ABSTRACT:A new ''0 isotope dilution technique is presented and examples are given of the determination of true gross oxygen evolution (photosynthesis) and oxygen uptake (respiration) in plankton samples from estuarine surface waters and in the seagrass Zostera marina. The technique involves enriching dissolved oxygen with 3602, and measuring changes in isotope enrichment and oxygen concentration with time. It is relatively simple to apply, and less expensive than previously used 180 methods. Oxygen uptake rates of plankton samples incubated under various light conditions ranged from 0.75 to 10 times that in darkness. Gross oxygen evolution rates determined by isotope dilution were from 0 to 80 % higher than gross oxygen evolution rates calculated from parallel light and dark bottle experiments and ranged between 180 and 800 FM O2 (FM chl a)-' h-'. Ratios of gross oxygen uptake to gross oxygen evolution were between 0.12 and 0.67 and depended on both light intensity and sampling time. Occasionally, net oxygen evolution and isotope ddution occurred in darkened bottles, suggesting the existence of a non-photosynthetic process that produces new oxygen. In 2. marina, oxygen uptake at saturating light intensities was 4.7 times the rate in darkness. The data indicate the existence of significant, but variable, light stimulation of oxygen consumption in estuarine waters and higher rates of gross oxygen production than measured by the light and dark bottle technique.

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The effect of nitrogen starvation on photosynthetic pigments and energy coupling was compared in Synechococcus sp. (Cyanophyta) strains originating from oceanic (oligotrophic) or coastal (eutrophic) marine environments. A survey indicated that those of oceanic or subtropical origin retained a greater fraction (55 to 98 %) of their major phycobiliprotein during a 24 h nitrogen starvation period compared to coastal strains (30 to 44 % ) For 3 strains studied in detail, nitrogen starvation caused a significant (> 85 ':L) loss of phycoerythrin from Synechococcussp. WH8018 after 24 h, but only a minor loss (< 25 %) from Synechococcus sp. WH7803 or WH8103 after 3 d starvation. All 3 strains exhibited reduced gross oxygen evolution dunng the first 24 h of starvation, however, indicating a reduction in energy transfer from phycoerythrin to the electron transport chain. Changes during starvation in the in vivo fluorescence excitation and emission spectra indicated a small degree of uncoupling of phycoerythrin from allophycocyanin in Synechococcus sp. WH7803 and MrH8103 but not Synechococcus sp. WH8018. In neither case could it account for the measured loss in photosynthetic efficiency, however. The unusual nature of phycobiliprotein regulation in oceanic strains may reflect an adaptation to episodic (few days time scale) inputs of limiting nutnents to oligotrophic surface waters, thus providing a mechanism for more rapid rejuvenation.