Differences in the cellular dry weight per unit biovolume of Phormidium autumnale (Cyanobacteria) dependent on growth conditions

The influence of substrate, light intensity, temperature and growth phase on the dry weight per unit biovolume of both living Phormidium autumnale trichomes and living single cells was investigated microinterferometrically. With a Mach-Zehnder Interference Microscope, both the interference-stripe-field method and the phase-shift method were used to measure the optical path differences (OPD) of cells and trichomes. To calculate the cellular dry weight of trichomes, the trichome diameters have to be measured. Widths between 4 and 7 µm were determined. Thick trichomes are characteristic for growth on agar-solidified medium, whereas this was observed in single cases only from trichomes growing on soil surfaces. A reliable prediction of trichome width from growth conditions is not possible. The dry weights per unit biovolume (fg µm⁻³) are independent of the studied parameters during the exponential growth phase (296 ± 22 fg µm⁻³) with exception of the agar-based cultures growing at low light intensity (259 ± 16 fg µm⁻³). During the stationary phase, dry weights per unit biovolume increase independently of growth conditions (353 ± 39 fg µm⁻³). Two separate factors of 0.14 and 0.17 for converting biovolume (mm³) of cells to milligrams carbon could be determined by comparing the growth phase and stationary phase-dependent average values of dry weights per unit biovolume, respectively. These conversion factors could be used as species-specific factors for Phormidium growing on soil surfaces. Irrespective of the method, both the stripe-field and phase-shift method gave similar results. However, the phase-shift method measured lower variances of values. Additionally, detailed quantifying investigations of structures within cells are possible. Thus, the phase-shift method could be a powerful analytical tool in, e.g., ecotoxicological monitoring analyses.