Ecological Growth Strategies in the Seaweeds Gracilaria foliifera (Rhodophyceae) and Ulva sp. (Chlorophyceae): Photosynthesis and Antenna Composition

Observations have been made on photosynthesis (oxygen evolution) and seasonal fluctuations in antenna pigments in 2 seaweeds which CO-occur in the vicinity of Beaufort, North Carolina, USA. Gracilaria foljifera (Rhodophyceae) and Ulva sp. (Chlorophyceae) were grown in outdoor continuous-flow cultures a t ambient incident light (I,) and .13I,. Pigment contents and accessory pigment: chlorophyll a ratios were higher at ,131, than at I,. Total pigment levels were correlated with soluble N in seaweed tissue. During the spring/summer growing season, pigment levels were low and peaks i n pigment content followed nutrient pulses in the ambient seawater. Pigment contents in both Species were higher in winter In G. foliifera, the R-phycoerythrin: chl a ratio was highest in fall-winter and lowest in summer. The higher growth rates achieved by Ulva sp. reflected the higher rates of photosynthesis measured in this species. Photosynthesis-light curves showed that Ulva sp. had a higher photosynthetic capacity (P,, = 430 ymol O2 evolved g dry wt-' h-') and initial slope (in shadeacclimated plants) than G. folijfera (P,, = 160 pm01 0, evolved g dry wt-' h-'). Increased pigment contents in shade plants of both species resulted in enhanced photosynthetic performance at subsaturating light intensities. It appears that the effect of transient pigment increases in the summer was to increase P, , , temporarily while, in winter, the effect was to limit the decrease in integrated net photosynthesis in the face of decreased light and temperature.