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

Abstract

Observations have been made on photosynthesis (oxygen evolution) and seasonal fluctua-tions 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 shade-acclimated 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 sub-saturating 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.