Kelps are a group of large brown algae from the order of Laminariales which form vast forests in the shallow rocky coasts of cold-water and temperate marine habitats. However, kelp populations are declining all around the world mainly due to anthropogenic impacts, including harvest from wild populations. Restoration and aquaculture activities have the potential to prevent the degradation of these ecosystems. The optimization of juvenile sporophyte production in the nursery stage is essential for the success of kelp aquaculture and restoration. Algae-based biostimulants have been shown to improve the growth, reproduction, and thermal tolerance of plants and algae. However, they have never been tested in kelps. Here, we investigated the use of biostimulants for the growth and reproduction of Laminaria ochroleuca gametophytes, and the potential of biostimulants to enhance the thermal tolerance of young sporophytes. The results showed that biostimulants have no effect on gametophyte growth, and in absence of conventional nutrient solutions, gametophytes didn't become reproductive. However, biostimulants increase photosynthetic performance in absence of nutrients, and they promote faster gametogenesis in the presence of nutrients. When exposed to high temperatures, F1 sporophytes developed in presence of biostimulants were healthier and showed higher growth rates.
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