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The development of new natural biostimulant sources for a sustainable agriculture is a strategic issue aiming to improve the healthy food production. In this sense, the goal was to identify the biostimulant action of the calcareous alga Lithothamnium sp. (Rhodophyta) on onion in two experiments, (i) in pots and (ii) at field under organic system, both in factorial scheme with two onion cultivars and foliar sprays with micronized Lithothamnium sp. at concentration of 1.5 and 3.0 g L−1. The plant growth promotion and commercial yield improvement of two onion cultivars were achieved and related to changes on amino acid metabolism and source-to-sink flow stimuli. Better nutrient acquisition also was achieved related to the stimuli to thin roots growth, with N, Mg, Mn, Zn, and Cu content in bulbs improved at 3.0 g L−1, while 1.5 g L−1 allowed highest Ca, Mg, Mn, and Zn content in bulbs. Thus, the results presents Lithothamnium sp. as novel biostimulant source to grow onion, which effects related to the presence of bioactive humic acid that calcareous algae release by micronization.

This work is based on the hypothesis that the depositions of the calcareous alga Lithothamnium sp. at the bottom of the ocean contain bioactive humic substances which may promote plant growth. To achieve this, micronized Lithothamnium sp. powder was investigated in terms of the degree of humification using a colorimetric approach; the bioactivity in comparison with that of auxin IAA (3-indole acetic acid) using a bioassay with Vigna radiata and the effect of foliar sprays on organically grown tomatoes. Humification and formation of humic acid were identified to a high degree in micronized Lithothamnium sp., which was highly bioactive showing auxin hormone-like activity. When applied as foliar sprays, rooting, growth, and yield were improved and concentrations of amino acids, sugars, and protein of tomato plants increased. Thus, the results presented a novel use for Lithothamnium sp., whereby the calcareous algae depositions provided a highly active humic acid for use as a plant biostimulant.