Temporal variation of caspase 3-like protein activity in cultures of the harmful dinoflagellates Karenia brevis and Karenia mikimotoi.
Résumé
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-d period. The cellular abundance, maximum photochemical efficiency of photosystem II (Fv/Fm), percentage of dead cells (revealed by SYTOX-labelling of nucleus of cells with a compromised membrane) and the variation in caspase 3-like protein activity were monitored every seven days. During the cultures' late decline phase, DNA degradation was also confirmed in Karenia cells using a TUNEL assay. Karenia brevis reached its maximal cellular abundance after three weeks then declined to its lowest value on day 56. The caspase 3-like activity likely served some housekeeping functions as demonstrated by the constitutive level of activity observed for this species (~ 3,000 to 6,664 RFU/h/mgprotein). The significant increase in activity on days 21 and 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 on day 56. The caspase 3-like protein activity (< 1,300 RFU/h/mgprotein) was elevated at the beginning of the experiment and just before the decline in cellular abundance and likely played some housekeeping functions as well as a role in the death of the cells. This study demonstrates the implication of caspase-3 like protein activity in a variety of processes in cells of harmful K. brevis and K. mikimotoi.
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