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 degrees C) and photoperiod (from to h, light : dark) on growth and survival of eight isolates of the cool temperature red alga Phycodrys rubens from different latitudes along the NE Atlantic coasts were investigated. The isolates belong to two genetic groups: a 'North Sea/Baltic Sea' group with isolates from Helgoland, the Kattegat and Kiel, and an 'Eastern Atlantic' group with isolates 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 more tolerant to high temperatures (18 degrees C). All isolates died at 20 degrees C after 1-4 weeks and survived low temperatures (-1.5 degrees C) without obvious damage. All isolates had optimum temperatures for growth in the range of 5 to 10 (-15) degrees C but the southernmost isolates of both groups (Brittany isolate of 'Eastern Atlantic' group and Kiel isolate from 'North Sea/Baltic Sea' group) also grew well at 18 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 : h. We conclude that a slight increase in tolerance to high temperatures has evolved independently at southern localities in both genetic groups. Tolerance to high temperatures of the northernmost isolates and growth potential at low temperatures of the southernmost isolates were far in excess of temperature extremes experienced in nature. Most isolates have the capacity to grow for most of the year under local temperature regimes. This is in conflict with the marked spring peak in growth reported for this species.