Beschreibung: The fluorophore [2-(4-pyridyl)-5{[4-dimethylaminoethyl-aminocarbamoyl)-methoxy]phenyl}oxazole], in short PDMPO, is incorporated in newly polymerized silica in diatom frustules and thereby provides a tool to estimate Si uptake, to study diatom cell cycles but also to determine mortality-independent abundance-based species specific growth rates in cultures and natural assemblages. In this study, the theoretical framework and applicability of the PDMPO staining technique to estimate diatom species specific growth rates were investigated. Three key polar diatom species, Pseudonitzschia subcurvata, Chaetoceros simplex and Thalassiosira sp., chosen to cover a broad range of species-related frustule and life-cycle characteristics, were incubated over 24 hours in control (no PDMPO) and with 0.125 µM and 0.6 µM PDMPO addition, respectively. The main assumptions tested during this study were: 1) Addition of PDMPO does not affect division rates. 2) Newly divided cells (daughter cells) can be readily recognized by their fluorescent valves and PDMPO is taken up only in newly formed valves. 3) The populations do not divide synchronously (here the impact of light-dark cycles on division was also included). Assumptions 1 and 2 were tested by comparing cell concentration-based growth rates with those based on PDMPO stain in control incubations and in incubations where PDMPO was added. This was carried out for P. subcurvata (Ps), C. simplex (Cs) and Thalassiosira sp. (Ts) acclimated to 20 µmol photon/m²/s at 0.125 µM (all species) and 0.6 µM (for Ps and Cs) PDMPO final concentration. The impact of PDMPO addition was further tested on Thalassiosira sp. acclimated at 110 µmol photon/m²/s at both 0.125 µM and 0.6 µM PDMPO final concentration.