INORGANIC CARBON LIMITATION OF PHOTOSYNTHESIS IN ULVA-ROTUNDATA (CHLOROPHYTA)

A computerized oxygen electrode system was used to make rapid and accurate measurements of photosynthetic light and dissolved inorganic carbon (DIC) response curves with a macroalga. Ulva rotundata Blid. was grown in an outdoor, continuous flow system in seawater under sunlight or 9% of sunlight at Beaufort, North Carolina. The light compensation points in the shade- and sun-grown plants, measured in seawater, were at photon flux densities (PFDs) of 16 and 27-mu-mol photons.m-2.s-1, respectively, but the quantum yield of O2 evolution was not significantly different. Rates of photosynthesis in seawater per unit area of thallus under saturating light and rates of dark respiration were about 1.5-fold higher in sun- than in shade-grown plants. The concentration of DIC in seawater (approximately 2 mM) limited photosynthesis at absorbed PFDs above 60-70-mu-mol photons.m-2.s-1. Addition of 20 mM inorganic carbon had no effect on quantum yield but caused about a 1.5-fold increase in the light-saturated photosynthetic rate in both shade- and sun-grown Ulva. The effect of DIC supplementation was greatest in plants grown in October and least in plants grown in June. The light- and DIC-saturated rate of photosynthesis in seawater was similar to the maximum rate obtained by exposing Ulva to 10% CO2 in the gas phase. The carbon isotope values (delta-C-13, reflecting the C-13/C-12 ratio compared to a standard) of Ulva grown in the same seawater supply were dependent on light and agitation. Samples from Beaufort Inlet were more negative (delta-C-13 value, -20.03 parts-per-thousand) than those grown in bright light with agitation (delta-C-13 value, -17.78 parts-per-thousand outdoors; -17.23 parts-per-thousand indoors), which may indicate DIC supply limited carbon uptake in seawater.