Nutrient stimulation of spore settlement in the kelpsPterygophora californica andMacrocystis pyrifera

FertilePterygophora californica Rupr. andMacrocystis pyrifera (L.) C.Ag. were collected in California, USA, from 1987 to 1989. Settlement activity of the spores was stimulated by nutrients, but this was not constant over time. Nutrients had no effect on the settlement ofP. californica spores between 2 and 18 h after sunrise, but settlement activity was stimulated by a nutrient mixture between 20 and 24 h after sunrise (14 to 18 h after release). Settlement activity inM. pyrifera spores was unaffected at 2 to 3 h after release, but settlement activity was significantly stimulated from 5 to 12 h after release. A variety of individual nutrients significantly stimulated settlement in 8 to 9 h oldM. pyrifera spores: ammonium, nitrate, glycine, phosphate, manganese-EDTA (18µM), borate, ferrous iron-EDTA, and ferric iron-EDTA. Spores also settled readily in unenriched artificial seawater, and nutrient-stimulated settlement rates were usually 150% of the unenriched control levels. Neither EDTA alone, cobalt-EDTA, nor manganese-EDTA (2µM) had significant effects onM. pyrifera spore settlement. The effects of time and of several individual nutrients on spore settlement activity are different from previously reported chemotactic effects of nutrients onP. californica andM. pyrifera spores. It is suggested that nutrient settlement-stimulation is mechanistically different from nutrient chemotaxis. However, like chemotaxis, settlement stimulation is probably an adaptation which increases the likelihood of spore settlement in microhabitats suitable for subsequent growth and reproduction of gametophytes.