Investigating the Carbon Uptake Physiology of Southern Ocean Phytoplankton - From Kinetics to Metal Requirements

Abstract

In this thesis, I investigate how Southern Ocean (SO) phytoplankton adapt their cellular machinery for carbon acquisition and photosynthesis. These organisms contend with unique environmental conditions, including low temperatures, dynamic light regimes, low diffusion rates, high gas solubility, and unusual trace metal distributions. I focus on three main questions: (1) how the kinetics and architecture of carbon concentrating mechanisms (CCMs) and Rubisco properties have evolved to suit cold, high CO2 waters; (2) how extracellular carbonic anhydrase (eCA) modulates iron (Fe) and carbon uptake under varying Fe availability in natural phytoplankton communities; and (3) how metabolic zinc (Zn) requirements, and potential Zn substitution by cobalt (Co) or cadmium (Cd), influence photosynthesis and growth across different SO taxa. I first explore the inorganic carbon uptake and Rubisco kinetics of two SO diatoms (Proboscia inermis and Chaetoceros flexuosus) alongside the temperate model Phaeodactylum tricornutum. Through carbonic anhydrase inhibition assays, stable isotope analyses, and ultrastructural pyrenoid observations, we show significant divergence in Rubisco carboxylation efficiency, CO2 affinity, and reliance on CCMs. P. inermis exhibits a comparatively low CO2 affinity Rubisco and reduced CCM dependence, while C. flexuosus relies strongly on extracellular CA. Temperature dependence assays further indicate that SO diatoms have kinetic adaptations optimizing carbon fixation under cold, high CO2 conditions. Then, I investigate the role of eCA in Fe and carbon uptake across different size classes of phytoplankton communities in the SO. Shipboard bioassays were conducted at the Southern Ocean Time Series (SOTS) station and near a deep chlorophyll maximum at the Antarctic Polar Front, under both Fe-amended and Fe-limited conditions. By inhibiting eCA with acetazolamide (AZ), I found that nano- and microeukaryotes displayed enhanced Fe and C uptake when Fe was abundant, whereas eCA inhibition variably affected Fe uptake under different Fe regimes. These findings highlight a complex interplay between eCA activity, Fe availability, and the efficiency of the biological carbon pump in high_latitude waters. Finally, I examine Zn requirements and the capacity for Co or Cd to substitute for Zn in carbonic anhydrase among eight diatom species and two strains of the haptophyte Phaeocystis antarctica, each isolated from distinct SO regions. Zn-dependent growth kinetics reveal pronounced biogeographical differences: subantarctic diatoms display much lower Zn requirements than polar diatoms, yet most SO diatoms exhibit limited Co substitution compared to temperate taxa. The two P. antarctica strains show moderate Zn/Co substitution, but they also require Co at high Zn concentrations, implying an essential metabolic role for Co beyond Zn replacement. Species-specific responses to Cd as a Zn surrogate were equally varied, suggesting that the elevated reliance on Zn-finger proteins in SO phytoplankton may constrain the extent of metal substitution. Collectively, these results underscore how low temperatures, and distinctive trace metal regimes (particularly with respect to Zn) shape the physiology of SO phytoplankton. An improved mechanistic understanding of these adaptations will be crucial for predicting how changing ocean conditions, such as warming, acidification, and shifting nutrient distributions, may alter carbon cycling and ecosystem function in the Southern Ocean.