Membrane lipid remodeling and autophagy to cope with phosphorus deficiency in the dinoflagellate Prorocentrum shikokuense.

Dinoflagellates, which are responsible for more than 80% of harmful algal blooms in coastal waters, are competitive in low-phosphate environments. However, the specific acclimated phosphorus strategies to adapt to phosphorus deficiency in dinoflagellates, particularly through intracellular phosphorus metabolism, remain largely unknown. Comprehensive physiological, biochemical, and transcriptomic analyses were conducted to investigate intracellular phosphorus modulation in a model dinoflagellate, Prorocentrum shikokuense, with a specific focus on membrane lipid remodeling and autophagy in response to phosphorus deficiency. Under phosphorus deficiency, P. shikokuense exhibited a preference to spare phospholipids with nonphospholipids. The major phospholipid classes of phosphatidylcholine and phosphatidylethanolamine decreased in content, whereas the betaine lipid class of diacylglyceryl carboxyhydroxymethylcholine increased in content. Furthermore, under phosphorus deficiency, P. shikokuense induced autophagy as a mechanism to conserve and recycle cellular phosphorus resources. The present study highlights the effective modulation of intracellular phosphorus in P. shikokuense through membrane phospholipid remodeling and autophagy and contributes to a comprehensive understanding of the acclimation strategies to low-phosphorus conditions in dinoflagellates.

Systematic exploration of transcriptional responses of interspecies interaction between Karenia mikimotoi and Prorocentrum shikokuense.

Karenia mikimotoi and Prorocentrum shikokuense (also identified as P. donghaiense Lu and P. obtusidens Schiller) are two important harmful algal species which often form blooms in the coasts of China. Studies have shown that the allelopathy of K. mikimotoi and P. shikokuense plays an important role in inter-algal competition, though the underlying mechanisms remain largely unclear. Here, we observed reciprocal inhibitory effects between K. mikimotoi and P. shikokuense under co-cultures. Based on the reference sequences, we isolated RNA sequencing reads of K. mikimotoi and P. shikokuense from co-culture metatranscriptome, respectively. We found the genes involved in photosynthesis, carbon fixation, energy metabolism, nutrients absorption and assimilation were significantly up-regulated in K. mikimotoi after co-cultured with P. shikokuense. However, genes involved in DNA replication and cell cycle were significantly down-regulated. These results suggested that co-culture with P. shikokuense stimulated cell metabolism and nutrients competition activity of K. mikimotoi, and inhibited cell cycle. In contrast, genes involved in energy metabolism, cell cycle and nutrients uptake and assimilation were dramatically down-regulated in P. shikokuense under co-culture with K. mikimotoi, indicating that K. mikimotoi could highly affect the cellular activity of P. shikokuense. In addition, the expression of PLA2G12 (Group XII secretory phospholipase A2) that can catalyze the accumulation of linoleic acid or linolenic acid, and nitrate reductase that may be involved in nitric oxide production were significantly increased in K. mikimotoi, suggesting that PLA2G12 and nitrate reductase may play important roles in the allelopathy of K. mikimotoi. Our findings shed new light on the interspecies competition between K. mikimotoi and P. shikokuense, and provide a novel strategy for studying interspecific competition in complex systems.

High throughput sequencing of 18S rRNA and its gene to characterize a Prorocentrum shikokuense (Dinophyceae) bloom.

Comparing phytoplankton non-bloom and bloom communities using rRNA and its coding gene can help understand the shift of dominant species and its driving processes (e.g., intrinsic growth or grazing). Here we conducted high-throughput sequencing of 18S rRNA and its coding gene for studying non-bloom and bloom plankton communities in East China Sea. The non-bloom community was dominated by diatoms whereas during the bloom it was dominated by the dinoflagellate Prorocentrum shikokuense (formerly P. donghaiense). P. shikokuense rRNA abundance and rRNA:rRNA gene ratio both increased markedly in the bloom, indicating that the bloom arose from active growth. In contrast, some non-bloom species showed low DNA abundances during the bloom albeit high rRNA:rRNA gene ratios, suggesting that cell loss processes such as grazing might have prevented these species from blooming or that these species might be at an early stage of bloom development. Furthermore, Pearson's correlation analysis showed that dinoflagellate abundance was positively correlated with temperature and negatively related to dissolved inorganic phosphate (DIP) concentrations, suggesting warm and DIP-poor environment as a niche space for P. shikokuense. Our results demonstrate the usefulness of combined analysis of rRNA and its gene in characterizing phytoplankton bloom development to shed light on the complex phytoplankton dynamics and regulating mechanisms in the course of bloom development.

First bloom event of the small dinoflagellate Prorocentrum shikokuense in the Mediterranean Sea: cryptogenic or introduced?

A bloom of putatively non-indigenous species (NIS) Prorocentrum shikokuense was detected for the first time in the Mediterranean Sea at the Brindisi harbor (Southern Adriatic Sea) on September 2016, in the context of EU Marine Strategy Framework Directive monitoring in the ports. This species is usually observed in the East China Sea and Japanese and Korean waters. In the Brindisi harbor this dinoflagellate reached the concentration 105 cell/L and represented from 30 to 50% of the total phytoplankton population. Besides this event, Prorocentrum shikokuense has not been found blooming until today in Mediterranean waters. This study suggests the necessity to improve the monitoring surveys in areas that are known vulnerable systems to alien and invasive species, such as ports.