted cultures of K. mikimotoi. The down-regulated glutamate synthase genes were also identified under N deficiency, suggesting suppression of primary amino acid synthesis to save N resource. Taken together, results of this study show enhanced multiple N resource acquisition and reuse of multiple N resources constitute a comprehensive strategy to cope with N deficiency in a dinoflagellate. [ABSTRACT FROM AUTHOR]