Allelopathic effects of Margalefidinium polykrikoides on the growth of Pyrodinium bahamense in different nutrient concentrations

Co-occurrence of the dinoflagellates Margalefidinium polykrikoides, a known fish killer, and
the neurotoxic species Pyrodinium bahamense is commonly observed in the coastal waters of
Sabah, Malaysia. During most of these events, M. polykrikoides dominated the bloom, apparently
suppressing the growth of P. bahamense. To increase our understanding on the nutrient conditions
of this phenomenon, a study was conducted to explore the interaction between these species.
The specific aim was to document the allelopathic effects, if any, of M. polykrikoides on P.
bahamense when varying ratios of the two species were co-cultured under different nitrogen (N)
and phosphorus (P) concentrations. The bioassay experiments started with three cell abundance
proportions, which were 5:5 (500 cells mL-1 of each species, M. polykrikoides, and P. bahamense);
1:5 (100 cells mL-1 of M. polykrikoides and 500 cells mL-1 of P. bahamense); and 5:1 (500 cells
mL-1 of M. polykrikoides and 100 cells mL-1 of P. bahamense). Additionally, culture filtrates (10,
20 and 50 mL) from the late exponential phase of M. polykrikoides were added to 150 mL of P.
bahamense to determine if cell filtrates were allelopathic. Results indicate that M. polykrikoides
was allelopathic to P. bahamense when nutrients were abundant, but not when nutrients were
limiting or N was limiting relative P. The production of allelopathic compounds was supported
by abnormal morphological changes in P. bahamense when co-cultured with M. polykrikoides.
This capacity to suppress P. bahamense growth, combined with the inherently faster growth rate
of M. polykrikoides relative to P. bahamense can account for why M. polykrikoides forms nearly
monospecific blooms when nutrients are high. The filtration studies indicated the allelopathic
capacity of M. polykrikoides required direct cell contact or that the allelopathic compounds
degraded rapidly and were inactive when added to P. bahamense cultures. These results are
important in understanding the bloom mechanisms of these two harmful algal blooms (HABs)
species.