PHOTOAUTOTROPHIC METABOLISM OF CAULERPA-TAXIFOLIA (CHLOROPHYTA) IN THE NW-MEDITERRANEAN

Oxygen production and consumption by the green alga Caulerpa taxifolia was measured in the NW Mediterranean at Monaco using a submersible re spirometer. Photosynthesis versus irradiance (P-I) curves were constru cted for populations at depths of 10, 15 and 25 m in summer and 10 m i n winter. C. taxifolia did not exhibit many of the photoadaptive respo nses to bathymetric changes in irradiance that have been reported in o ther algae. The initial slope of the P-I curve (alpha) was inversely p roportionate to depth; the irradiance required for respiratory compens ation (I-c), the concentrations of chlorophylls a and b and their rati os (chl a/b) did not vary significantly with depth. Changes in the rat e of dark respiration (-R), the gross and net photosynthetic capacitie s (P-m(g), P-m(n)), alpha, and the irradiance required for the onset o f saturation (I-k) were greater between 15 and 25 m than between 10 an d 15 m, despite the change in irradiance being smaller. P-m(n) decreas ed from 119 to 34 mu mol O-2 g(-1) dry wt h(-1) between 10 and 25 m. N et 24 h productivity (P-d(n)), given optimum atmospheric and ocean tra nsparency, was estimated to decrease from 790 to 89 mu mol O-2 g(-1) d ry wt d(-1) between 10 and 25 m indicating a maximum photoautotrophic growth Limit of 29 m in summer. At 10 m in winter, -R, I-c, I-k and ch l a/b were lower than in summer, P-m(g) and P-m(n) were similar, and t he concentrations of chlorophylls a and b were higher, indicating adju stment of the photosynthetic apparatus to seasonal changes in light an d/or temperature. Maximum P-d(n) was estimated to be 436 mu mol O-2 g( -1) dry wt d(-1). By adjusting the y-intercept of the summer bathymetr ic model to fit this rate, a maximum photoautotrophic growth limit of 24 m was indicated in winter. Although these theoretical photoautotrop hic limits reasonably correlate with the distribution of dense populat ions of C. taxifolia at Monaco, they are greatly inferior to its maxim um reported growth depth of 99 m. This ability to grow far deeper than the photoautotrophic limit implies significant carbon acquisition by heterotrophy.