The smallest phytoplankton may be bigger than we first thought
The ability of flow cytometry to sort hundreds of thousands of phytoplankton cells in minutes has been used in marine science for over 30 years. However, the differentiation of these cells into different types and then further into size distributions and optical properties still requires the manual interpretation of skilled analysts.
We have developed and implemented an automated scheme on the large Atlantic Meridional Transect flow cytometric database consisting of around 104 samples and 109 cells. This unique, well-calibrated dataset covers 100° of latitude between the UK and the Falklands, and has multiple samples between the surface and 200m. The results clearly show that Prochlorococcus, which are very small marine cyanobacteria, are consistently larger (>0.65 µm) than previously thought and have a distinctive double peak (0.75 µm and 1.75 µm) in their size distribution, which varies strongly with depth. This is coupled with changes in their optical properties: a term we have coined as “opto-types”. Synechococcus by contrast are strongly monodispersed and are typically 1.5 µm in diameter.
This work has uncovered new information regarding the size distribution of the smallest phytoplankton and has implications for how energy is transferred between different biological organisms.
Tim Smyth, Glen Tarran and Shubha Sathyendranath from the Plymouth Marine Laboratory, UK