Environmental context. Isoprene, a natural product of both terrestrial vegetation and marine organisms, is rapidly oxidised in the atmosphere, and thereby plays a key role in the regional budget of oxidants. Although isoprene production from terrestrial plants has been extensively investigated, production processes and emission rates from marine species are still poorly understood. We present results from laboratory experiments showing that isoprene is emitted from living phytoplankton cells at variable rates depending on the light intensity, cell volume, and carbon content of the plankton cells.Abstract. We report here isoprene emission rates determined from various phytoplankton cultures incubated under PAR light which was varied so as to simulate a natural diel cycle. Phytoplankton species representative of different phytoplankton functional types (PFTs) namely: cyanobacteria, diatoms, coccolithophorides, and chlorophytes have been studied. Biomass normalised isoprene emission rates presented here relative to the chlorophyll-a (Chl-a) content of the cultures showed that the two cyanobacteria (Synechococcus and Trichodesmium) were the strongest emitters with emission rates in the range of 17 to 28 mg C 5 H 8 g À1 Chl-a h À1 . Diatoms produced isoprene in a significantly lower emission range: 3 to 7.5 mg C 5 H 8 g À1 Chl-a h À1 and Dunaliella tertiolecta was by far the lowest emitter of our investigated plankton cultures. Despite the group specific differences observed, a high emission rate variance was observed to occur within one phytoplankton group. However, a combination of literature and our own data showed a clear relationship between the actual cell volume and the isoprene emission rates. This relationship could be a valuable tool for future modelling approaches of global isoprene emissions.

1. I n erythrocytes of man and rabbits methylene blue catalysis is limited by the following factors at the pH optimum 7.5; primarily by the cellular concentration of NADP+, next by that of glucose-6-P. Possible further limiting steps are glucose-6-phosphate dehydrogenase, phosphogluconate dehydrogenase and reduced-NADP dehydrogenase.2. Calculation of the intracellular NADPf concentration with the two-substrate equation for the glucose-6-phosphate dehydrogenase-reaction does not yield reasonable results. The existence of inhibiting factors of the methylene blue catalysis is postulated. Their amount is presumably lower in intact cells as compared with hemolysates.3. There is no pH dependence of the glucose-6-P concentration in presence of methylene blue. There exists a competition between the oxidative pentose-phosphate-pathway and the EmbdenMeyerhof pathway, in which the phosphofructokinase plays a key role.4. The proportions of the two pathways a t different pH values are calculated from data on oxygen and glucose consumption. At pH values below 7.6 glucose is utilized practically exclusively via the oxidative pentose phosphate pathway, whereas the Embden-Meyerhof pathway is inhibited. At pH 8.2 the participation of the oxidative pentose-phosphate pathway amounts to 40°/, of the total glucose utilization, while the Embden-Meyerhof pathway is little if any reduced as compared to cells withouth methylene blue.5 . With increased degree of oxidation of the pyridine nucleotides there occur decreases in the levels of hexose-and triose-phosphates. With these changes and the decreased flow to lactate an increased formation of 2,3-diphosphoglycerate occurs.