Marine Bacterial Biopolymers, Cyanobacteria and Seaweed Biomasses as Soil Amendments to Enhance Soil Wetting Properties and Water Retention

Soil water retention is a key factor in ecological processes regulating ecosystem stability and resilience under environmental stress. In this regard, marine-derived additives may provide sustainable strategies to enhance soil water dynamics. Here, novel biopolymers derived from thermophilic bacteria, including six exopolysaccharides (EPS1–EPS6) and four biosurfactants (BS1-BS4), and biomasses from seaweed (BM1–BM4) and marine cyanobacteria (BC1–BC2), were investigated for their wetting properties and soil water retention. Wetting properties, including reduction in contact angle (RCA) and atmospheric-air moisture uptake (AMU), were monitored for 36 h at constant temperature (30 °C). The effect on soil water retention was evaluated in terms of water loss of soil samples treated with two different concentrations (0.5 and 1% w/w) of either biopolymers or biomasses in a microcosm consisting of 10 g of soil and 10 mL of water, kept at a stable temperature of 22 °C for 200 h (until complete evaporation occurred). BC2 derived from Leptolyngbya sp. 43.3 was the best wetting agent (RCA = 39.44%), while the EPS4 produced by Bacillus horneckiae SBP3 was the best humectant agent (AMU = 179.63%). Soils amended with bacterial biopolymers (EPS4, EPS5, EPS6, BS1 and BS3), as well as biomasses derived from cyanobacteria BC2 and seaweed BM1–BM4, produced better improvement in soil water retention, with marked effects at the concentration of 1% w/w. The lipopeptide BS1 was the most effective in water loss reduction over a specific time of 96–125 h at both concentrations. These findings highlight the potential of these materials as nature-based solutions to improve soil-mediated ecosystem resilience to drought under climate change.