Dynamics of cell death across growth stages and the diel cycle in the dinoflagellate Karenia brevis

Diurnal-Rhythmic Relationships between Physiological Parameters and Photosynthesis- and Antioxidant-Enzyme Genes Expression in the Raphidophyte Chattonella marina Complex

Diurnal rhythms in physiological functions contribute to homeostasis in many organisms. Although relationships between molecular biology and diurnal rhythms have been well studied in model organisms like higher plants, those in harmful algal bloom species are poorly understood. Here we measured several physiological parameters and the expression patterns of photosynthesis-related and antioxidant-enzyme genes in the Chattonella marina complex to understand the biological meaning of diurnal rhythm. Under a light-dark cycle, Fv/Fm and expression of psbA, psbD, and 2-Cys prx showed significant increases in the light and decreases during the dark. These rhythms remained even under continuous dark conditions. DCMU suppressed the induction of psbA, psbD, and 2-Cys prx expression under both light regimes. Oxidative stress levels and H2O2 scavenging activities were relatively stable, and there was no significant correlation between H2O2 scavenging activities and antioxidant-enzyme gene expression. These results indicate that the Chattonella marina complex has developed mechanisms for efficient photosynthetic energy production in the light. Our results showed that this species has a diurnal rhythm and a biological clock. These phenomena are thought to contribute to the efficiency of physiological activities centered on photosynthesis and cell growth related to the diurnal vertical movement of this species.

Novel insights into toxin changes associated with the growth of Alexandrium pacificum: Revealing active toxin-secretion ability and toxin cell quota variation

Paralytic shellfish toxins (PSTs) are widely distributed globally and are produced by Alexandrium pacificum in marine system. However, the characteristics of toxins producing and secreting associated with growth phases are still unclear, especially whether A. pacificum has the ability to actively secrete PSTs is controversial. In this study, variation characteristics of intracellular and extracellular PSTs contents associated with A. pacificum growth phases were investigated thoroughly. The results showed that intracellular and extracellular PSTs contents increased sharply during the exponential phase. But during the stationary phase, the intracellular PSTs content increased by only 26 %, and the extracellular PSTs content did not increase significantly. Since the increase in extracellular PSTs content mainly occurred at the exponential phase, when most cells were living, we speculated that active PSTs secretion of living cells might be an important production pathway of extracellular toxins besides leakage from dead cells. Furthermore, toxin cell quota variation associated with the growth phase was analysed. In the exponential phase, the toxin cell quota first increased and then decreased, with a maximum of 19.02 ± 1.80 fmol/cell at 6 d. However, after entering the stationary phase, this value slowly increased again, suggesting that vigilance should be raised for the plateau of Alexandrium blooms. In addition, cells in the exponential phase mainly produced O-sulfated components such as GTX1&4, cells in the stationary phase mainly produced O-sulfate-free components such as GTX5. In this study, the toxigenic rules of A. pacificum were comprehensively uncovered, which provided theoretical guidance for the prevention and mitigation of A. pacificum blooms.

Algal cell viability assessment: The role of environmental factors in phytoplankton population dynamics

The viability of algal cells is one of the most fundamental issues in marine ecological research. In this work, a method was designed to identify algal cell viability based on digital holography and deep learning, which divided algal cells into three categories: active, weak, and dead cells. This method was applied to measure algal cells in surface waters of the East China Sea in spring, revealing about 4.34 %-23.29 % weak cells and 3.98 %-19.47 % dead cells. Levels of nitrate and chlorophyll a were the main factors affecting the viability of algal cells. Furthermore, algal viability changes during the heating and cooling were observed in laboratory experiments: high temperatures led to an increase in weak algal cells. This may provide an explanation for why most harmful algal blooms occur in warming months. This study provided a novel insight into how to identify the viability of algal cells and understand their significance in the ocean.

Cell death responses to acute high light mediated by non-photochemical quenching in the dinoflagellate Karenia brevis

Programmed cell death (PCD) can be induced in microalgae by many abiotic challenges via generation of reactive oxygen species (ROS). Marine phytoplankton live in a highly variable light environment, yet the potential for excess photosynthetically available radiation to trigger PCD has not been examined. On the other hand, photoprotective non-photochemical quenching (NPQ) is hypothesized to counteract intracellular ROS, potentially preventing cell death. The main objective of this study is to investigate high-light-induced death processes and their relationship with photosynthesis in bloom-forming dinoflagellate Karenia brevis. Here, we characterized the prevalence of ROS, caspase-like enzyme activity and cell death as well as photosynthetic status under acute irradiance of 500, 750 or 1000 µmol m-2 s-1. PCD only occurred at the largest light shift. Although depressed photosynthetic capacities and oxidative stress were apparent across the stress gradient, they did not necessarily lead to cell death. NPQ exhibited dose-dependent activation with increasing light stress, which enabled cells to resist or delay PCD. These results highlight the important role of the balance between ROS generation and NPQ activation on determining cell fates in Karenia under acute irradiance stress. This research also provides insights into potential survival strategies and mechanisms of cell loss under a changeable light environment.

The Effects of the Harmful Algal Bloom Species Karenia brevis on Survival of Red Porgy (Pagrus pagrus) Larvae

The harmful algal bloom species, Karenia brevis, forms annual, often intense blooms in the Gulf of Mexico, particularly along the west Florida shelf. Though the ability of K. brevis blooms to cause mass mortalities in juvenile fish are well documented, the direct effect of bloom concentrations on larval fish has not been studied extensively. To better understand the potential effect of K. brevis on larval fish survival, laboratory spawned red porgy (Pagrus pagrus) larvae from 4-26 days post-hatch were exposed to concentrations of K. brevis observed in the field for either 24 or 48 h. This species is representative of fish which spawn in regions of the Gulf of Mexico and whose larvae are epipelagic and may encounter K. brevis blooms. In this study, three different K. brevis strains varying in the amount of brevetoxin produced were tested. Larval survivorship was found to be inversely proportional to the amount of brevetoxin produced by each strain. The EC₅₀ value from the combined 24 h experiments was ~163,000 K. brevis cells L^-1, which corresponds to cell concentrations found in moderately dense blooms. Larval mortality also increased substantially in the 48 h versus 24 h exposure treatments. These findings indicate K. brevis blooms have the potential to contribute to natural mortality of fish larvae and further reduce inter-annual recruitment of fishery species whose stocks in the Gulf of Mexico may already be depleted.

A Novel Algicidal Bacterium and Its Effects against the Toxic Dinoflagellate Karenia mikimotoi (Dinophyceae)

The toxic dinoflagellate Karenia mikimotoi is a harmful algal bloom-forming species in coastal areas around the world. It produces ichthyotoxins and hemolytic toxins, with deleterious effects on marine ecosystems. In this study, the bacterium Pseudoalteromonas sp. FDHY-MZ2, with high algicidal efficiency against K. mikimotoi, was isolated from a bloom event. Based on the results, it completely lysed K. mikimotoi cells within 24 h 0.5% (vol/vol), with the algicidal activity of the supernatant of the bacterium culture. Algal cell wall fragmentation occurred, leading to cell death. There was a marked decline in various photochemical traits. When treated with the supernatant, cellulase, pheophorbide a oxygenase (PAO) and cyclin B genes were significantly increased, suggesting induced cell wall deterioration, chloroplast degradation and cell cycle regulation of K. mikimotoi cells. In addition, the expression levels of reactive oxygen species (ROS) scavenging gene was significantly inhibited, indicating that the ROS removal system was damaged. The bacterial culture was dried to obtain the spray-dried powder, which showed algicidal activity rates of 92.2 and 100% against a laboratory K. mikimotoi culture and a field microcosm of Karlodinium sp. bloom within 24 h with the addition of 0.04% mass fraction powder. Our results demonstrate that FDHY-MZ2 is a suitable strain for K. mikimotoi and Karlodinium sp. blooms management. In addition, this study provides a new strategy for the anthropogenic control of harmful algal bloom-forming species in situ. IMPORTANCE K. mikimotoi is a noxious algal bloom-forming species that cause damaging of the aquaculture industry and great financial losses. Bacterium with algicidal activity is an ideal agency to inhibit the growth of harmful algae. In this approach application, the bacterium with high algicidal activity is required and the final management material is ideal for easy-to-use. The algicidal characteristics are also needed to understand the effects of the bacterium for managing strategy exploration. In this study, we isolated a novel algicidal bacterium with extremely high lysis efficiency for K. mikimotoi. The algicidal characteristics of the bacterium as well as the chemical and molecular response of K. mikimotoi with the strain challenge were examined. Finally, the algicidal powder was explored for application. The results demonstrate that FDHY-MZ2 is suitable for K. mikimotoi and Karlodinium sp. blooms controlling, and this study provides a new strategy for algicidal bacterium application.