Dataset 1. Methane production of seaweeds and coastal plants from Port Phillip Bay, Australia

RESEARCH HYPOTHESIS
Methane production potential of different seaweed and coastal plant species is predictable from their methylated osmolyte content (DMSP, TMA, TMAO, choline). High sulfate concentrations in coastal waters suppress competing methanogenic pathways, making methylotrophic methanogenesis — methane production from methylated compounds — dominant in these environments. Osmolytes stored in marine plants are released on cell breakdown and used as substrates by methanogens in the sediment.

DATA COLLECTION
This dataset contains time-series headspace methane concentrations from laboratory sand slurry microcosm experiments. Sand was collected from the intertidal zone at Port Phillip Bay, Victoria, Australia. Three seaweeds — Ulva lactuca, Ecklonia radiata (kelp), and Ceramiales (red filamentous algae) — were collected on 2 October 2023. Three coastal plants — Avicennia marina (mangrove leaves), Amphibolis antarctica (seagrass), and Salicornia quinqueflora (saltmarsh) — were collected on 1 August 2024.

Slurries were prepared in triplicate in 160 mL serum vials with 30 g sand, 70 mL seawater, and ~100 mg wet weight biomass, sealed under anoxic conditions. Headspace gas samples were taken at 0, 50, 120, 220, 320, and 460 h (seaweeds) and 0, 21, 72, 96, 120, 170, and 180 h (coastal plants). Methane was measured by GC-PDHID with NATA-accredited calibration standards and quality-control blanks every 20-30 samples. Controls included no-biomass slurries and pure DMS or TMA additions.

FINDINGS AND INTERPRETATION
Among seaweeds, Ulva produced the most methane (1200 ± 300 ppm at 460 h), followed by red filamentous algae (200 ± 200 ppm) and kelp (100 ± 30 ppm). Among coastal plants, mangrove leaves produced the highest concentrations (5500 ± 200 ppm at 180 h), followed by seagrass (1000 ± 100 ppm) and saltmarsh (400 ± 300 ppm).
Seaweed methane production correlated strongly with DMSP content (adj. R² = 0.81) and total methyl group content (adj. R² = 0.82, p < 0.001), with stoichiometry near 100% conversion of methyl groups to methane. Mangrove leaves, despite producing the highest methane, had osmolytes accounting for less than 10% of observed production; methanol generated through pectin hydrolysis during decomposition is implicated as the primary substrate.

This dataset can be used to compare methane production rates across species and validate stoichiometric osmolyte-to-methane models for seaweeds. Results also show that methane production is a transient phenomenon, plateauing after 7-10 days, consistent with substrate depletion rather than ongoing production. The dataset provides a basis for projecting how eutrophication-driven Ulva blooms, kelp forest decline, and coastal restoration efforts may alter shallow marine methane budgets, and highlights that simple osmolyte measurements are insufficient predictors of methane potential for coastal plants such as mangroves.