Naturally occurring protoplasts in two <i>Ulva</i> spp. reveal a previously underestimated proliferation process

Abstract Interest in Ulva cultivation grows worldwide, but associated bottlenecks exist, including ineffective seeding methods, spontaneous reproduction or biomass loss, and high cultivation costs. Spontaneous biomass degradation causes significant losses, and the underlying biological process is still poorly understood. During a spontaneous degradation event in Ulva lacinulata and a reproduction event in Ulva compressa , production of natural protoplasts was observed. U. lacinulata produced high protoplast yields and parts of the original biomass regenerated. In U. compressa , protoplasts were found in fertile thalli, but the original biomass was lost. Protoplast germination rates were low (2.01 ± 0.48 % in U. lacinulata , 4.14 ± 3.31 % in U. compressa ), and resulted in three morphologies: unattached germlings, unattached discs, and cell masses. Discs and cell masses became fertile early and released gametes. Our results provide the first evidence of natural production of protoplasts in Ulva spp. We estimate that higher seeding yields can potentially be obtained by natural protoplast production (5.95 ± 4.50 × 10 10 individuals g −1 ) than by gametogenesis (2.03 ± 1.15 × 10 9 individuals g −1 ), thus closing an important knowledge gap in the life cycle of Ulva species. These results provide important insights into the reproductive cycle of Ulva spp. relevant for large-scale cultivation.