Dynamic macromolecular composition and high exudation rates in Prochlorococcus

Abstract

Every living cell is composed of macromolecules such as proteins, DNA, RNA and pigments. The ratio between these macromolecular pools depends on the allocation of resources within the organism to different physiological requirements, and in turn affects biogeochemical cycles of elements such as carbon, nitrogen and phosphorus. Here, we present detailed measurements of the macromolecular composition of Prochlorococcus MIT9312, a representative strain of a globally abundant marine primary producer, as it grows and declines due to nitrogen starvation in laboratory batch cultures. As cells reached stationary stage and declined, protein per cell decreased by [~]30% whereas RNA per cell and pigments per cell decreased by [~]75%. The decline stage was associated with the appearance of chlorotic cells which had higher forward scatter (a proxy for cell size) but lower chlorophyll autofluorescence, as well as with changes in photosynthetic pigment composition. Specifically, during culture decline divinyl-chlorophyll-like pigments emerged, which were not observed during exponential growth. These divinyl-chlorophyll-like pigments were also observed in natural samples from the Eastern Mediterranean. Around >80% of the carbon fixed by Prochlorococcus MIT9312 (but not of a different strain, NATL2A) was released into the growth media as dissolved organic carbon under these laboratory conditions. Variations in RNA/protein indicate that, broadly defined, the macromolecular composition of Prochlorococcus MIT9312 is more similar to eukaryotic phytoplankton than to marine heterotrophic bacteria, possibly due to the significant investment in photosynthetic machinery of phototrophs.