TEMPERATURE ADAPTATION IN STRAINS OF THE AMPHI-EQUATORIAL GREEN-ALGA UROSPORA-PENICILLIFORMIS (ACROSIPHONIALES) - BIOGEOGRAPHICAL IMPLICATIONS

The temperature requirements for growth and the upper survival tempera tures (UST's) of the amphi-equatorial green alga Urospora penicillifor mis collected from several localities within its distribution area bet ween 1986 and 1991 were determined. Ecotypic variation, both with rega rd to growth ranges and optima and to survival temperatures, was demon strated. In the polar strains of U. penicilliformis, temperature growt h ranges were narrower and the growth optima and UST's were at lower t emperatures compared to cold-temperate strains. In particular, the pol ar strains grew between 0 and 15 degrees C with optimal growth at 0 or 5 degrees C, whereas the cold-temperate isolates grew between 0 and ( 15) 20 degrees C with almost equal growth rates or a growth optimum be tween 5 and 15 degrees C. The Arctic strains survived 23 to 24 degrees C, and the Antarctic isolate only 19 degrees C, while the UST's of th e cold-temperate isolates were between (24) 25 to 26 degrees C. The da ta strongly indicate that a cold water history of ca. 3 million yr in the Arctic can be sufficient for changes in the temperature growth ran ges and optima as well as for small changes of UST as shown in the Arc tic populations of U. penicilliformis. For stronger reduction of upper survival temperatures, longer time periods are necessary as exemplifi ed in the isolate from Antarctica, where low temperatures have existed for at least 14 million yr. The significantly lower UST of the Antarc tic strain, points to an early contact of the alga with the cold water of the Antarctic region and may indicate an origin of U. penicillifor mis in the Southern Hemisphere. The UST's of the cold-temperate isolat es (24 to 26 degrees C) would have allowed a migration across the equa tor during Pleistocene lowerings of the seawater temperatures in the t ropics. Growth, however, would not have been possible during the passa ge across the equator due to the narrow temperature-growth window. The nature of the geographical boundaries and the control of seasonal dev elopment of U. penicilliformis by the temperature conditions in the va rious geographical regions are discussed in relation to the present lo cal temperature regime.