In situ nitrogen fixation associated with seagrasses in the Gulf of Elat (Red Sea)

In situ nitrogen fixation associated with the seagrass Halophila stipulacea, at the northern Gulf of Elat (Red Sea), is eight to ten times higher than that of nearby plant-free areas. A daily cycle of nitrogen fixation is evident, with rates during the day being seven times greater than during the night. Removal of seagrass leaves only from a patch within a seagrass bed gradually decreases nitrogen fixation activity, reaching the rates of plant-free areas after ten hours. A method devised for the in situ measurement of nitrogen fixation rates using belljars is described in detail. Nitrogen fixation rates in situ are higher than in the laboratory and lack the lag period typical to laboratory measurements. In laboratory experiments using intact plant samples, glucose enhances nitrogen fixation rates both in light and dark. Photosystem II inhibitor (3-3,4-dichloro-phenyl-1,1-dimethylurea) doubles nitrogen fixation rates in light. Both field and laboratory results indicate that light is essential for nitrogen fixation activity in the H. stipulacea bed possibly through its effect on cyanobacterial population that occupy the aerobic niches of the phyllosphere and on photosynthetic Rhodospirillacean bacteria that inhabit the anaerobic ones. Nitrogen fixation rates evident in H. stipulacea beds in situ account for a considerable portion of the biomass production by the seagrasses. The dependence of high nitrogenase activity by the diazotrophs on the presence of the seagrasses indicates the great importance of the seagrass community to the nitrogen cycle in its highly oligotrophic surroundings of the Gulf of Elat.     

Retinal isomerization in bacteriorhodopsin is controlled by specific chromophore-protein interactions. A study with noncovalent artificial pigments.

It has previously been shown that, in mutants lacking the Lys-216 residue, protonated Schiff bases of retinal occupy noncovalently the bacteriorhodopsin (bR) binding site. Moreover, the retinal-Lys-216 covalent bond is not a prerequisite for initiating the photochemical and proton pump activity of the pigment. In the present work, various Schiff bases of aromatic polyene chromophores were incubated with bacterioopsin to give noncovalent pigments that retain the Lys-216 residue in the binding site. It was observed that the pigment's absorption was considerably red-shifted relative to the corresponding protonated Schiff bases (PSB) in solution and was sensitive to Schiff base linkage substitution. Their PSB pK(a) is considerably elevated, similarly to those of related covalently bound pigments. However, the characteristic low-pH purple to blue transition is not observed, but rather a chromophore release from the binding site takes place that is characterized by a pK(a) of approximately 6 (sensitive to the specific complex). It is suggested that, in variance with native bR, in these complexes Asp-85 is protonated and Asp-212 serves as the sole negatively charged counterion. In contrast to the bound analogues, no photocycle could be detected. It is suggested that a specific retinal-protein geometrical arrangement in the binding site is a prerequisite for achieving the selective retinal photoisomerization.     

Catabolism of indole-3-acetic acid to indole-3-methanol in a crude enzyme extract and in protoplasts from Scots pine (Pinus sylvestris)

The antagonistic effects of ethylene and Ag⁺ on the metabolism of [1-¹⁴C]indole-3-acetic acid (IAA) and on the rates of ethylene production were studied in tobacco leaf discs ( Nicotiana rustica var. Brasilia ). During the first 10 h of incubation, Ag⁺-pretreated leaf discs contained more free [¹⁴C]IAA than untreated ones due to decreased oxidative decarboxylation, and the discs also produced more ethylene. Exogenously supplied ethylene nullified these effects of Ag⁺. However, the most pronounced effect of Ag⁺ in increasing ethylene production, as well as the strongest antagonistic effect of exogenous ethylene, were found between 24 and 48 h of incubation. During this time span no effect on the level of free IAA and on its decarboxylation could be observed. It is suggested that ethylene exerted its autoinhibitory effect by a feedback control on the IAA-induced ethylene biosynthesis. Possible mechanisms for the autoinhibitory effect of ethylene are discussed.