Dataset 2. Methylated osmolyte content of seaweeds and coastal plants from Port Phillip Bay Australia

RESEARCH HYPOTHESIS

Methylated osmolytes — DMSP, TMA, TMAO, and choline — are the primary methanogenic substrates in shallow coastal sediments receiving seaweed and plant biomass. We hypothesised that osmolyte concentrations would differ substantially between species and predict methane production potential, providing a simple chemical proxy for estimating methane emissions from different biomass types.

DATA COLLECTION

This dataset contains quantified concentrations of four methylated osmolytes in six species of seaweed and coastal plant biomass collected from Port Phillip Bay, Victoria, Australia. Three seaweeds — Ulva lactuca, Ecklonia radiata (kelp), and Ceramiales (red filamentous algae) — were collected on 2 October 2023 from five sites (Werribee, Altona, St Kilda, Elwood, Green Point). Three coastal plants — Avicennia marina (mangrove leaves), Amphibolis antarctica (seagrass), and Salicornia quinqueflora (saltmarsh) — were collected on 1 August 2024 from Hastings and Flinders.

Biomass samples were frozen immediately after collection, then crushed in liquid nitrogen and 20–50 mg extracted in 2:6:1 chloroform:methanol:water with internal standards (CHAPS, CAPS, PIPES, TRIS at 2 µM). Extracts were analysed in triplicate by HILIC-HRMS on a Vanquish Horizon–Q Exactive Plus Orbitrap, using a seven-point calibration curve. Limits of detection were: choline 0.08 µM, DMSP 1.51 µM, TMA 0.46 µM, TMAO 0.05 µM. Peak integration was performed in Skyline 24.1. All concentrations are normalised to biomass dry weight (nmol/mg).

FINDINGS AND INTERPRETATION

DMSP dominated osmolyte composition in seaweeds, particularly in Ulva, where concentrations were substantially higher than in any other species or osmolyte measured. Coastal plants showed a different composition profile, with choline and TMA as the dominant osmolytes and very low DMSP. When osmolyte data were related to methane production from companion microcosm experiments (Dataset 1), DMSP content correlated strongly with seaweed methane production (adj. R² = 0.81, p < 0.001), and summed methyl group content explained methane production at near 100% stoichiometric efficiency (adj. R² = 0.82, p < 0.001). For coastal plants, particularly mangroves, analysed osmolytes accounted for less than 10% of observed methane production, indicating that other substrates — likely methanol from pectin hydrolysis — are the dominant methanogenic precursors.

This dataset enables comparison of osmolyte profiles across species and can be used alongside Dataset 1 to evaluate osmolyte content as a predictor of methane production potential. It demonstrates that DMSP is a reliable proxy for seaweed methane potential, while osmolyte measurements alone are insufficient for coastal plants.