Temporal variation of caspase 3-like protein activity in cultures of the harmful dinoflagellates Karenia brevis and Karenia mikimotoi

To better understand the role of caspase-like proteins in the cellular activities of harmful dinoflagellates, cultures of Karenia brevis and Karenia mikimotoi were monitored over a 56-day period. The cellular abundance, maximum photochemical efficiency of photosystem II (Fv/Fm), percentage of dead cells (revealed by SYTOX-labelling of nuclei of cells with a compromised membrane) and the variation in caspase 3-like protein activity were monitored every 7 days. During the cultures’ late decline phase, DNA degradation was also confirmed in Karenia cells using a terminal deoxynucletidyl transferase-mediated dUTP nick end labelling assay. Karenia brevis reached its maximal cellular abundance after 3 weeks then declined to its lowest value on Day 56. In cells of K. brevis the caspase 3-like activity likely served some housekeeping functions as demonstrated by the constitutive level of activity observed throughout the whole experiment ( 3000– 6664 RFU/h/mg protein). The increase in activity which occurred on Days 21 and 28 ( 38 000 RFU/h/mg protein on Day 28) coincided with the appearance cyst-like structures in the cultures and suggests a role in the encystment progression of K. brevis. For K. mikimotoi, the highest cellular abundance was reached on Day 42 after which it declined to reach its lowest value, along with the highest percentage of dead cells, on Day 56. The caspase 3-like protein activity (,1300 RFU/h/mg protein) was elevated at the beginning of the experiment (on Days 0, 7 and 14) and just before the decline in cellular abundance (on Days 35 and 42) and likely played some housekeeping functions as well as a role in the death of the cells. This study demonstrates the involvement of caspase 3-like protein activity in a variety of processes in cells of the harmful algae K. brevis and K. mikimotoi.