Temperature ecotypes and biogeography of Acrosiphoniales (Chlorophyta) with Arctic-Antarctic disjunct and Arctic/cold-temperature distributions

The temperature requirements for growth and the upper survival temperatures (USTs) of the Antarctic-Arctic disjunct green alga Acrosiphonia arcta and of the Arctic/cold-temperature A. sonderi (Acrosiphoniales) from several localities within their distribution areas were determined. Ecotypic variation with regard to growth optima as well as survival temperatures was demonstrated in both species. While cold-temperature strains had relatively high or optimal growth rates at 15°C, polar isolates had very low rates at this temperature and showed growth optima between 0 and 10°C. The UST of the polar isolates of A. arcta is 22°C, i.e. slightly lower than those of the cold-temperate strains at 23–25°C. The cold-temperate isolate of A. sonderi survived 25°C, whereas Arctic strains had USTs of 22–24°C. The data indicate that changes in growth responses to temperature as well as small changes in UST can be achieved in relatively short time periods of exposure to low temperatures (3 million years) as exemplified in Arctic populations of A. arcta as well as A. sonderi. The UST of A. arcta (22–25°C) indicate the possibility of dispersal across the equator into the other hemisphere during the Pleistocene lowering of seawater temperatures in the tropics. Growth, however, would not have been possible during passage across the tropics due to the narrow temperature-growth window. Possible migration routes, seasonal 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.