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The in vivo fluorescence yield of chlorophyll a in Chlorella pyrenoidosa is shown to vary with the availability of nitrate, trace elements, and with the presence of mercury. The fluorescence yield of Phaeodactylum tricornutum also varies with culture conditions and comparisons of Cyclotella nana and Chaetoceros galvestonensis show a species‐dependent variation. Both types of yield fluctuations are eliminated when photosynthetic electron transport is blocked by adding 3‐(3,4dichlorophenyl)‐1,1‐dimethylurea (DCMU) to phytoplankton: the in vivo fluorescence yield becomes maximal and a constant function of cellular chlorophyll a, regardless of growth conditions or of the species examined. Preliminary observations of samples from the Caribbean Sea also indicate a constant in vivo fluorescence yield in the presence of DCMU.

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Anaerobiosis depresses the light-and bicarbonate-saturated rates of 02 evolution in intact spinach (Spinacia oleracea) chloroplasts by as much as 3-fold from those observed under aerobic conditions. These lower rates are accelerated 2-fold or more by the addition of 1 ,UM antimycin A or by low concentrations of the uncouplers 0.3 mM NH4CI or 0.25 jaM carbonyl cyanide m-chlorophenylhydrazone. Oxaloacetate and glycerate 3-phosphate reduction rates are also increased by antimycin A or an uncoupler under anaerobic conditions. At intermediate light intensities, the rate accelerations by either antimycin A or uncoupler are inversely proportional to the adenosine 5'-triphosphate demand of the reduction process for the acceptors HCO3-, glycerate 3-phosphate, and oxaloacetate. The acceleration of bicarbonate-supported 02 evolution may also be produced by adding an adenosine 5'-triphosphate sink (nbose 5-phosphate) to anaerobic chloroplasts. The above results suggest that a proton gradient back pressure resulting from antimycin A-sensitive cyclic electron flow is responsible for the depression of light-saturated photosynthesis under anaerobiosis.Photosynthetic CO2 fixation requires that intact chloroplast electron transport supply 3 moles of ATP and 2 moles of NADPH for each mole of CO2 fixed in the Calvin cycle (4). However, uncertainty in the in vivo coupling ratio (11, 26) and the fact that average quantum requirements of 10 to 12 quanta/ CO2 fixed (10) exceed the expected value of 8, provide doubt that 3 ATP molecules may be formed for every 2 NADP reduced by the linear scheme of photosynthetic electron transport (26). If insufficient ATP is generated during NADP reduction, the extra quanta must be required in some other coupled photoreaction. There is evidence (9) that 02 reduction may be coupled to ATP formation through a portion of the electron carrier sequence normally involved in NADP reduction. A ferredoxin-dependent cycle around PSI has been shown to generate ATP under anaerobic conditions (3) and may also function aerobically in vivo. Several investigators (3,14,25,26) have suggested the existence of a cycle in which electrons from the primary acceptor of PSI reduce Cyt b-563 and are subsequently returned to the oxidized PSI trap, P700+, in a process coupled to the production of ATP. The inhibitor antimycin A of mitochondrial b and c type Cyt interaction (23) has previously been used to study the function of b type Cyt in both bacterial and green plant cyclic electron transport (1,3,(13)(14)(15)19).