PHYSIOLOGICAL COMPARISON OF THE ISOMORPHIC LIFE-HISTORY PHASES OF THE HIGH INTERTIDAL ALGA ENDOCLADIA-MURICATA (RHODOPHYTA)

The isomorphic phases of Endocladia muricata (Post. & Rupr.) J. Ag. were compared for photosynthetic and respiratory differences in response to a variety of environmental manipulations. Photosynthetic light response during submergence at 15-degrees-C and the pattern of respiratory recovery following prolonged emergence (3 h) at either 15-degrees or 30-degrees-C were similar between gametophytes and tetrasporophytes. The phases showed the same ability to photosynthesize and respire during emergence at each temperature tested (15-degrees 25-degrees, and 35-degrees-C fully hydrated thalli) and at various desiccation states (measured at 25-degrees-C only). Submerged rates of photosynthesis following prolonged emergence at 15-degrees and 30-degrees-C were, however, slightly greater (17%) for tetrasporophytes as compared to gametophytes. Regardless of the life history phase, plants incubated at 15-degrees-C during emergence recovered more completely than plants incubated at 30-degrees-C. Photosynthetic recovery after 1 h in plants incubated at 15-degrees-C often ''spiked'' and yielded rates as great as 185% of pretreatment rates. Increased photosynthetic rates during recovery were absent from the 30-degrees-C incubations. The initial photosynthetic recovery of plants collected from the upper limits of distribution was greater than that of plants collected from the lower limits. Recovered rates of respiration were highly variable over time. Respiration often exceeded pretreatment values more than threefold, and the elevated rates were sustained for 12 h. Photosynthesis and respiration in air were comparable to rates in seawater and varied slightly with increasing temperature. Photosynthetic and respiratory rates also decreased with increasing tissue water loss. Thus, only slight differences in physiological performance were observed between phases and individuals collected from different vertical positions. Metabolic differences were transient and apparent only under experimental conditions that modeled extreme environmental conditions.