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The relationship between gross photosynthesis (GPS) and electron transport rate (ETR) in marine algae has been shown to vary as a function of irradiance; however, little is known about the effect of nutrients on the this relationship in seagrasses. The objective of this study was to evaluate the effect of nitrate concentration on the GPS (measured as O2 evolution) vs ETR (estimated by fluorescence quenching analysis) relationship of the seagrass Cymodocea nodosa from the Spanish Mediterranean Sea. Carbon levels in the tissue increased 6.5% when nitrate in the culture medium augmented from 0 to 100 µM. Nitrogen in the tissue, however, increased more than 60% when nitrate concentration in the medium reached 100 µM. Chlorophyll a + b levels increased approximately 30%, while absorptance augmented 15% when nitrate increased from 0 to 100 µM. In general, maximum oxygenic photosynthesis and maximum ETR values increased when nitrate in the medium increased. The relationship between GPS and ETR did not show a linear response at low nitrate levels and high irradiances. In contrast, a linear relationship was observed at nitrate levels above 50 µM, even at high irradiances. The results from this study suggest that the lack of correlation between ETR and GPS is the result of low nitrogen levels in the tissue of marine macrophytes. They also suggest that seasonal fluctuations in nitrate levels or nitrogen pulses, such as those observed during upwelling events, may affect the relationship between GPS and ETR in C. nodosa or other marine macrophytes.
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