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Cutin, the lipid polymer, which is the structural component of cuticle, was isolated from Vicia faba leaves by a combination of enzymatic and chemical techniques. Exhaustive hydrogenolysis of powdered cutin followed by thinlayer chromatography and a combination of gas chromatography and mass spectrometry showed that this cutin was composed of 10,16-dihydroxypalmitic acid (77.8 %), 9,16dihydroxypalmitic acid (7.

N # fixation, measured as acetylene reduction, was studied in laboratory cultures and in natural assemblages (both as a mixed population and as individually picked colonies) of the heterocystous cyanobacteria Aphanizomenon sp. and Nodularia spp. from the Baltic Sea. During a diurnal cycle of alternating light and darkness, these organisms reduced acetylene predominantly during the period of illumination, although considerable activity was also observed during the dark period. In both laboratory cultures and natural populations N # fixation was saturated below a photon flux density of 600 µm −# s −" . In cyanobacterial blooms in the Baltic Sea, nitrogenase activity was mostly confined to the surface layers. Samples collected from greater depths did not possess the same capacity for acetylene reduction as samples from the surface itself, even when incubated at the photon flux density prevailing in surface waters. This suggests that, with respect to N # fixation, Baltic cyanobacteria are adapted to the intensity of illumination that they are currently experiencing.