Long-term unialgal seaweed cultivation in artificial seawater without water change. I. Laboratory investigations of Ulva

Unialgal stock cultures for seeding the tank water can be used for the mass cultivation of seaweeds in artificial seawater, as in microalgal mass cultivation. High costs for artificial sea salt mixtures and distilled water would be required, however, to use artificial seawater in big tanks for commercial purposes, but frequent medium changes may not be required, since the major ions contained in artificial seawater, such as sodium, will barely be utilised by the cultured algae during long periods of time, even over years. Similar growth rates were obtained at the start and end of an experiment conducted with Ulva pseudocurvata cultivated unialgally for two years without water change, at a low algal density and with weekly addition of Provasoli Enrichment. Growth in this experiment was increasingly impeded due to gamete swarming events causing dark greenish water every few weeks. The experimental water had then to be boiled in order to kill the gametes. In a second experiment, this time performed at a high algal density, maximum yield values were 20–30 g fresh weight m−2 d−1, both for 2-year-old and newly prepared artificial seawater. These results verified the hypothesis that long-term Ulva cultivation in artificial seawater is possible without frequent medium changes.