Insights into Stress-Induced Death Processes during Aging in the Marine Bloom-Forming Dinoflagellate Karenia brevis

Harmful algal blooms (HABs) globally affect marine ecosystems and human health. Significant attention has been paid to understanding the initiation processes of HABs, while much less is known of the mechanisms causing cell demise and bloom decline. Recent evidence reveals that programmed cell death (PCD) can be a possible pathway for HAB termination. However, it is still not clear how PCD expression varies with the growth of marine phytoplankton. In order to characterize how susceptibility to stress-induced cell death changes with the growth of marine harmful algae, we examined the prevalence of PCD markers during 48 h after the addition of H2O2 in dinoflagellate Karenia brevis cultures at the early-log, mid-log and stationary phase. The results show that acute susceptibility to cell death (before time 0.5 h) increased as cultures aged, reflecting a chronological decrease in stress acclimation abilities. However, the youngest cultures showed strong PCD expression and the fastest overall rate of cell loss within the first 24 h, hypothesized to result from the cell-density-dependent H2O2 detoxifying process. This research highlights the existence of PCD in HAB species and that aging marine phytoplankton are more susceptible to exogenous stress, which agrees with previous observations of significant PCD during bloom decline.