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The effect of freezing on seaweed photosynthesis
The effect of freezing on photosynthesis was studied in a variety of brown and red seaweeds from the Gulf of Maine, USA. Photosynthesis in sublittoral fringe and rock pool species was adversely affected by a single 6 or 12 h exposure to -20 °C even after a 7 d recovery period in sea water at 5 °C, whereas most intertidal forms were unaffected by this treatment. Three h at -20 °C resulted in an immediate reduction in photosynthesis of most intertidal seaweeds, with the degree of inhibition corresponding to zonation on the shore. For example, photosynthesis in the upper shore fucoid Fucus spiralis was unaffected by 3 h at -20 °C, whereas that of the low-intertidal Fucus edentatus was reduced by 97 %. The percentage of frozen tissue water after 3 h at -20 °C was similar in all species, suggesting that differences in susceptibility to freezing are attributable to physiological tolerance rather than avoidance. Freezing intolerant species exhibited massive amino acid release on re-immersion in sea water following freezing. In contrast, amino acid release was much lower in freezing tolerant species. The release of amino acids is believed to be due to loss of plasmalemma integrity, suggesting that the freezing tolerance of seaweeds may be controlled by the plasmalemma. Overall, the results of this study suggest that freezing may play an important role in structuring sub-arctic and boreal intertidal communities. Freezing can operate in 2 ways: (1) as a disturbance excluding sub-littoral and rock pool species from the eulittoral and (2) as a physiological stress affecting relative competitive abilities of eulittoral forms via differential reduction in photosynthesis and hence growth.