Use of cultivars with certain agronomical traits has been throughout the development history of kelp Saccharina japonica farming in China. Pursuit of high yield had been on top of all the parameters. However, in recent years, demand for better blade quality has steadily become intensive, especially in the northern farming region of this country. In this article, we report a newly bred, intraspecific hybrid E25 (as designation code), of its biological features and its applicability as a potential cultivar in the related farming region. A fully grown sporophyte of E25 could reach 4–5 m in length, 0.6–0.8 m in width, and 3–5 kg in fresh biomass depending on the farming zones and growing density within ca. 7-month cultivation period from November to May–June of next year. Unlike most of the cultivars or naturally grown sporophytes, sporophyte of E25 lacks fascia in the middle of the strip and possesses smooth blades. It has thicker ruffled undulate margins on both longitudinal sides and relatively low water content in its blade. Due to its much higher chlorophyll a content, blanched product is greener than most of the other cultivars, much favored by the processing industry. Seedling production of E25 relays on a pair of vegetatively propagated unialgal male and female gametophytes. High polymorphic microsatellite markers could accurately identify its uniformed genotype and distinguish it from other conventional cultivars. The first filial sporophytic generation of E25 (E25-F1) maintains the same yield level as its parent, reaching 23–25 kg m−1 cultivation rope, but with larger deviations in individual sizes as reflected by its higher coefficient of variations (CV).

In aquaculture of the kelp Saccharina japonica, hybrid cultivars could improve both the yield and quality of the farmed products. Hybrid cultivars refer to the diploid macroscopic offspring resulted from crossing a pair of haploid vegetative gametophyte cell lines. When these two cell lines are derived from individuals that are geographically isolated, the adult sporophytic offspring mostly remain immature in their second year of growth from June to September in the sea. The absence of sori in the adult sporophytes of the hybrid during this period prevents them from being used as parental ones for releasing spores to produce “summer seedlings” in hatchery. This is due to the seasonal restriction of hatchery operation which requires the availability of matured sori exclusively in August in the temperate zone in the northern hemisphere. The reasons for appearance of nonsporulating adult individuals has never been investigated although raising adult sporophyte in land-based tank filled with recirculated cold water at 10 °C has been a routine operation in hatchery enterprises in the subtropical zone in the south region of China for more than 20 years. While in the north region of China, parental sporophytes have been always raised in the sea until they are used for hatchery. From 2017 to 2019, we have conducted a series of culture experiments, both in beakers with isolated thallus in incubators and in large concrete tanks with intact individuals, trying to understand this phenomenon and look for a solution. The results showed that, (1) the hybrid cultivars demonstrated a feature of photoperiodic short-day plant and that it could form sorus within 25 days when exposed to SD at higher temperature (18 °C); (2) the hybrid cultivar maintains unique genetic structures different from the conventional ones as revealed by use of ten polymorphic microsatellite markers; (3) the sensitivity of the hybrids to the shortening of day-length differs in at least 10 days among individuals and could be inherited to its offspring; (4) sori induction in the intact adult sporophytes could be achieved within a month when supplied with SD, applicable to all current cultivars including the hybrids. The discovery and solution presented in this article has opened a gateway to utilization of hybrid cultivars in the giant kelp farming industry in China.

Although the original Saccharina japonica populations in China are assumed to have been accidentally introduced from Japan in 1920s, few investigations have been performed to prove it. Genetic structure analysis is expected to shed light on this question. In the present study, 11 highly polymorphic microsatellite markers were employed to analyze the genetic structure of representative populations of S. japonica in the Northwest Pacific, including the four varieties from Japan. Populations from China had a much lower level of genetic diversity than the ones from Japan in terms of the number of alleles, private alleles and locally common alleles, and the values of heterozygosity, suggestive of the founder effect in the introduced populations. Both the Bayesian model-based and genetic distance-based analyses revealed three major clusters: (1) S. japonica var. japonica, var. religiosa, and var. ochotensis from Hokkaido, as well as the populations from Korea and Russia; (2) populations from China; and (3) var. diabolica. Saccharina japonica var. diabolica harbored the highest number of private alleles, with three of them having a frequency of 1.00 at three loci, which have the potential to be used as specific markers to distinguish this variety from others. Significant genetic differentiation was found between almost all the populations except for a few cultivated populations from China. Populations from China were most closely related to S. japonica var. japonica, which was in turn most close to var. religiosa and the Korean population. The microsatellite markers used in this study were shown to be polymorphic enough to decipher the subtle genetic structure of S. japonica, including the different varieties.

The giant kelp farming industry in China, with an annual production of 1.4 million t (dry weight, DW) in a farming area of 130,564 ha, depends on a safe and constant supply of sporelings each year that are nurtured in ca. 20 commercial hatchery enterprises in Shandong and Fujian provinces in the north and south, respectively. Extension of the kelp growing period in the sea relies on the use of the so-called summer sporeling. Its production is based on using recirculated cold water (5–10 °C) to allow gametogenesis to happen and thereafter the young sporophytes to grow in summer when the ambient seawater temperature (above 20 °C) is too high. This allows sporophytic sporelings to be transferred to open water in early autumn, 2 months earlier than naturally occurring plants in coastal populations. Although historically commercially viable and successful, there have been few reports to describe this hatchery process. In this case study, the entire process of a commercial hatchery manipulation in the 2015 production season at Lvshun, Dalian, northern China, is presented, including pre-treatment of parental plants, control of spore release, and control and adjustment of solar irradiance and temperature at different developmental stages of the plants as well as the nutrient supply regime over the entire course. Analyses of the data and process in this report is expected to shed light on future optimization of the technique as well as providing useful clues for other farmed kelp species in other regions of the world.

Seedling production of brown alga Saccharina japonica in China happens in recirculated cold seawater to provide sufficiently large young plants for open sea cultivation when the seawater temperature drops below 20℃. In order to clarify whether there was a variety contamination problem in the sharing recirculated cold water system when more than one variety was produced in this investigation,a female gametophyte clone was used to test whether hybrid sporophytes could be obtained at the inlet of the water system and inside the seedling culture tanks, using parthenogenetic sporophytes as the control. Four microsatellite loci were used to further analyze the genotypes of the sporophytes. Results showed that the sporophytes obtained at the water inlet and inside the seedling production tanks were composed of alien genes indicating clearly the presence of variety contamination. Thus,this paper suggested that an independent water system was required to cultivate the parental plants in order to maintain purity and superior characteristics of each variety.

During sexual reproduction of seaweeds, spermatozoid (sperm) discharge is triggered by chemical messengers (pheromones) released by the female gametes. The chemotactic ability of the sperm ensures fertilization success. Using unialgal male and female gametophyte material under designated standard gametogenesis testing (SGT) conditions, the potential life-span of the sperm of two seaweeds, Saccharina japonica and Undaria pinnatifida, was assessed by their ability to fertilize eggs. Results show that within 20–30 min after being discharged, sperm of both species could complete fertilization without an apparent decline in fertilization rate. Although fertilization rate 60–120 min after sperm discharge dropped significantly in both species, some sperm were viable enough to fertilize the eggs. In S. japonica, at 12°C, some sperm were able to fertilize eggs up to 12 h after discharge. In both species, egg discharge rates (EDR) in the male and female mixed positive controls were significantly higher than those of all the sperm-testing groups. Doubling the seeded male gametophytes of S. japonica in the SGT tests significantly increased the EDR, further confirming the effect of the presence of the male on the female in terms of facilitating egg discharge from oogonia.