Photoacclimation by phytoplankton determines the distribution of global subsurface chlorophyll maxima in the ocean

Yoshio Masuda,Hideyuki Nakano,Sherwood Lan Smith,Hiroshi Sumata,Yasuhiro Yamanaka,Takafumi Hirata,Akira Oka

doi:10.1038/s43247-021-00201-y

Abstract

Subsurface chlorophyll maxima are widely observed in the ocean, and they often occur at greater depths than maximum phytoplankton biomass. However, a consistent mechanistic explanation for their distribution in the global ocean remains lacking. One possible mechanism is photoacclimation, whereby phytoplankton adjust their cellular chlorophyll content in response to environmental conditions. Here, we incorporate optimality-based photoacclimation theory based on resource allocation trade-off between nutrient uptake and light harvesting capacity into a 3D biogeochemical ocean circulation model to determine the influence of resource allocation strategy on phytoplankton chlorophyll to carbon ratio distributions. We find that photoacclimation is a common driving mechanism that consistently explains observed global scale patterns in the depth and intensity of subsurface chlorophyll maxima across ocean regions. This mechanistic link between cellular-scale physiological responses and the global scale chlorophyll distribution can inform interpretation of ocean observations and projections of phytoplankton responses to climate change.

Highlights

Subsurface chlorophyll maxima are widely observed in the ocean, and they often occur at greater depths than maximum phytoplankton biomass
Photoacclimation is a dynamic physiological response to light availability, manifested as variations of the intracellular concentrations of light-harvesting pigments, which are most commonly observed as chlorophyll
Photoacclimation is recognized as an important mechanism underlying subsurface chlorophyll maxima (SCM) formation[21,22,23,24], where it is attributed mostly to increases in cellular chl:phytoplankton carbon biomass concentration (phyC) with depth

Summary

Introduction

Subsurface chlorophyll maxima are widely observed in the ocean, and they often occur at greater depths than maximum phytoplankton biomass. We find that photoacclimation is a common driving mechanism that consistently explains observed global scale patterns in the depth and intensity of subsurface chlorophyll maxima across ocean regions. Photoacclimation is recognized as an important mechanism underlying SCM formation[21,22,23,24], where it is attributed mostly to increases in cellular chl:phyC with depth. Under low light and nutrient-replete conditions, phytoplankton increase their resource allocation for light harvesting (chloroplast) while decreasing that for nutrient uptake (Supplementary Fig. 1b) This resource allocation theory has recently provided the first theoretical derivation[31] of the widely applied empirical Droop cell quota model[32]. Pahlow’s model[28] has explained the results of incubation experiments[33]

Objectives

Methods

Results