Abstract
A global rise in kelp aquaculture has resulted in an increased demand for high quality and quantity kelp seedstock, yet many bottlenecks remain to sustainable scale-up solutions. One such bottleneck is increasing biomass quickly and efficiently in the nursery phase of the kelp cultivation cycle. The microscopic gametophyte is leveraged during this phase, using red light to stimulate vegetative growth and delay onset of gametophyte fertility until deployment. However, red light may make the gametophyte less photosynthetically efficient and thus slow growth. A solution may be growing gametophytes in blue light, which can increase production, while changing the nutrient composition of the media to repress fertility. Here we test the efficacy of light source (blue and red), two common media types (PES and f/2) and iron supplementation on the growth, fertility and photosynthetic parameters of Alaria esculenta gametophytes, a commercial kelp species. We found the highest biomass was achieved under blue light with iron-free f/2. Similarly, pigment concentration was higher in blue light and iron-free f/2, while gametophytes were found to have more shade-adapted pigment characteristics under red light. Red light strongly supressed formation of sporophytes and while withholding iron did not supress fertility in blue light, it did result in less malformed sporophytes. Based on this research, we recommend growing A. esculenta gametophytes in iron-free f/2. Failure of iron-free media to supress fertility in blue light may be due to iron retention of gametophytes or trace levels of iron in seawater, which warrants further research. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1007/s10811-025-03665-z.