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Abstract Meiospores of Laminariales macroalgae must select a benthic substratum suitable for their attachment and survival, but also suitable for the development of the sessile sporophyte stage which can grow metres in length. In a controlled four month experiment, meiospores of Saccharina latissima were allowed to settle and develop on twelve different polymer surfaces. Highest meiospore settlement was seen where the attachment force of the developing macroscopic sporophytes was weak (<0.3 N), leading to the eventual detachment of the juveniles before they can grow 100 mm. The sporophyte holdfast cover (%) was strongly related to the biomass achieved (R2 = 0.68) and negatively correlated to the water contact angle (θ w) of the polymer (R2 = 0.45). Yet, meiospore settlement was positively correlated to θ w (R2 = 0.24). The study shows that the selective settlement of the meiospore conflicts with the requirements of the macroscopic sporophyte to attach firmly. It is hypothesised that higher θ w is used by kelp meiospores as a cue for recently disturbed environments, allowing gregarious settlement in areas with reduced interspecific competition. Graphical abstract Unlabelled Image Highlights • Saccharina latissima meiospores favour surfaces with high water contact angles (θ w). • The holdfast bioadhesive of the sporophytes binds weakly to high θ w surfaces. • Juvenile sporophytes are then easy detached once 100 mm, so mortality is high. • Settlement selection can conflict with the needs of the sporophyte to attach firmly. • High θ w , may indicate recently disturbed surfaces without intraspecific competition. [ABSTRACT FROM AUTHOR]

The macroalga Saccharina latissima holds promise as a widespread crop in northern Europe. Currently, seeded lines are cultured for 1–2 months within a hatchery until they become ≥1 mm sporophytes, which are then outplanted to a farm. Textiles are being developed as a cultivation substrate for macroalgae, however due to their large surface area, these need either direct in-situ seeding at the farm or a very short, high turnover hatchery period. Two materials, Kuralon twine and non-woven textile, were seeded using three S. latissima developmental stages: meiospore, gametophyte or juvenile sporophyte. The gametophyte and sporophyte stages were applied using a binder solution to adhere them to the materials. These were outplanted at a seaweed farm in the Sound of Kerrera, UK. After 5 weeks, fronds were significantly larger (45 ± 25 mm) and more abundant (20 ± 8·20 cm −1 ) when seeded with sporophytes over gametophytes or meiospores (8 ± 10 mm and 2 ± 2·20 cm −1 ). This reflects the growth advantage of outplanting juvenile sporophytes, since they are larger and more developed when outplanted. Higher fouling of filamentous algae was seen on the non-woven textile but this did not appear to affect growth. After four months, sporophyte seeded materials had the largest fronds and the greatest fresh mass of 2.1 ± 0.8 kg·20 cm −1 , equivalent to simultaneously deployed hatchery reared twine (2.0 ± 0.1 kg·20 cm −1 ). Gametophyte seeding achieved 0.8 ± 0.6 kg·20 cm −1 while meiospore seeding achieved only 0.1 ± 0.1 kg·20 cm −1 . No difference was found between growth on Kuralon twine or non-woven textile ( p  > 0.05), showing that both are suitable growth substrates. Seeding of juvenile sporophytes onto textile using the binder is demonstrated to be a successful method for the cultivation of S. latissima , and may require only 1% of the hatchery tankage, once optimised. It is expected that this method is transferable to the cultivation of other kelp species. [ABSTRACT FROM AUTHOR]

For the cultivation of the European phaeophyte macroalgae Laminaria digitata and Saccharina latissima , meiospores are settled onto twines within a hatchery, where they are grown for several months. The twine used is often a customarily selected synthetic polymer, polyamide or polypropylene. However, little is known about the impacts of this choice on hatchery performance. To test the effect of substrate material, we settled and cultured meiospores from both L. digitata and S. latissima , independently on twelve polymer blocks for 4 mo. They were first grown for 2 mon under laboratory conditions, then a further 2 mon in outdoor tanks. Meiospore settlement varied significantly between polymers by up to 15-fold ( p < 0.0001) with some species-specific differences also observed ( p < 0.0001). Tufnol was the least suitable polymer, as formaldehyde leachate reduced settlement and inhibited juvenile growth/development. After 8 wk, all polymers excluding Tufnol, were performing similarly with generally ~ 1 mm sporophytes present at a density of 1–2 mm − 2 , A negative density-dependent effect of sporophyte size and density was observed in both species ( p < 0.05). At the end of the experiment, two distinct grouping of polymers were identified regarding S. latissima . Those that initially had very high settlement (high density polyethylene, polymethyl methacrylate, polyoxymethylene copolymer/homopolymer and polytetrafluoroethylene) had the lowest final mean lengths, % cover and biomass (< 0.2 g wet weight·block − 1 ) at the end of the experiment. Conversely many of the polymers with the lowest initial settlement (polyamide, polycarbonate, medium density polyethylene and polyvinylchloride) had the highest final mean lengths, % cover and biomass (1.7–4.9 g wet weight·block − 1 ). This reversal of fortunes is discussed regarding discriminatory meiospore settlement, differences in apparent adhesion strength of the seaweed holdfast and the transition of the growing sporophytes from a viscous force dominated boundary layer environment to a turbulent dominated environment with increasing drag as the sporophyte grows. [ABSTRACT FROM AUTHOR]