Lipid droplets of bacteria, algae and fungi and a relationship between their contents and genome sizes as revealed by BODIPY and DAPI staining

We have developed a convenient method to visualize triacylglycerol-filled lipid droplets (LDs) in some species of bacteria, algae and fungi by staining with borondipyrromethene difluoride (BODIPY). When BODIPY was excited by blue light, LDs emitted green fluorescence, which was distinguished easily from the red autofluorescence of chloroplasts. This makes BODIPY staining suitable for the identification of small amounts of LDs, especially in plants. We first ensured that in Chlamydomonas reinhardtii cells growing in nitrogen-replete (+N) and -deficient (-N) media, the spots of BODIPY-stained LDs coincided with those of Nile Red-stained LDs. In addition, it was shown that the LD content per cell in N-starved cells was 200-fold higher than those of the control (+N) using a video-intensified microscope photoncounting system (VIMPCS). BODIPY staining was applied to visualize LD in bacteria, algae and fungi, and included those algae regarded as nonoleaginous. We identified LD spots in unicellular and multicellular bacteria and eukaryotes, namely Cyanidioschyzon merolae, Cyanidium caldarium delta, Chlamydomonas reinhardtii, Klebsormidium nitens and Penicillium sp., but not in Anabaena flos-aquae. We also examined the relationship between the contents of LDs and the genome size in the algae and fungi using VIMPCS but were unable to find a strong relationship between genome size and production of LDs. Finally, the location of LDs was considered in relation to organelles including the endoplasmic reticulum and chloroplasts, which are related to the formation of LDs.