TEMPERATURE ECOTYPES AND BIOGEOGRAPHY OF ACROSIPHONIALES (CHLOROPHYTA) WITH ARCTIC-ANTARCTIC DISJUNCT AND ARCTIC COLD-TEMPERATE DISTRIBUTIONS

The temperature requirements for growth and the upper survival tempera tures (USTs) of the Antarctic-Arctic disjunct green alga Acrosiphonia arcta and of the Arctic/cold-temperate A. sonderi (Acrosiphoniales) fr om several localities within their distribution areas were determined. Ecotypic variation with regard to growth optima as well as survival t emperatures was demonstrated in both species. While cold-temperate str ains had relatively high or optimal growth rates at 15-degrees-C, pola r isolates had very low rates at this temperature and showed growth op tima between 0 and 10-degrees-C. The UST of the polar isolates of A. a rcta is 22-degrees-C, i.e. slightly lower than those of the cold-tempe rate strains at 23-25-degrees-C. The cold-temperate isolate of A. sond eri survived 25-degrees-C, whereas Arctic strains had USTs of 22-24-de grees-C. The data indicate that changes in growth responses to tempera ture as well as small changes in UST can be achieved in relatively sho rt time periods of exposure to low temperatures (3 million years) as e xemplified in Arctic populations of A. arcta as well as A. sonderi. Th e UST of A. arcta (22-25-degrees-C) indicate the possibility of disper sal across the equator into the other hemisphere during the Pleistocen e lowering of seawater temperatures in the tropics. Growth, however, w ould not have been possible during passage across the tropics due to t he narrow temperature-growth window. Possible migration routes, season al development and the nature of geographical boundaries are discussed in relation to the data obtained for temperature requirements in the context of the present local temperature regimes.