Bloom formation of the toxic raphidophyte Heterosigma akashiwo in response to ocean acidification

This master's thesis investigated the effects of acidified conditions on movement behaviors of the harmful algal bloom causing raphidophyte Heterosigma akashiwo. Individual movements of cells (i.e., swimming speed, turning rates, vertical velocity, declination, and diffusivity) were quantified using 3-D digital video analysis for a 6-h period and were compared among 3 pCO2 treatments of 280, 380, and 750 ppm reflecting ocean carbonate chemistry in pre-industrial, current, and year 2100 at 2 different pH-acclimation conditions. We discovered that an abrupt decrease in pH levels significantly induced downward swimming of H. akashiwo (all p < 0.001), by exhibiting faster net vertical velocity (-31, -46, -66 µm s-1 at 280, 380, 750 ppm, respectively) and an increase in downward moving fractions (68, 78, 87% at 280, 380, 750 ppm). However, pH-acclimation reduced this strong vertical downward bias, by increasing net vertical velocity (-23, -12, -47 µm s-1 at 280, 380, 750 ppm) and decreased downward swimming populations (59, 56, 75% at 280, 380, 750 ppm) regardless of pCO 2 levels. Pre-acclimation significantly increased swimming speed (144 to 150 um s-1) at all pCO2 levels (all p < 0.001) compared to swimming speed (110 to 119 um s-1) in pH-shock condition and resulted in 2.3 to 2.5 fold higher diffusivity (1.8×105 to 2.2×105 µm2 s-1) compared to diffusivity (8.0×104 to 8.7×10 5 µm2 s-1) in pH-shock. Either pCO2 alteration or acclimation had no effects on turning rates (38.7 to 44.8 deg s-1). Dispersal length of H. akashiwo bloom during 10-h implied a decreasing bloom dispersal in abrupt pH decreases but an increasing bloom dispersal in gradual pH decreases, emphasizing the importance of taking pH-acclimation status into account.