FUNCTIONAL IMPLICATIONS OF THE FORM OF CODIUM-BURSA, A BALLOON-LIKE MEDITERRANEAN MACROALGA

The constraints imposed by the geometry of Codium bursa, a balloon-lik e Mediterranean macroalga with a thick (almost-equal-to 5.6 mm) thallu s enclosing a lumen filled with water, on its functional properties we re tested by examining the scaling of its form, composition (pigmentat ion and nutrient content), light absorption, metabolism (respiration r ate, light use efficiency, and maximal photosynthetic rate), and growt h to individual size. We found that C. bursa absorbs most (98 %) of th e incident light, due to its high areal pigment density (0.52 mug chl a mm-2), but that light absorption was very inefficient, as seen from the low values of absorption per unit carbon (0.045 M2 g-1 C), which s uggest a low intrinsic growth rate. The pigment and nutrient content o f C. bursa increase more slowly than the macroalgal weight, so that la rge C. bursa balloons have lower pigment and nutrient contents than sm aller individuals. As a consequence, photosynthetic capacity declines with increasing size, and light compensation points increase with incr easing individual size, suggesting increasing light requirements and r educed potential growth rate as C. bursa grows. This was confirmed by in situ estimates of growth rate that showed C. bursa to rank amongst the slowest-growing macroalgae (specific growth rate 1.8 +/- 0.3 x 10( -3) d-1) as well as by the significant decline of specific growth rate with increasing algal size. The results obtained confirm the slow gro wth rates and the strong size-dependence of biochemical composition an d metabolism imposed by the spherical geometry of C bursa.