INFLUENCE OF TEMPERATURE AND PHOTOPERIOD ON SURVIVAL AND GROWTH OF NORTH-EAST ATLANTIC ISOLATES OF PHYCODRYS RUBENS (RHODOPHYTA) FROM DIFFERENT LATITUDES

The influence of varying temperatures (from -1.5 degrees C to 23 degre es C) and photoperiod (from <0 : (24)over bar> to <24 : (0)over bar> h , light : dark) on growth and survival of eight isolates of the cool t emperature red alga Phycodrys rubens from different latitudes along th e NE Atlantic coasts were investigated. The isolates belong to two gen etic groups: a 'North Sea/Baltic Sea' group with isolates from Helgola nd, the Kattegat and Kiel, and an 'Eastern Atlantic' group with isolat es from the Barents Sea, the Kattegat, Ireland and Brittany. There was a high degree of overall similarity in the responses of the isolates but the southernmost isolates of both genetic groups were slightly mor e tolerant to high temperatures (18 degrees C). All isolates died at 2 0 degrees C after 1-4 weeks and survived low temperatures (-1.5 degree s C) without obvious damage. All isolates had optimum temperatures for growth in the range of 5 to 10 (-15) degrees C but the southernmost i solates of both groups (Brittany isolate of 'Eastern Atlantic' group a nd Kiel isolate from 'North Sea/Baltic Sea' group) also grew well at 1 8 degrees C. At 15 (and 18) degrees C maximum growth rates were higher at a photoperiod of L : (D) over bar = 16 : (8) over bar h than at L : (D) over bar = 10 : <(14)over bar> h. We conclude that a slight incr ease in tolerance to high temperatures has evolved independently at so uthern localities in both genetic groups. Tolerance to high temperatur es of the northernmost isolates and growth potential at low temperatur es of the southernmost isolates were far in excess of temperature extr emes experienced in nature. Most isolates have the capacity to grow fo r most of the year under local temperature regimes. This is in conflic t with the marked spring peak in growth reported for this species.