Photoautotrophic metabolism of Caulerpa taxifolia (Chlorophyta) in the NW Mediterranean

Abstract

Oxygen production and consumption by the green alga Caulerpa taxifolia was measured in the NW Mediterranean at Monaco using a submersible respirometer. Photosynthesis versus irradiance (P-I) curves were constructed for populations at depths of 10, 15 and 25 m in summer and 10 m in winter. C. taxifolia did not exhibit many of the photoadaptive responses to bathymetric changes in irradiance that have been reported in other algae. The initial slope of the P-I curve (α) was inversely proportionate to depth; the irradiance required for respiratory compensation (I(c)), the concentrations of chlorophylls a and b and their ratios (chl a/b) did not vary significantly with depth. Changes in the rate of dark respiration (-R), the gross and net photosynthetic capacities (P(m)/(g), P(m)/(n)), α, and the irradiance required for the onset of saturation (I(k)) were greater between 15 and 25 m than between 10 and 15 m, despite the change in irradiance being smaller. P(m)/(n) decreased from 119 to 34 μmol O2 g-1 dry wt h-1 between 10 and 25 m. Net 24 h productivity (P(d)/(n)), given optimum atmospheric and ocean transparency, was estimated to decrease from 790 to 89 μmol O2 g-1 dry 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 chl a/b were lower than in summer, P(m)/(g) and P(m)/(n) were similar, and the concentrations of chlorophylls a and b were higher, indicating adjustment of the photosynthetic apparatus to seasonal changes in light and/or temperature. Maximum P(d)/(n) was estimated to be 436 μmol O2 g-1 dry wt d-1. By adjusting the y-intercept of the summer bathymetric model to fit this rate, a maximum photoautotrophic growth limit of 24 m was indicated in winter. Although these theoretical photoautotrophic limits reasonably correlate with the distribution of dense populations of C. taxifolia at Monaco, they are greatly inferior to its maximum reported growth depth of 99 m. This ability to grow far deeper than the photoautotrophic limit implies significant carbon acquisition by heterotrophy.