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The transmitted double-beam interference microscope was used to determine the dry weight per unit biovolume of single living cells, trichomes and mucous sheaths of eight mainly terrestrial species of cyanobacteria from cultures and in situ samples. The minimum dry weight was 131.7 fg mm À3 whereas the maximum was 459.2 fg mm À3 from single cell measurements. The average (AESD) of all 72 measurements was 265 AE 46 fg mm À3. This value is lower than the average calculated from literature data by a factor of 1.8. Additional elemental measurements of the amount of carbon resulted in an average value (AESD) of 48 AE 3% of dry weight, which corresponds with literature data. Thus we recommend a new conversion factor of 0.127 for biovolume (mm 3 ) of cells to mg carbon, which could be used for cyanobacteria in respect to overall biomass calculations. Dry weight measurements were also carried out on the mucous sheaths of both trichomes (Phormidium) and coenobia (Gloeocapsa). Dry weights per unit volume of the sheaths varied greatly, ranging from 28 fg mm À3 (Phormidium) to 70 fg mm À3 and even 210 fg mm À3 (Gloeocapsa). In Gloeocapsa the dry mass of sheath material of a single coenobium exceeded the cellular dry weight 6-fold. As the interference microscopical technique is unique in its ability to determine dry masses of single living untreated cells, even in complex environmental samples, we intended to develop this method to make it available to a broad range of applications.

SummaryAsian soybean rust (Phakopsora pachyrhizi) causes a devastating disease in soybean (Glycine max). We tested the hypothesis that the fungus generates high turgor pressure in its hyaline appressoria to mechanically pierce epidermal cells.Turgor pressure was determined by a microscopic technique, called transmitted light double-beam interference Mach-Zehnder microscopy (MZM), which was developed in the 1960s as a forefront of live cell imaging. We revitalized some original microscopes and equipped them for modern image capturing. MZM data were corroborated by cytorrhysis experiments.Incipient cytorrhysis determined the turgor pressure in appressoria of P. pachyrhizi to be equivalent to 5.13 MPa. MZM data revealed that osmotically active sugar alcohols only accounted for 75% of this value. Despite having a lower turgor pressure, hyaline rust appressoria were able to penetrate non-biodegradable polytetrafluoroethylene (PTFE) membranes more efficiently than do melanized appressoria of the anthracnose fungus Colletotrichum graminicola or the rice blast fungus Magnaporthe oryzae.Our findings challenge the hypotheses that force-based penetration is a specific hallmark of fungi differentiating melanized appressoria and that this turgor-driven process is solely caused by metabolic degradation products. The appressorial turgor pressure may explain the capability of P. pachyrhizi to forcefully invade a wide range of different plants and may pave the way to novel plant protection approaches.

An Arctic clone of Phaeocystispouchetii LAGERHEIM was compared to Phaeocystis globosa SCHERFFEL isolated from the southern North Sea with regard to temperature tolerance and colony shapes. Already young P.pouchetii colonies (< 100/~m) show the typical distribution of the cells in groups, separated from each other by wide zones of cell-free mucilage; the maximum colony size is ca 2 mm in diameter. P.pouchetii colonies form clouds with bubble-like vesicles, spherical colony-shapes are seldom found. P.g/obosa colonies are spherical up to a size of 2 mm; the cells are distributed homogeneously over the periphery of the colonies. A 'pouchetii'-Iike distribution of cells never occurs either in the spherical young colonies or in the pear-shaped old colonies (size up to 8 mm). A development from the colony shape of the 'globosa'-type to the 'pouchetii'-type or vice versa was never found. Therefore the colony shape has to be considered a constant distinctive character. Single cells of P, pouchetii and P.globosa cannot be separated from each other by using the light microscope; this also holds for the flagellates and the non-motile cells. P.pouchetii grows well between 0 ~ and 14 ~ P.g/obosa between 4 ~ and 22 ~ respectively. Because of the distinctive differences in the morphology of the colonies and the differences in temperature tolerances we propose that Phaeocystis globosa should no longer be considered conspecific with Phaeocystis pouchetii.