Carbon Metabolism in Seagrasses

Abstract

Patterns of initial photosynthetic CO₂ incorporation were determined for some seagrasses and were related to activities of primary carbon fixing enzymes, carbonic anhydrase activities, and δ¹³C values. According to the incorporation patterns, Cymodocea nodosa was a C4 species while Thalassia hemprichli and Thalassodendron ciliatum were C₃ plants. Halophila stipulacea showed an unusual incorporation pattern which could be viewed as intermediate between typical C₃ and C₄ pathways. The activity ratios of ribulose-l, 5-bisphosphate carboxylase (RUBPcase) to phosphoenolpyruvate carboxylase (PEPcase) were about 3 for Thalassodendron ciliatum and 1 for Cymodocea nodosa and Halophila stipulacea. The latter value, which is intermediate to ratios found in terrestrial C₃ and C₄ plants, may correlate with the incorporation patterns found for Halophila stipulacea. Since the C₄ seagrass lacked the Kranz anatomy, it may, in addition, point to a flexible incorporation potential for these plants. The high δ¹³C values found in these and other seagrasses did not correlate with their photosynthetic pathways as in terrestrial plants. This discrepancy is probably due to a ‘closed system’ type of photosynthesis in which CO₂ is efficiently utilized. The C₃ species which utilize CO₂ enzymatically must convert exogenous HCO-₃ to CO₂ internally. Even though carbonic anhydrase activities were very low, conversion rates seemed to be sufficient for high rates of photosynthesis. Since enzymatic fixation rates approached photosynthetic rates even at CO₂ saturation, the limitation for these seagrasses to express their high photosynthetic potential is most probably the HCO⁻₃ uptake system.