Linking flow cytometric cell sorting and compound-specific 13C-analysis to determine population-specific isotopic signatures and growth rates in cyanobacteria-dominated lake plankton

A novel methodology was applied to determine the delta(13)C signatures of natural cyanobacterial and algal populations by combined compound-specific isotope ratio mass spectrometry and pyrolytic methylation-gas chromatography (Py-GC-IRMS) of the fatty acids released from phytoplankton fractions collected using fluorescence-activated cell sorting. Py-GC-IRMS provided direct analysis of the very small samples (<200 ng total C) derived from the cell sorting of individual phototrophic populations, while minimizing the chances on contamination and loss in sample handling. Despite trichome lengths exceeding the diameter of the sort droplets, filamentous cyanobacteria were amenable to population-specific cell sorting. In concert with C-13-CO2 labeling, the combined use of flow cytometric cell sorting and Py-GC-IRMS enabled both the assessment of standing stocks and of population-specific growth rates of the predominant cyanobacterial and algal taxa in Lake Loosdrecht (The Netherlands). Filamentous prochlorophytes, formerly the dominant cyanobacterial taxon in the lake, appeared less abundant in recent years and exhibited growth rates 30%-40% lower than the rates recorded for oscillatorioid populations. Diatom and green algal populations grew at rates 4- to 10-fold higher than filamentous cyanobacteria and are thus important for the lake's carbon budget. This approach offers new possibilities in studying plankton dynamics at a resolution not feasible in the past.