Reply to Dees et al.: Ocean warming promotes species-specific increases in the cellular growth rates of harmful algal blooms

Aerosolized algal bloom toxins are not inert

Harmful algal blooms (HABs) are projected to become increasingly prevalent, extending over longer periods and wider geographic regions due to the warming surface ocean water and other environmental factors, including but not limited to nutrient concentrations and runoff for marine and freshwater environments. Incidents of respiratory distress linked to the inhalation of marine aerosols containing HAB toxins have been documented, though the risk is typically associated with the original toxins. However, aerosolized toxins in micrometer and submicrometer particles are vulnerable to atmospheric processing. This processing can potentially degrade HAB toxins and produce byproducts with varying potencies compared to the parent toxins. The inhalation of aerosolized HAB toxins, especially in conjunction with co-morbid factors such as exposure to air pollutants from increased commercial activities in ports, may represent a significant exposure pathway for a considerable portion of the global population. Understanding the chemistry behind the transformation of these toxins can enhance public protection by improving the existing HAB alert systems.

Nutrient Alteration Drives the Impacts of Seawater Acidification on the Bloom-Forming Dinoflagellate Karenia mikimotoi

Seawater acidification and nutrient alteration are two dominant environmental factors in coastal environments that influence the dynamics and succession of marine microalgae. However, the impacts of their combination have seldom been recorded. A simulated experimental system was set up to mimic the effects of elevated acidification on a bloom-forming dinoflagellate, Karenia mikimotoi, exposed to different nutrient conditions, and the possible mechanism was discussed. The results showed that acidification at different pH levels of 7.6 or 7.4 significantly influenced microalgal growth (p<0.05) compared with the control at pH 8.0. Mitochondria, the key sites of aerobic respiration and energy production, were impaired in a pH-dependent manner, and a simultaneous alteration of reactive oxygen species (ROS) production occurred. Cytochrome c oxidase (COX) and citrate synthase (CS), two mitochondrial metabolism-related enzymes, were actively induced with acidification exposure, suggesting the involvement of the mitochondrial pathway in coping with acidification. Moreover, different nutrient statuses indicated by various N:P ratios of 7:1 (N limitation) and 52:1 (P limitation) dramatically altered the impacts of acidification compared with those exposed to an N:P ratio of 17:1 (control), microalgal growth at pH 7.4 was obviously accelerated with the elevation of the nutrient ratio compared to that at pH 8.1 (p<0.05), and nutrient limitations seemed beneficial for growth in acidifying conditions. The production of alkaline phosphatase (AP) and acid phosphatase (AcP), an effective index indicating the microalgal growth status, significantly increased at the same time (p<0.05), which further supported this speculation. However, nitrate reductase (NR) was slightly inhibited. Hemolytic toxin production showed an obvious increase as the N:P ratio increased when exposed to acidification. Taken together, mitochondrial metabolism was suspected to be involved in the process of coping with acidification, and nutrient alterations, especially P limitation, could effectively alleviate the negative impacts induced by acidification. The obtained results might be a possible explanation for the competitive fitness of K. mikimotoi during bloom development.

Effects of ocean acidification and phosphate limitation on physiology and toxicity of the dinoflagellate Karenia mikimotoi

This work demonstrated a 10-day batch culture experiment to test the physiology and toxicity of harmful dinoflagellate Karenia mikimotoi in response to ocean acidification (OA) under two different phosphate concentrations. Cells were previously acclimated in OA (pH = 7.8 and CO₂ = 1100 μatm) condition for about three months before testing the responses of K. mikimotoi cells to a two-factorial combinations experimentation. This work measured the variation in physiological parameters (growth, rETR) and toxicity (hemolytic activity and its toxicity to zebrafish embryos) in four treatments, representing two factorial combinations of CO₂ (450 and 1100 μatm) and phosphate concentration (37.75 and 4.67 umol l^-1). Results: OA stimulated the faster growth, and the highest rETR_max in high phosphate (HP) treatment, low phosphate (LP) and a combination of high CO₂ and low phosphate (HC*LP) inhibited the growth and E_k in comparison to low CO₂*high phosphate (LCHP) treatment. The embryotoxicity of K. mikimotoi cells enhanced in all high CO₂ (HC) conditions irrespective of phosphate concentration, but the EC₅₀ of hemolytic activity increased in all high CO₂ (HC) and low phosphate (LP) treatments in comparison of LCHP. Ocean acidification (high CO₂ and lower pH) was probably the main factor that affected the rETR_max, hemolytic activity and embryotoxicity, but low phosphate was the main factor that affected the growth, α, and E_k. There were significant interactive effects of OA and low phosphate (LP) on growth, rETR_max, and hemolytic activity, but there were no significant effects on α, E_k, and embryotoxicity. If these results are extrapolated to the aquatic environment, it can be hypothesized that the K. mikimotoi cells were impacted significantly by future changing ocean (e.g., ocean acidification and nutrient stoichiometry).