Unravelling Hydrogen Isotope Fractionation in Marine Macroalgae: Insights from Macrocystis pyrifera

Marine ecosystems play a critical role in global photosynthetic carbon fixation, with approximately 5.36 Pg C exported annually via the biological pump. Macroalgae alone sequester around 200 million tons of CO₂ annually, though these estimations are largely based on indirect calculations. Hydrogen isotope (δ²H) analyses offer a promising avenue to refine such estimates while advancing our understanding of macroalgal carbon and energy metabolism.

Stable isotope studies have been instrumental in ecological and biogeochemical research, yet the application of δ²H analyses to marine algae remains limited. Most prior studies have focused on salinity-driven δ²H variations in algae, overlooking the potential of δ²H to reveal key biochemical processes. Recent findings suggest that δ²H values of organic molecules are significantly influenced by biosynthetic fractionation (²H-ε_bio), governed by the interplay between photosynthetic (²H-ε_λ) and post-photosynthetic (²H-ε_Η) processes. This metabolic signal, previously observed in terrestrial plants, is strongly modulated by the photosynthetic carbohydrate supply rate, impacting δ²H variability in organic compounds.

The giant kelp Macrocystis pyrifera provides an ideal model system to investigate these processes in marine environments. Unlike terrestrial plants, M. pyrifera offers a simplified isotopic system due to: (i) access to water with stable δ²H values, (ii) exclusion of evaporative ²H-fractionation, and (iii) a primitive vascular system that minimizes isotopic exchange across its structure. These unique features allow us to isolate and examine the variability of ²H-ε_λ under different light conditions, shedding light on the metabolic processes underlying δ²H variability in marine photoautotrophs.

This study highlights the potential of δ²H analyses to bridge the gap between isotopic and biochemical research in marine systems. By focusing on M. pyrifera, we aim to provide critical insights into the drivers of δ²H variability and their broader implications for understanding marine carbon dynamics and the role of macroalgae in global biogeochemical cycles. This work lays the groundwork for advancing isotopic methodologies and applying them to ecological and palaeoenvironmental studies in marine ecosystems.