Thrombolites as a potential nature-based solution for carbon dioxide removal

ABSTRACT Achieving net-zero targets requires emerging carbon dioxide removal (CDR) pathways capable of contributing to long-term carbon storage. Microbialite communities, including stromatolites and thrombolites, are promising but under‑explored biological platforms with potential for scalable CDR. Their deployment has been limited by uncertainties surrounding survivability in seawater, biomineralisation rates, net CO₂ drawdown, and compatibility with engineered substrates. Here, we address four conceptual barriers using thrombolites from hypersaline Lake Clifton (Western Australia). First, long-term experiments show that thrombolite communities not only survive but persist and grow in natural seawater for >18 months, overturning assumptions that they are restricted to extreme hypersaline environments. After 12 weeks, seawater‑acclimatised thrombolites showed increases in mass (+7.78%,) and density (+2.69%), whereas hypersaline controls showed slight mass loss. Second, these changes were accompanied by strong geochemical signatures of biomineralisation, including, an ∼11-fold rise in carbonate alkalinity, 93% depletion of soluble Ca²⁺, and directional increases in thrombolite total inorganic carbon (TIC) under seawater conditions. Third, we quantify rapid net headspace CO₂ drawdown under controlled mesocosm conditions. In sealed mesocosms, seawater thrombolites reduced headspace CO₂ from ambient levels (∼0.04%) to near-zero (< 10⁻⁷%) within 2–4 hours, sustaining near‑zero concentrations for seven days, whereas controls showed only gradual decline and did not reach near-zero concentrations. Finally, we demonstrate bioengineering potential using Ca-modified polyvinyl-alcohol nanomembranes, which supported rapid microbialite colonisation and enhanced carbonate deposition relative to unmodified substrates. Together, these results show that thrombolites can acclimatise to marine conditions, exhibit rapid CO₂ drawdown in closed mesocosm systems, and mineralise engineered substrates, providing mechanistic insight into microbialite-driven carbon cycling processes relevant to emerging CO 2 removal strategies.