The effect of growth irradiance on the coupling of carbon and nitrogen metabolism in Chaetomorpha linum (Chlorophyta)

The influence of growth irradiance on the nonsteady-state relationship betw een photosynthesis and tissue carbon (C) and nitrogen (N) pools in Chaetomo rpha linum (Muller) Kutzing in response to abrupt changes in external nitro gen (N) availability was determined in laboratory experiments. For a given thallus N content, algae acclimated to low irradiance consistently had a hi gher rate of light-saturated photosynthesis (P-max normalized to dry weight ) than algae acclimated to saturating irradiance; for both treatments, P-ma x was correlated to thallus N, Both P-max and the photosynthetic efficiency (alpha(dw)) were correlated in C. linum grown at either saturating or limi ting irradiance over the range of experimental conditions, indicating that variations in electron transport were coupled to variations in C-fixation c apacity despite the large range of tissue N content from 1.1% to 4.8%, Opti mizing both a and P-max and thereby acclimating to an intermediate light le vel may be a general characteristic of thin-structured opportunistic algae that confers a competitive advantage in estuarine environments in which bot h light and nutrient conditions are highly variable, Nitrogen-saturated alg ae had the same photosynthesis-irradiance relationship regardless of light level. When deprived of an external N supply, photosynthetic rates did not change in C, linum acclimated to low irradiance despite a two-fold decrease in tissue N content, suggesting that the active pools of chlorophyll and R ubisco remained constant, Both alpha and P-max decreased immediately and co ntinuously in algae acclimated to high irradiance on removal of the N suppl y even though tissue N content was relatively high during most of the N-sta rvation period, indicating a diversion of energy and reductant away from C fixation to support high growth rates. Carbon and nitrogen assimilation wer e equally balanced in algae in both light treatments throughout the N-satur ation and -depletion phases, except when protein synthesis was limited by t he depletion of internal N reserves in severely N-starved high-light algae and excess C accumulated as starch stores. This suggests that the ability f or short-term adjustment of internal allocation to acquire N and C in almos t constant proportions may be especially beneficial to macroalgae living in environments characterized by high variability in light levels and nutrien t supply.