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Rapid ocean warming is affecting kelp forests globally. While the sporophyte life stage has been well studied for many species, the microscopic life stages of laminarian kelps have been understudied, particularly regarding spatial and temporal variations in thermal tolerance and their interaction. We investigated the thermal tolerance of growth, survival, development, and fertilization of Ecklonia radiata gametophytes, derived from zoospores sampled from two sites in Tasmania, Australia, throughout a year, over a temperature gradient (3–30°C). For growth we found a relatively stable thermal optimum at ~20.5°C and stable thermal maxima (25.3–27.7°C). The magnitude of growth was highly variable and depended on season and site, with no consistent spatial pattern for growth and gametophyte size. Survival also had a relatively stable thermal optimum of ~17°C, 3°C below the optimum for growth. Gametophytes grew to single cells between 5 and 25°C, but sporophytes were only observed between 10 and 20°C, indicating reproductive failure outside this range. The results reveal complex effects of source population and season of collection on gametophyte performance in E. radiata, with implications when comparing results from material collected at different localities and times. In Tasmania, gametophytes grow considerably below the estimated thermal maxima and thermal optima that are currently only reached during summer heatwaves, whereas optima for survival (~17°C) are frequently reached and surpassed during heatwaves, which may affect the persistence and recruitment of E. radiata in a warmer climate. [ABSTRACT FROM AUTHOR]

The swimming of male gametes of brown algae is controlled by two heterokont flagella, regardless of reproductive system, but the patterns of flagellar movement can vary. We investigated sperm chemotaxis and phototaxis in four oogamous Australasian fucalean species. Sperm morphology was similar to that of male gametes of isogamous and anisogamous species but their chemotactic behaviours were much closer to those of other oogamous species than those of isogamous and anisogamous species. Moreover, unlike anisogamous species, sperm chemotaxis of the oogamous species, Hormosira banksii, was not inhibited by a phosphodiesterase inhibitor, theophylline, suggesting variability in regulatory mechanisms of sperm chemotaxis amongst brown algae. In our previous work, lower extracellular Ca2+, supernatant of female gametes and theophylline induced the reversal of phototactic signs in anisogamous male gametes. In contrast, lower extracellular Ca2+ and theophylline did not significantly affect phototaxis in sperm of the oogamous H. banksii. Moreover, high extracellular Ca2+ and supernatant of eggs disrupted the negative phototaxis but did not reverse their phototactic sign. These results suggest that the regulation of phototaxis also varies amongst brown algal gametes. Further investigation of brown algal species is warranted to assess the generality of these patterns of chemotaxis and phototaxis among anisogamous and oogamous gametes. [ABSTRACT FROM AUTHOR]

Drifting, fertile thalli are well documented to be the primary long-distance dispersal vector for many marine macroalgae, but little information about reproductive viability of drift is known. This study examined the reproductive viability and longevity of floating fragments of the intertidal Australasian fucoid Hormosira banksii (Turner) Decne. Beach wrack surveys and field experiments were conducted to test the model that long-distance dispersal is achieved in H. banksii via floating, fertile fronds. High densities of beach wrack fragments were evident during summer compared to autumn. The majority of beach wrack occurred on sandy beaches rather than rocky shores. Both male and female fragments were present in the beach wrack. Detached fronds were capable of releasing gametes up to 8 weeks after detachment. Beach wrack produced high fertilization rates and recruited successfully onto artificial panels. Results suggest that detached fragments are reproductively viable and that floating, fertile fronds may be an important mechanism for facilitating long-distance dispersal in this species. Nevertheless, the frequency of fronds reaching a suitable habitat and contributing to gene flow between populations, or colonizing new populations, may not be proportional to the total density of beach wrack. [ABSTRACT FROM AUTHOR]

Seeding twines are convienient for outplanting cultured seaweeds, facilitating seeding in high densities. Twines are typically made of synthetic polymer fibres, likely for increased durability and economic benefits relative to natural fibres. However, synthetic twines can contribute to oceanic plastic pollution if fragmented or lost at sea, whereas natural-fibre twines will biodegrade over time. We compared the seeding success of zygotes and germlings of the fucoid Phyllospora comosa among different twines varying in diameter and composition (nylon (1 mm & 3 mm), Kuralon (2 mm & 3 mm) and Algae Twine™ (1 mm) and hemp (1 mm) in three separate experiments (A, B & C). For experiments A and B (both six weeks duration), equal densities of P. comosa embryos were seeded directly onto twines wrapped tightly around 20-cm lengths of 50-mm diameter PVC pipe and cultured indoors in controlled conditions. For experiment C (three-months duration) equal sections of each twine were wrapped in a randomised order around replicate quadrats (50 cm × 50 cm) and cultured in large flow through outdoor tanks. Successful seeding of P. comosa zygotes occurred on all twines in all three experiments, but differences in length, percentage cover and densities varied among twine types across times, and between experiments. Germlings had reached ~ 1-mm lengths by 9 weeks of age in Experiment A, but similar lengths were reached by 4 weeks in Experiment B. In Experiment C, germlings had attained ~1.7 mm by 12 weeks on all twines except hemp. This study demonstrates that whilst 2-mm Kuralon twine generally performed well, a range of twine types may be suitable for seeding and subsequent hatchery growth of P. comosa embryos, including the natural twine, hemp. Farmers may thus effectively base their choice of seeding twine on the cost of material versus environmental trade-offs of using natural versus synthetic twines. However, performance of the different twines at sea remain to be tested in future studies. • Demand for seaweeds is creating opportunities for culture of novel seaweed species. • Synthetic twines for seeding seaweeds contribute to oceanic plastic pollution. • Phyllospora comosa seeded on twines varying in composition and diameter. • Farmers may base twine choice on material cost versus environmental trade-offs. • Future studies of performance of different seeded twines deployed at sea are needed. [ABSTRACT FROM AUTHOR]