Biodiversity and Stoichiometric Plasticity Increase Pico‐Phytoplankton Contributions to Marine Net Primary Productivity and the Biological Pump

Abstract

Earth System Models generally predict increasing upper ocean stratification from 21st century warming, which will cause a decrease in the vertical nutrient flux forcing declines in marine net primary productivity (NPP) and carbon export. Recent advances in quantifying marine ecosystem carbon to nutrient stoichiometry have identified large latitudinal and biome variability, with low‐latitude oligotrophic systems harboring pico‐sized phytoplankton exhibiting large phosphorus to carbon cellular plasticity. The climate forced changes in nutrient flux stoichiometry and phytoplankton community composition are thus likely to alter the ocean's biogeochemical response and feedback with the carbon‐climate system. We have added three pico‐phytoplankton functional types within the Biogeochemical Elemental Cycling component of the Community Earth System Model while incorporating variable cellular phosphorus to carbon stoichiometry for all represented phytoplankton types. The model simulates Prochlorococcus and Synechococcus populations that dominate the productivity and sinking carbon export of the tropical and subtropical ocean, and pico‐eukaryote populations that contribute significantly to productivity and export within the subtropical to mid‐latitude transition zone, with the western subtropical regions of each basin supporting the most P‐poor stoichiometries. Subtropical gyre recirculation regions along the poleward flanks of surface western boundary currents are identified as regional hotspots of enhanced carbon export exhibiting C‐rich/P‐poor stoichiometry, preferentially inhabited by pico‐eukaryotes and diatoms. Collectively, pico‐phytoplankton contribute ∼58% of global NPP and ∼46% of global particulate organic carbon export below 100 m through direct and ecosystem processing pathways. Biodiversity and cellular nutrient plasticity in marine pico‐phytoplankton combine to increase their contributions to ocean productivity and the biological carbon pump.

Observed declines in upper ocean phosphate-to-nitrate availability

“…), mesozooplankton (200-2000 μm, smaller copepod, large dinoflagellates) and macrozooplankton (>2000 μm, larger copepod, krill). Besides the modified plankton types, the 8P4Z model also has a better representation of the phytoplankton group-specific variable stoichiometries, both for iron (35) and for the macronutrients (N, P, Si) (68).…”

Global niche partitioning of purine and pyrimidine cross-feeding among ocean microbes

“…To explore this using a bounded model system, we have begun characterizing the production of organic carbon by the highly abundant oceanic cyanobacterium Prochlorococcus (17) and assessing its fate. By performing ∼10-15% of oceanic CO 2 -fixation (18,19), Prochlorococcus is a major source of organic carbon to ocean ecosystems, and over geologic timescales is thought to have evolved metabolic interdependencies with co-occurring heterotrophs (20). In recent experiments, we noticed that pyrimidines and purines are both abundant exudates in Prochlorococcus (21).…”

Simultaneous acclimation to nitrogen and iron scarcity in open ocean cyanobacteria revealed by sparse tensor decomposition of metatranscriptomes

“…Microbial communities play essential roles in every Earth biome and drive global biogeochemical cycles through their collective metabolic activity (1). In open ocean ecosystems, the abundant cyanobacteria Prochlorococcus and Synechococcus are keystone photosynthetic microbes that account for an estimated ⇠ 20 25% of total marine primary production (2,3). Although descended from a common ancestor, these two genera encompass a diversity of genetically and phenotypically differentiated subpopulations (4).…”

“…Although descended from a common ancestor, these two genera encompass a diversity of genetically and phenotypically differentiated subpopulations (4). Prochlorococcus numerically dominates in the nutrient-poor tropical and subtropical open ocean waters that span from about 40°S to 40°N, whereas Synechococcus thrives across a greater range of habitats and is more abundant in colder, more nutrient-rich regions (2,3). Within each genus, genetically differentiated clades are further adapted to occupy distinct ecological niches, such as high-light adapted Prochlorococcus clades encountered near the surface, and low-light adapted Prochlorococcus clades generally found in deeper waters (5).…”