This study investigates the relationship between biovolume and biomass of naturally derived marine bacterioplankton. Researchers collected and cultured marine bacterioplankton in particle-free seawater to estimate the biovolume-to-biomass conversion factor and the C:N ratio. They found that the average per-cell carbon biomass was relatively constant at 20 ± 0.8 fg of C, and the biovolume-to-biomass conversion factor averaged 0.38 ± 0.05 g of C cm⁻³, which is three times higher than previously estimated from Escherichia coli. The C:N ratio was 3.7 ± 0.2. These results suggest that natural marine bacterial biomass and production may be higher than previously thought, and variations in bacterial size may not reflect variations in biomass per cell. The study also highlights the importance of accurate size measurement and biovolume estimation in determining bacterial biomass. They used fluorescent microspheres to calibrate their size measurement criteria and found that their method underestimated the real linear dimensions of fluorescent microspheres by about 8%. They corrected for this by using a factor of 1.28. The study also found that smaller cells tend to have more carbon and nitrogen per biovolume than larger cells, which contradicts the expectation that biovolume and biomass would be positively correlated. The researchers concluded that the use of a constant biovolume-to-biomass conversion factor for field data may lead to incorrect estimation of bacterial biomass. They suggest that a conversion factor based on cell number rather than biovolume may be more practical and accurate for biomass estimation, as long as the cells are in the size range of those reported here. The study also found that the C:N ratio of marine bacteria was 3.7, which is similar to that found by Seiderer et al. with cultured marine bacteria and higher than the 3.4 ratio found by Heldal et al. with Escherichia coli. The study's results suggest that the biomass of small bacterioplankton may be more significant than previously thought, despite their high abundance. The findings have implications for understanding the role of bacteria in nutrient dynamics in marine ecosystems, where nitrogen is often limiting. The study also emphasizes the importance of accurate size measurement and biovolume estimation in determining bacterial biomass and highlights the need for further research in this area.This study investigates the relationship between biovolume and biomass of naturally derived marine bacterioplankton. Researchers collected and cultured marine bacterioplankton in particle-free seawater to estimate the biovolume-to-biomass conversion factor and the C:N ratio. They found that the average per-cell carbon biomass was relatively constant at 20 ± 0.8 fg of C, and the biovolume-to-biomass conversion factor averaged 0.38 ± 0.05 g of C cm⁻³, which is three times higher than previously estimated from Escherichia coli. The C:N ratio was 3.7 ± 0.2. These results suggest that natural marine bacterial biomass and production may be higher than previously thought, and variations in bacterial size may not reflect variations in biomass per cell. The study also highlights the importance of accurate size measurement and biovolume estimation in determining bacterial biomass. They used fluorescent microspheres to calibrate their size measurement criteria and found that their method underestimated the real linear dimensions of fluorescent microspheres by about 8%. They corrected for this by using a factor of 1.28. The study also found that smaller cells tend to have more carbon and nitrogen per biovolume than larger cells, which contradicts the expectation that biovolume and biomass would be positively correlated. The researchers concluded that the use of a constant biovolume-to-biomass conversion factor for field data may lead to incorrect estimation of bacterial biomass. They suggest that a conversion factor based on cell number rather than biovolume may be more practical and accurate for biomass estimation, as long as the cells are in the size range of those reported here. The study also found that the C:N ratio of marine bacteria was 3.7, which is similar to that found by Seiderer et al. with cultured marine bacteria and higher than the 3.4 ratio found by Heldal et al. with Escherichia coli. The study's results suggest that the biomass of small bacterioplankton may be more significant than previously thought, despite their high abundance. The findings have implications for understanding the role of bacteria in nutrient dynamics in marine ecosystems, where nitrogen is often limiting. The study also emphasizes the importance of accurate size measurement and biovolume estimation in determining bacterial biomass and highlights the need for further research in this area.