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

The constraints imposed by the geometry of Codium bursa, a balloon-like Mediterranean macroalga with a thick (almost-equal-to 5.6 mm) thallus enclosing a lumen filled with water, on its functional properties were tested by examining the scaling of its form, composition (pigmentation and nutrient content), light absorption, metabolism (respiration rate, light use efficiency, and maximal photosynthetic rate), and growth to individual size. We found that C. bursa absorbs most (98 %) of the 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 suggest a low intrinsic growth rate. The pigment and nutrient content of C. bursa increase more slowly than the macroalgal weight, so that large C. bursa balloons have lower pigment and nutrient contents than smaller individuals. As a consequence, photosynthetic capacity declines with increasing size, and light compensation points increase with increasing individual size, suggesting increasing light requirements and reduced 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 growth rates and the strong size-dependence of biochemical composition and metabolism imposed by the spherical geometry of C bursa.