Salinity decline promotes growth and harmful blooms of a toxic alga by diverting carbon flow.

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.

Metabarcoding revealed a high diversity of Amphidomataceae (Dinophyceae) and the seasonal distribution of their toxigenic species in the Taiwan Strait.

The dinophyte family Amphidomataceae includes the genera Azadinium and Amphidoma. Four of these species are known to produce azaspiracids, which are lipophilic phycotoxins accumulating in shellfish. The diversity and biogeography of Amphidomataceae is far from yet resolved. Here we performed a time series sampling of both water and sediments in the Taiwan Strait from Nov. 2018 to April 2021. Metabarcoding was performed to unveil the diversity of Amphidomataceae targeting internal transcribed spacer (ITS1) region and partial large subunit ribosomal DNA (LSU rDNA D1-D3), followed by quantitative PCR (qPCR) with modified primers for Az. poporum ribotypes. The diversity of Amphidomataceae was revealed from the water samples with the aid of ITS1 and LSU based molecular phylogeny. The LSU based approach detected only a few species. In contrast, ITS1 based dataset showed eight new Azadinium clades and several ZOTUs (zero-radius operational taxonomic units) grouping together with Am. languida. Moreover, eleven known Azadinium species including three ribotypes of Az. poporum and Az. dexteroporum, and two ribotypes of Az. spinosum, were detected. The latter two species have not been reported in China before. Among these toxigenic species, Az. poporum was relevantly abundant whereas others were rare. The maximum of 209 cells L -1 of Az. poporum ribotype A was estimated using qPCR nearby Quanzhou in Nov. 2018 and 172 cells L 1 of Az. poporum ribotype B was detected far off coast in Apr. 2021. Metabarcoding on sediment samples revealed Az. poporum ribotypes B and C, but strains obtained with sediment incubation experiments yielded only ribotype B. Using qPCR about 0.2 cysts g -1 of Az. poporum ribotype B were quantified in May 2019 but cysts of Az. poporum ribotype C were not detected. Our results suggest that metabarcoding targeting ITS1 region is powerful to uncover the diversity of harmful dinophytes. Our results also highlight the rich diversity of Amphidomataceae and risk potential of azaspiracids in the Taiwan Strait and surrounding waters.

Co-occurrence of Alexandrium minutum (Dinophyceae) ribotypes from the Chinese and Malaysian coastal waters and their toxin production.

The bloom-forming dinophyte Alexandrium minutum comprises biogeographic inferred, global and Pacific clades with both toxic and nontoxic strains reported. A. minutum has a wide distribution in the Western Pacific, but to date only a few strains have available DNA sequences. To fully understand its genetic diversity, sampling was undertaken from the Yellow Sea, the East and South China Sea, and five strains of A. minutum and two strains of its sister species, A. tamutum, were established. Their morphology was examined by light and scanning electron microscopy. In addition, sequences were obtained from both large subunit (LSU) ribosomal DNA and/or internal transcribed spacer (ITS) region. Strains of A. minutum are morphologically indistinguishable, characterized by a smaller cell size and a narrow sixth precingular plate. In contrast, A. tamutum has a wider sixth precingular plate. High nucleotide divergences of LSU (D1-D3) rDNA and ITS were revealed amongst strains of A. minutum (10% and 25%, respectively), and A. tamutum (3% and 13%, respectively). Molecular phylogenies based on LSU rDNA and ITS revealed three ribotypes (B-D) of A. minutum, and two ribotypes of A. tamutum in the Western Pacific. Seasonal sampling in the East China Sea to detect A. minutum using the DNA metabarcoding targeting ITS1 region was also performed. Our results showed that the ribotypes B and C of A. minutum co-occurred in the water. Paralytic shellfish toxin (PSTs) of all seven strains was analysed using liquid chromatography with tandem mass spectrometry (LC-MS/MS). PSTs were detected only in A. minutum ribotypes B and C with predominance of gonyautoxins 1/4. Our results suggest high diversity and risk potential of this toxic species in this region.

Isolation of an algicidal bacterium and its effects against the harmful-algal- bloom dinoflagellate Prorocentrum donghaiense (Dinophyceae).

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.