Hydrogen Isotope Dynamics in Macrocystis pyrifera: Implications for Compound-Specific Isotope Analyses

Marine macroalgae are central to coastal carbon cycling and represent a significant portion of global primary production and organic matter export. Giant kelp (Macrocystis pyrifera) forests, in particular, serve as major carbon sinks and influence regional nutrient dynamics. However, the isotopic and biochemical pathways that define these contributions remain poorly constrained. While hydrogen isotope (δ²H) analyses are widely utilised in terrestrial ecology to integrate environmental water signals and metabolic fractionation, their application in marine macroalgae, particularly at the compound-specific level, currently remains underutilised.

Our previous research demonstrated that δ²H values in the soluble sugars of M. pyrifera are highly sensitive to light intensity, which indicates a distinct metabolic imprint tied to photosynthetic carbohydrate supply. We have now expanded this investigation to include lipid biomarkers, specifically focusing on fatty acids (analysed as methyl derivatives) and sterols (analysed as acetate derivatives). Samples were collected across six kelp forest sites in Carmel Bay, California. Preliminary Gas Chromatography-Mass Spectrometry results from three fully processed sites show complex profiles of C12–C26 saturated and unsaturated fatty acids, alongside a range of cholest-, ergost-, and stigmast-based sterols. These molecular distributions vary systematically with site and depth, offering early evidence of biochemical partitioning between photosynthetic and post-photosynthetic pathways under varying natural light regimes.

This presentation will explore new compound-specific δ²H measurements performed on the aforementioned compounds. By doing so, we aim to determine whether δ²H signatures in fatty acids and sterols primarily track photosynthetic fractionation or are shaped by downstream metabolic adjustments. By synthesising isotopic and molecular data, we seek to disentangle external environmental drivers, such as light and water isotopic composition, from intrinsic biochemical controls on compound-specific δ²H values. Refining these relationships is vital for the development of robust δ²H-based paleoenvironmental proxies and for assessing the role of modern and ancient kelp forests as dynamic carbon sinks.