The relationships between morphology and photosynthetic parameters within the polymorphic genus Caulerpa

The functional form theory predicts that surface area vs. volume ratio (SA:V) and maximum productivity (P-max) decrease as morphological complexity of an algal thallus increases. The genus Caulerpa demonstrates a tremendous range of morphological forms as well as interspecific plasticity which make it ideal to study intrageneric relationships between photosynthesis and morphology. We used two approaches to assess functional-morphology relationships: 1) an extensive survey of P-max for 15 Caulerpa taxa in relation to SA:V from species and forms of the Caribbean and 2) an intensive study of photosynthetic attributes [P-max, photosynthetic efficiency (alpha), compensation irradiance (I-c), saturation irradiance (I-k)] and respiration (R) for 4 Caulerpa taxa from the Indian River Lagoon, Florida, focusing on SA:V and surface area vs. biomass (SA:B) ratios. These two sites represent areas of relatively low and high nutrient water respectively. SA:V ratios for Caribbean Caulerpa ranged from 0.5+/-0.10 to 11.7+/-3.53 cm(2) . cm(-3) and were highly correlated (r(2)=0.74, p=0.01) to net photosynthetic rates (values from 0.4 to 4.6 mg C . g ODW-1. h(-1)). However, for clumped forms, e.g., C. sertularioides var. brevipes (J. Ag.) Svedelius, from the Indian River Lagoon, P-max Values were lower than predicted possibly due to an auto-shading effect of thin-overlapping branches. In these forms, SA:B ratio was more highly correlated to photosynthetic attributes than SA:V ratio. Our data from the Caribbean, with a mean P-max of 2.23 mg C . g ODW-1. h(-1), suggest that the genus Caulerpa, in low nutrient systems, approaches the lower mid-range of net primary production of other algal forms. Conversely, Indian River Lagoon values are much higher, reaching a maximum of 17.2 mg C . g ODW-1. h(-1), possibly due to nutrient enriched waters of this system that enhance P-max. Photosynthetic efficiency ranged between 0.007 to 0.042 mg C g ODW-1. h(-1) per mu M m(-2). s(-2). s(-1) PAR. P-max was mantained at 2500 mu M m(-2). s(-1) and only one species, C. prolifera f.zosterifolia Borgesen,showed a photoinhibitory effect at 3000 mu M m(-2). s(-1). Our data indicate that photosynthetic attributes of Caulerpa from shallow water in the Indian River Lagoon are characteristic of sun-adapted plants, i.e., high saturation and compensation irradiances, low efficiencies for capturing low photon flux densities and high exposure resistance to elevated light irradiances.