Dynamic macromolecular composition and high exudation rates in Prochlorococcus

Every living cell is composed of macromolecules such as proteins, DNA, RNA and pigments or cofactors. The ratio between these macromolecular pools depends on the allocation of resources within the organism to different physiological requirements, and in turn determines the elemental composition of the organism and, potentially, how it may affect 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 N starvation in laboratory batch cultures. While the mean protein/cell was mostly stable, RNA/cell and pigments/cell decreased by an order of magnitude as cells reached stationary stage and declined. 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 80-85% 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. Broadly defined, the macromolecular composition of Prochlorococcus MIT9312 is more similar to eukaryotic phytoplankton than to marine heterotrophic bacteria, suggesting a different set of physiological constraints determines the macromolecular composition of these two broad classes of marine microorganisms.