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

Abstract

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 (a), 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 μM · m −2 · s −1 PAR. P max was mantained at 2500 μM m −2 · s −1 and only one species, C. prolifera f. zosterifolia Børgesen, showed a photoinhibitory effect at 3000 μM 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.