In this study, we test the applicability of the Droop/Caperon internal stores model to describe the growth and decline of the globally abundant marine cyanobacterium Prochlorococcus in batch culture as a function of internal and external inorganic and organic carbon and nitrogen. A rigorous parameter fitting exercise, constrained by the measured cell density, ammonium and inorganic carbon concentrations, revealed many different combinations of parameter values that provided equally good model‐data fit. Introducing data on C : N ratio from the published literature provided additional constraints which could only be satisfied with a structural change to the model. The simplest addition that satisfied these constraints and improved quantitative overall model‐data agreement was to explicitly represent excretion, which was predicted to account for 0.03–8.5% of the daily primary productivity; somewhat lower than previous estimates based on radiotracer incorporation. We argue that this significant and biologically meaningful process should not be neglected in models of marine phytoplankton.

Many microorganisms produce resting cells with very low metabolic activity that allow them to survive phases of prolonged nutrient or energy stress. In cyanobacteria and some eukaryotic phytoplankton, the production of resting stages is accompanied by a loss of photosynthetic pigments, a process termed chlorosis. Here, we show that a chlorosis-like process occurs under multiple stress conditions in axenic laboratory cultures of Prochlorococcus, the dominant phytoplankton linage in large regions of the oligotrophic ocean and a global key player in ocean biogeochemical cycles. In Prochlorococcus strain MIT9313, chlorotic cells show reduced metabolic activity, measured as C and N uptake by Nanoscale secondary ion mass spectrometry (NanoSIMS). However, unlike many other cyanobacteria, chlorotic Prochlorococcus cells are not viable and do not regrow under axenic conditions when transferred to new media. Nevertheless, cocultures with a heterotrophic bacterium, Alteromonas macleodii HOT1A3, allowed Prochlorococcus to survive nutrient starvation for months. We propose that reliance on co-occurring heterotrophic bacteria, rather than the ability to survive extended starvation as resting cells, underlies the ecological success of Prochlorococcus. IMPORTANCE The ability of microorganisms to withstand long periods of nutrient starvation is key to their survival and success under highly fluctuating conditions that are common in nature. Therefore, one would expect this trait to be prevalent among organisms in the nutrient-poor open ocean. Here, we show that this is not the case for Prochlorococcus, a globally abundant and ecologically important marine cyanobacterium. Instead, Prochlorococcus relies on co-occurring heterotrophic bacteria to survive extended phases of nutrient and light starvation. Our results highlight the power of microbial interactions to drive major biogeochemical cycles in the ocean and elsewhere with consequences at the global scale.

In the oceans and seas, environmental conditions change over multiple temporal and spatial scales. Here, we ask what factors affect the bacterial community structure across time, depth and size fraction during six seasonal cruises (2 years) in the ultraoligotrophic Eastern Mediterranean Sea. The bacterial community varied most between size fractions (free-living (FL) vs. particle-associated), followed by depth and finally season. The FL community was taxonomically richer and more stable than the particleassociated (PA) one, which was characterized by recurrent 'blooms' of heterotrophic bacteria such as Alteromonas and Ralstonia. The heterotrophic FL and PA communities were also correlated with different environmental parameters: the FL population correlated with depth and phytoplankton, whereas PA bacteria were correlated primarily with the time of sampling. A significant part of the variability in community structure could, however, not be explained by the measured parameters. The metabolic potential of the PA community, predicted from 16S rRNA amplicon data using PICRUSt, was enriched in pathways associated with the degradation and utilization of biological macromolecules, as well as plastics, other petroleum products and herbicides. The FL community was enriched in predicted pathways for the metabolism of inositol phosphate, a potential phosphorus source, and of polycyclic aromatic hydrocarbons.

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