Photosynthetic responses of Florida seaweeds to light and temperature: a physiological survey

The photosynthetic responses of 37 tropical seaweeds (14 Chlorophyceae,S Phaeophyceae and 18 Rhodophyceae) were measured in a Gilson Warburg Apparatus under a variety of light and temperature regimes. The brown algae Padina vickersiae and Sporochnus pedunculatus exhibited the lowest saturation light intensity (263 ILE/m2/sec), while five green algae (Acetabularia crenulata. Cladophora coelothrix. Dictyosphaeria cavernosa. Monostrorna oxysperrnurn and Codiurn repens) had the highest light optima (3,843-4,258ILE/m2/sec). Overall, the Chlorophyceae exhibited the broadest range oflight optima; in contrast, the Phaeophyceae primarily had low light optima, while several Rhodophyceae had higher light optima. The thermal optima for 34 seaweeds ranged from 15-30°C. Catoglossa leprierii, Botryocladia occidentalis. Codiurn taylorii, Soliera tenera and Codiurn intertexturn exhibited relatively broad thermal optima, with C. leprierii having the most eurythermal response. The Chlorophyceae exhibited thermal optima between IS-30°C, the Phaeophyceae between IS-27°C, and most Rhodophyceae between] 8-24°C. Few taxa, except for Cladophora coelothrix and Dictyosphaeria cavernosa, had broad physiological tolerances to both high temperature and light regimes. Overall, the Phaeophyceae exhibited the most restricted temperature and light optima, while the Chlorophyceae and Rhodophyceae exhibited broader tolerances. Until recently there have been few comparative physiological investigations of Horida seaweeds. Previous studies of photosynthetic and respiratory responses of Horida seaweeds have been concerned with the estuarine intertidal red alga Bostrychia binderi (Dawes et a1., 1978; Durako and Dawes, 1980; Hoffman and Dawes, 1980; Davis and Dawes, 1981), the euryhaline subtidal red algae Graci/aria tikvahiae (Lapointe et a1., 1984) and Hypnea musciformis (Dawes et al., 1976; Durako and Dawes, 1980) as well as a few open coastal species, including the brown alga Sargassum (Prince, 1980), the green alga Batophora oerstedii (Morrison, 1984) and the red alga Eucheuma (Mathieson and Dawes, 1974; Moon and Dawes, 1976). Overall, the photosynthetic responses of these seaweeds show a broad tolerance to light and temperature, comparable to northern intertidal and shallow subtidal species (Stocker and Holdheide, 1938; Kanwisher, 1966; Mathieson and Burns, 1971; Mathieson and Norall, 1975a; 1975b; Brinkhuis et al., 1976). Even so the limited number of Horida seaweeds studied and the varied techniques employed make it difficult to generalize. The present study was initiated in order to compare the photosynthetic responses to light and temperature of a number of subtropical and tropical species from Horida. Of particular interest was whether the physiological responses of the plants could be correlated with their known distribution (both horizontal and vertical) and/or seasonal occurrence. METHODS AND MATERIALS Most of the samples for this study were collected during a series of autecologica] and floristic studies of Florida marine algae (Dawes et aI., ]974a; ]974b; Mathieson and Dawes, ]974; ]975); the remainder were collected during the same period (197 ]-]972) on the west coast of Florida (Table I). After being collected, the plants were maintained in an ice chest and were transported to the laboratory within a 0.5-7.0 h of collection. Standard size sections (i.e., 2.5-3.0 cm) were cut from terminal frond portions