Rapid determination of the dry weight of single, living cyanobacterial cells using the Mach-Zehnder double-beam interference microscope

Abstract

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 µm−3 whereas the maximum was 459.2 fg µm−3 from single cell measurements. The average (±SD) of all 72 measurements was 265 ± 46 fg µm−3. 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 (±SD) of 48 ± 3% of dry weight, which corresponds with literature data. Thus we recommend a new conversion factor of 0.127 for biovolume (mm3) 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 µm−3 (Phormidium) to 70 fg µm−3 and even 210 fg µm−3 (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.