Dynamic macromolecular composition and high exudation rates inProchlorococcus

Abstract

10Every living cell is composed of macromolecules such as proteins, DNA, RNA and pigments or 11 cofactors. The ratio between these macromolecular pools depends on the allocation of resources within 12 the organism to different physiological requirements, and in turn determines the elemental composition 13 of the organism and, potentially, how it may affect biogeochemical cycles of elements such as carbon, 14 nitrogen and phosphorus. Here, we present detailed measurements of the macromolecular composition 15 of Prochlorococcus MIT9312, a representative strain of a globally abundant marine primary producer, 16 as it grows and declines due to N starvation in laboratory batch cultures. As cells reached stationary 17 stage and declined, protein/cell decreased by ~30% and RNA/cell and pigments/cell decreased by an 18 order of magnitude. The decline stage was associated with the appearance of chlorotic cells which had 19 higher forward scatter (a proxy for cell size) but lower chlorophyll autofluorescence, as well as with 20 changes in photosynthetic pigment composition. Specifically, during culture decline divinyl-21 chlorophyll-like pigments emerged, which were not observed during exponential growth. These 22 divinyl-chlorophyll-like pigments were also observed in natural samples from the Eastern 23 Mediterranean. Around 80-85% of the carbon fixed by Prochlorococcus MIT9312 (but not of a 24 different strain, NATL2A) was released into the growth media as dissolved organic carbon under these 25 laboratory conditions. Broadly defined, the macromolecular composition of Prochlorococcus 26 MIT9312 is more similar to eukaryotic phytoplankton than to marine heterotrophic bacteria, suggesting 27 a different set of physiological constraints determines the macromolecular composition of these two 28 broad classes of marine microorganisms. 29 30

Dynamic Allocation of Carbon Storage and Nutrient-Dependent Exudation in a Revised Genome-Scale Model of Prochlorococcus

“…We note that this modeling approach over-simplifies the complex process of photosynthesis; for example, we do not account for the dynamics of photoprotective pigments, which allow some of the incident photons to be dissipated as heat. Indeed, the ratio of the photoprotective pigment zeaxanthin to divinyl chlorophyll a increases under nitrogen starvation, suggesting that, under these conditions, some of the photon flux may be diverted from the reaction centers ( Steglich et al, 2001 ; Roth-rosenberg et al, 2019 ). Nevertheless, Prochlorococcus undergo photoinhibition at high light intensities ( Moore et al, 1995 ; Mella-Flores et al, 2012 ), despite the presence of photoprotective pigments and other protection mechanisms such as cyclic electron flow [which is represented in the model ( Casey et al, 2016 )].…”

“…Another innovation employed by these organisms is an increased metabolic rate that in turn manifest in the exudation of organic compounds ( Fogg et al, 1965 ; Mague et al, 1980 ; López-Sandoval et al, 2013 ; Braakman et al, 2017 ; Braakman, 2019 ; Moran and Durham, 2019 ). Typically, 2–25% of the carbon fixed by photosynthesis is released by exudation from the cell, although values as high as 90% have been reported ( Bertilsson et al, 2005 ; López-Sandoval et al, 2013 ; Roth-rosenberg et al, 2019 ; Szul et al, 2019 ). This exudation, combined with cell death, lytic viral infections, and grazing debris made by predators (“sloppy feeding”), makes dissolved organic matter of phytoplankton origin omnipresent in natural waters ( Thornton, 2014 ).…”

“…Experimental evidence and theoretical considerations indicate that Prochlorococcus exudes different metabolites in a way that strongly depends on environmental conditions ( Dubinsky and Berman-Frank, 2001 ; Szul et al, 2019 ) as well as on the strain’s genetic makeup ( Becker et al, 2014 ; Roth-rosenberg et al, 2019 ). While GEMs can be used to predict these fluxes, they require modifications to deal with processes not usually considered in FBA, including: (a) the special nature of photon fluxes [which, unlike molecular fluxes, cannot easily be “shut off” at short time scales ( Dubinsky and Berman-Frank, 2001 )]; (b) the buffering role of intracellular storage molecules such as glycogen.…”

Phytoplankton exudates provide full nutrition to a subset of accompanying heterotrophic bacteria via carbon, nitrogen and phosphorus allocation

“…organic acids, organo-halogens, and isoprene (Shaw et al, 2003;Bertilsson et al, 2005) as well as lipids, proteins, and fragments of DNA and RNA (Biller et al, 2015). The strain Prochlorococcus MIT9312 used in our experiments, releases ~ 90% of the fixed carbon as DOCp (Roth-Rosenberg et al, 2021a).…”

“…In the Eastern Mediterranean, the structure of the free-living heterotrophic bacterial community is weakly but significantly correlated with the presence of divinyl Chlorophyll A, a diagnostic pigment of Prochlorococcus, further supporting a potential link through DOM production and uptake (Roth-Rosenberg et al, 2021b). The specific strain used, MIT9312, was selected because it is the most abundant ecotype globally (Johnson et al, 2006), although not in the Mediterranean (Mella-Flores et al, 2011), and has recently been show to exude large amounts of DOC (Roth-Rosenberg et al, 2021a).…”