Biochemical responses and photosynthetic performance of Gracilaria sp. (Rhodophyta) from Cádiz, Spain, cultured under different inorganic carbon and nitrogen levels

Abstract

Photosynthetic acclimation and the interactions between carbon (C) and nitrogen (N) metabolism have been studied in the red macroalga Gracilaria sp. from Cádiz, Spain, cultured under different inorganic C and N levels. The use of chemostats and buffered medium allowed continuous restoration of the alkaline reserve and constancy of pH during the experiments. The N∶C ratios and phycobiliprotein, chlorophyll a and soluble protein contents decreased when Gracilaria sp. was grown at low N levels. Algae grown in a high inorganic C concentration (5% CO₂) displayed a higher soluble carbohydrate concentration and maximum photosynthesis rates but a lower photosynthesic affinity for inorganic C, and lower phycobiliprotein and Rubisco contents, than those cultured at low inorganic C levels (air CO₂). The inorganic C enrichment also affected the N uptake and assimilation in Gracilaria sp., causing a decrease in the N uptake rate even under conditions of N sufficiency. These results reflect the significant influence of the inorganic C growth regime on N assimilation in Gracilaria sp.