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released by lysing algae but rather being produced in the presence of intact cells. The activity of the soluble cellular contents was only 1 % compared to the activity of the whole sample (extract lysis (%) = 0.010 x [total lysis (%)] -0.015, n = 22, r2 = 0.930, P*").

Correlation of haemolysis with phytoplankton groups

Correlations with haemolysis were determined for the most abundant phytoplankton groups. Haemolysis for whole mesocosms samples correlated with total P. pouchetii numbers (lysis (%) = 0.001203 * [cell density ml-1 sample] + 3.43, n = 31, r2 = 0.674, P*"). For the non-motile P. pouchetii cell abundance, i.e., cells from colonies this regression equation was almost the same (% lysis = 0.001248 x [cell density ml-1 sample] + 3.63, n = 31, r2 = 0.677, P*", Fig. 4, Table 2). Interestingly, motile P. pouchetii numbers (Fig. 1) did not correlate with the haemolytic activity. This was most evident in M1 with low haemolytic activity and highest densities of motile cells (Table 1, Fig. 1). The relation between diatoms and haemolytic activity was negative because the haemolysis started just after the diatom bloom (P**). There was no correlation between the haemolytic activity and heterotrophic Xagellates or the small phototrophic Xagellates (mainly unidentified flagellates and a few small developing cells of Scrippsiella trochoidea with their distinctive chloroplasts, Table 2). There was, however, a positive correlation between the hae-molytic activity and the abundance of both the larger heterotophs (P*) as well as the larger phototrophs (P*). The abundance of these groups, however, were positively correlated with the non-motile Phaeocystis cells abundance (heterotrophs, P*** and phototrophs P, Table 2) which complicates assessment of which groups were responsible for the haemolytic activity. The phototrophs in Ml and M3 reached comparable densities while Ml had much lower haemolytic activity (Fig. 5a). In addition, in M2 the phototrophs displayed low densities in the second half of the mes-ocosm period while haemolytic activity was high. These observations suggest that the larger photo-trophs were not responsible for haemolytic activity. In all bags the larger heterotrophs were present during the diatom blooms and increased during the second half of the experiment, while there was hardly any haemolytic activity present during the diatom blooms and the P. pouchetii period in Ml (Fig. 5b). Therefore, the larger heterotrophs are unlikely candidates for haemolysis as well.

Dose response curves at different temperatures

Dose-response curves from dilution series of samples taken at March 24 from the three mesocosms showed a linear relationship between 10 and 70% lysis. For the whole data set the activity at 4°C was approximately half the activity measured at 15°C (value at 4°C = 0.44 x [value at 15°C] + 3.24, n = 21, r2 = 0.963, P*", Fig. 6).

Light effects

Aanesen et al. 1998 measured a light dependent negative effect of P. pouchetii on fish larvae. To

Fig. 4 Relation between the abundance of phytoplankton and the haemolytic activity measured in whole mesocosm samples. The regression line displayed is for non-motile P. pouchetii (i.e., cells present in colonies)

Fig. 4 Relation between the abundance of phytoplankton and the haemolytic activity measured in whole mesocosm samples. The regression line displayed is for non-motile P. pouchetii (i.e., cells present in colonies)

test possible influence of light intensity the hae-molytic activity of whole mesocosms samples was analyzed not only at 7 but also at 40 mmol photons m~2s_1 on experimental day 21. For the three mesocosms the haemolytic activity increased twofold at higher light conditions (13 and 28% lysis for M1, 24 and 57% for M2; 23 and 55% for M3,). A similar experiment also including a dark incubation was performed on day 24 (Fig. 7). There was no difference between the dark and standard light conditions. Similar to the experiment on 20 March, at 40 mmol photons m~2 s_1 the haemo-lytic activity was higher compared to 7 mmol photons m~2s_1, although not twice the amount observed in the previous experiment. The measurements at high light in this case could be an underestimation because they are higher than 70% where the response is no longer linear (Fig. 6, previous section). Extracts were also more

percentage of culture diluted in seawater

Fig. 6 Dose response curves for whole mesocosm samples taken at 24 March, 2003. Dilution series were measured at ambient temperature (4°C, dotted lines) and at standard experimental temperature (15°C, solid lines). Bars represent standard deviation percentage of culture diluted in seawater

Fig. 6 Dose response curves for whole mesocosm samples taken at 24 March, 2003. Dilution series were measured at ambient temperature (4°C, dotted lines) and at standard experimental temperature (15°C, solid lines). Bars represent standard deviation active at the high-light conditions (data not shown).

Discussion

Haemolytic activity was observed in samples during Phaeocystis pouchetii blooms in mesocosms. The activity measured using incubations of whole mesocosm samples mixed with blood suspensions was proportional with the amount of sample added. The haemolytic activity in the mesocosms correlated best with the density of non-motile P. pouchetii but also with the abundance of the larger phototrophs and heterotrophs. The last two were also positively related to P. pouchetii numbers. Biomass during the P. pouchetii bloom was dominated by non-motile P. pouchetii cells and

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