Ultrastructure of cell death in Prorocentrum redfieldii, a bloom-forming dinoflagellate

Abstract

Phytoplankton are primary producers in oceanic food chains and are very important in the ocean’s biogeochemical cycles (Azam, 1998; Falkowski, 1994; Wilken et al., 2018). During nutrient replete conditions, population growth in phytoplankton communities can surpass external loss factors (e.g. grazing, sinking and lysis) leading to bloom formation (Brussaard et al., 1995; Choi et al., 2017). In recent years there has been an increase in the occurrence of HABs, which are exacerbated by anthropogenic inputs. The involvement of programmed cell death (PCD) as a loss factor in collapsing blooms has become an area of interest in phytoplankton ecology. The study examined, for the first time, the ultrastructure of cell mortality in a collapsing anaerobic HAB of Prorocentrum redfieldii, an armoured dinoflagellate and ascertain whether there was structural evidence of PCD as a driver of the bloom collapse. These ultrastructural responses were compared with those of replete cultures subjected to stressors known to induce PCD, i.e., macronutrient deprivation nitrogen (NO3-) and phosphorus (PO4 3-) and heat stress. Biochemical assays associated with PCD, such as caspase-like activity, ROS and cell viability staining as well as DNA laddering, were carried out. The findings of this study established that cells dying from a collapsing bloom exhibited morphological characteristics like extensive vacuolation, ER derived double membrane structures and lytic vacuoles, which resemble the process of autophagy. There was an upregulation of the endomembrane system, and interaction between the endomembrane and autophagic systems. Structural alterations were observed in organelles known to be sources of ROS like the mitochondria and chloroplasts. Similar changes in ultrastructure were observed in the nutrient deprived and mild heat-shocked cells (32 ºC). Biochemical activity further revealed an overproduction of ROS, loss of cell viability and no caspase activity in the dying cells. The study concluded that P. redfieldii cells in a collapsing bloom were dying a caspase independent autophagic cell death mediated by oxidative stress.