Growth and reproduction of the green macroalga Ulva prolifera

The aims of this study were to investigate the growth and reproductive strategies of Ulva prolifera that contribute to its omnipresence and annual blooms. In addition, the influence of the position in the alga, length and integrity, on growth, sporulation, regeneration and development of Ulva species was investigated. Growth, regeneration, and reproduction of segments and tissues of U. prolifera were studied systematically by shallow liquid cultivation under laboratory conditions. The results, observed with an inverted microscope, showed that the basal and mid segments could generate rhizoids at lower ends, while the upper ends developed into branches. However, the upper ends of small basal segments could generate rhizoids instead of branches. We concluded that the polarity of regeneration is not absolute, and 1.00 mm in length was critical to maintain polarity of regeneration. A number of cells in the basal tissue developed into sporangia. We also observed that the basal tissues could form rhizoids without thalli, or thalli without rhizoids. Similar to the basal segments, about 70% of mid segments generated rhizoids at their lower ends, and leaf-shaped branches at their upper ends. Many cells of mid segments transformed into sporangia. Cells in most mid tissues either died or developed into sporangia. Only 36% of mid tissues generated rhizoids and branches. Many basal and mid segments formed an air chamber, which is one of characteristics of rapid growth. Cells in the apical segments had an ability to produce and release spores. The lower ends of the apical segments had a tendency to generate rhizoids at the early stage of culture, but they disintegrated quickly because of the production and release of spores. The apical tissues of U. prolifera dispersed quickly, some cells transformed into sporangia, and others died. The spores released from segments and tissues could germinate intoseedlings immediately. Cells of small pieces of tissue could germinate directly into filamentous fronds. The results of data analysis indicated that growth rates were closely related to the initial length and the position in the thallus from which they came. The regeneration rate of segments and tissues was related to their position in the thallus and the integrality of the thallus. That is, growth and regeneration of Ulva segments had polarity. There were significant differences in regeneration rates between segments or tissues from the same position in the thallus. For basal