A strategy for the proliferation of Ulva prolifera, main causative species of green tides, with formation of sporangia by fragmentation

Abstract

Ulva prolifera, a common green seaweed, is one of the causative species of green tides that occurred frequently along the shores of Qingdao in 2008 and had detrimental effects on the preparations for the 2008 Beijing Olympic Games sailing competition, since more than 30 percent of the area of the games was invaded. In view of the rapid accumulation of the vast biomass of floating U. prolifera in green tides, we investigated the formation of sporangia in disks of different diameters excised from U. prolifera, changes of the photosynthetic properties of cells during sporangia formation, and development of spores. The results suggested that disks less than 1.00 mm in diameter were optimal for the formation of sporangia, but there was a small amount of spore release in these. The highest percentage of area of spore release occurred in disks that were 2.50 mm in diameter. In contrast, sporangia were formed only at the cut edges of larger disks (3.00 mm, 3.50 mm, and 4.00 mm in diameter). Additionally, the majority of spores liberated from the disks appeared vigorous and developed successfully into new individuals. These results implied that fragments of the appropriate size from the U. prolifera thalli broken by a variety of factors via producing spores gave rise to the rapid proliferation of the seaweed under field conditions, which may be one of the most important factors to the rapid accumulation of the vast biomass of U. prolifera in the green tide that occurred in Qingdao, 2008.

Figure 1. The germination in situ of the spores of U. prolifera floating along the shores of Qingdao in the summer of 2008.

The scale bars represent 10 µm.

Figure 2. The green tide that occurred along the shores of Qingdao in the summer of 2008.

Figure 3. The phases of the formation of sporangia in excised disks from U. prolifera.

(A) newly excised disks; (B) chloroplasts dispersed; (C) chloroplasts aggregating in the center of cells; (D) vacuoles swollen; (E) spores within the sporangia; (F) partial spores released from sporangia. The scale bars represent 10 µm.

Figure 4. The values of (A) the effective PS II quantum yield [Y (II)] and (B) the optimum PS II quantum yield [F_v/F_m] of each phase of the formation of sporangia.

1, Newly excised disks; 2, chloroplast dispersed; 3, chloroplasts aggregated; 4, vacuoles swollen; 5, pyriform spores within sporangia. The values of (C) Y (II) and (D) F_v/F_m of the vegetative cells (1) and that of the mature sporangia (2) in the same excised disks. In (A) and (B), different letters (a, b, c) represent significant differences between the phases of the formation of sporangia (p<0.05, ANOVA, followed by Tukey test for post-hoc comparisons). In (C) and (D), asterisks represent significant differences between the vegetative cells and the mature sporangia (p<0.05, two-sample t-test).

Figure 5. The percentage of area of sporangia and that of spore release to the whole disks.

Different letters (a, b, c) represent significant differences between the sizes of disks treatments (p<0.05, ANOVA, followed by Tukey test for post-hoc comparisons).

Figure 6. The phases of the early development of U. prolifera.

The scale bars represent (A–J) 10 µm and (K–N) 20 µm.

Figure 7. The values of Y (II) and F_v/F_m of the phases of the development of U. prolifera.

Asterisks represent significant differences between the apical and the rhizoid in the same sporeling (p<0.05, two-sample t-test).