Elucidation of Maturation Mechanisms of Kelp Sporophytes × SDGs
The coast is a habitat for diverse organisms and a source of fishery resources. These are supported by seaweeds that grow in seagrass beds. We contribute to SDG 14 (LIFE BELOW WATER) by understanding the life of seaweeds and aiming to conserve them.
The United Nations has designated the decade starting in 2021 as the "Decade of Ocean Science," with the aim of contributing to SDGs. Ocean science, as defined by the UN, includes the field of fisheries.
Kelps have a microscopic generation (gametophytes) and macroscopic generation (sporophytes), and we cultivate and harvest the macroscopic generation for use.
Sporophyte maturation is a vital phenomenon essential not only for supporting reproduction in natural seaweed beds and maintaining seaweed beds, but also for securing the seed stock required for the mass seedling production for seaweed cultivation.
Therefore, we are conducting research on the various conditions and mechanisms that lead to the success of sporophyte maturation, as well as the development of artificial control technology.
The kelp we see in our daily life corresponds to the macroscopic generation (called sporophytes) in the kelp life cycle.
The maturation of the macroscopic generation plays an important role in the transition from the macroscopic generation to microgeneration (gametophytes).
The surface of the body rises as the sporophyte matures, forming a group of reproductive organs called the sorus. In the process, meiosis produces germ cells with two flagella, called zoospores.
The released zoospores transition to the microgeneration (gametophytes) by landing on a new substrate.
The gametophyte generation is sexual, and the male and female gametophytes mature, returning to macroscopic generation after the fertilization of each germ cell.
The function of seaweed beds
Seaweed forms seaweed beds in coastal areas, playing various roles in the ecosystem.
Seaweed beds have a wide range of functions, ranging from providing habitat and breeding grounds of coastal organisms to serving as a source of feed and conserving the environment (such as the preservation of nutrient salts in coastal areas, carbon fixation and water purification, and stabilization of the sediment).
The continuous maintenance and management of the seaweed beds are needed to ensure they continue to fulfill these functions.
The purpose of this research is to answer the question of how kelps survive and reproduce in an ever-changing environment.
Particularly, success in the maturation of the macroscopic generation is a life phenomenon essential not only for supporting reproduction in natural seaweed beds and maintaining seaweed beds but also for securing the seed stock required for mass seedling production for cultivation. Therefore, by conducting research on the various conditions and mechanisms that lead to the successful maturation of kelp, as well as the development of its artificial control technology, our research aims to be useful not only for kelp cultivation but also for seaweed bed maintenance and environmental conservation.
Growth patterns of the macroscopic generation
Let’s take Laminaria japonica growing in southern Hokkaido as an example and see the growth process of its macroscopic generation.
Laminaria japonica sporophytes grow from winter to spring. They absorb the abundant nutrients of the Oyashio (cold current) to grow larger.
They store excess nutrients in their body during this period (in the form such as glutamic acid, which supports the Japanese food culture). They grow approximately 5 cm a day during their maximum growth period.
Then, when the nutrients in the seawater are depleted after the large growth (bloom) of phytoplankton in spring, they use the stored nutrients to continue growing for some time.
Once the stored nutrients are exhausted, recovery in the stronger light available from spring to summer promotes the accumulation of polysaccharides, such as alginic acid.
As the stored nutrients are exhausted, they wither from the tip (Suegare or Sakigare in Japanese), and then reach the maturation stage to form a reproductive organ (sorus).
The timing and degree of its formation greatly affect the biomass of the next generation.
The zoospores released from the sorus have chemotaxis, settling in a better microscopic environment, although the area available is narrow.
Then, they become male and female gametophytes, returning to the sporophyte generation after the fertilization of the germ cells.
The withering tissues at the tip serve as a supply source of resources for the sorus parts and meristem, and as a result, they die without reaching maturity.
Thus, the tip indeed plays the role of “keeping the base alive by cutting themselves.”
The meristem of the surviving base regenerates with the benefit of the Oyashio and grows as an individual in the second year.
Cultivation of the macroscopic generation and maturation ability
Species in the family of kelps distributed all over the world have lengths ranging from tens of centimeters to tens of meters, depending on the type of the macroscopic generation.
They are therefore difficult to culture in the laboratory.
Thus, the detailed mechanisms of their maturation are investigated by culturing tissue segments of the macroscopic generation in the laboratory.
Cultures were performed in nutrient-enriched media, and all the segments obtained from the sporophyte formed sori.
Many parts of the natural thalli flow out without reaching the maturity!
All parts of the sporophyte blade have the ability to form the sorus, but many tissues at the tip flow out without reaching maturity in the natural environment.
The formation of the sorus requires several conditions, one of which is the accumulation of resources (nutrients).
Therefore, it was clarified that if the accumulation of resources in the natural environment does not meet the maturity conditions (resource limitation occurs), they flow out without reaching maturity.
The sporophytes of kelps have various abilities to protect themselves and reproduce.
Their cell wall is composed of cellulose as a skeleton and a matrix containing alginic acid and fucoidan, a viscous polysaccharide that exhibits physiological activities, such as antibacterial activity, which serve for defense together with cuticle.
Epidermal cells have developed an intracellular structure called the physode, which includes phlorotannin that plays a role as a repellent for herbivore and UV protection, and it accumulates in large quantities in damaged parts of the kelp.
In addition, when oligosaccharides derived from alginic acid act as elicitors on the alga body, reactive oxygen is generated, inducing various defense reactions.
One such reaction is the activation of haloperoxidase, which releases iodine compounds with antibacterial activity to the outside of the body, playing a part in chemical defense.
This defense mechanism is also an important factor for the protection of the reproductive organ (sorus) of the sporophyte.
The sorus exerts various abilities, protects the zoospore (germ cells), and bridges to the next generation.
Mature individuals of kelp sporophytes are used as seaweed bed seed stock for mass seedling production at cultivation sites or as seed stock to be supplied for the construction of seaweed beds.
More efficient seedling production and management will be possible by securing the required amount of seedlings for anyone, anytime, and anywhere.
Based on the information on kelps obtained in this research, more practical applications to production sites are expected.
Kelp has a mechanism to protect, survive, and reproduce against various environmental stresses throughout its life cycle.
We are focusing on many resource seaweeds, including kelps, to maximize their potential.
1. Mizuta & Yasui, Bot. Mar., 2010 (DOI : https://doi.org/10.1515/bot.2010.047)
2. Mizuta & Yasui, J. Appl. Phycol., 2012 (DOI : https://doi.org/10.1007/s10811-011-9750-8)
3. Mizuta et al., Aquacult. Res. 2007 (DOI : https://doi.org/10.1111/j.1365-2109.2007.01809.x)
4. Kai et al., J. Appl. Phycol., 2006 (DOI : https://doi.org/10.1007/s10811-005-9020-8)
5. Shimizu et al., J. Appl. Phycol, 2018 (DOI : https://doi.org/10.1007/s10811-017-1320-2)
6. Mizuta et al., Fisheries Sci., 1999 (DOI : https://doi.org/10.2331/fishsci.65.104)
・Like the relationship between the growth and maturation of living organisms, there are still many unclear points including the mechanisms involved in switching between the growth and maturation of kelp sporophytes and the relationship between defense and growth/maturation.
We believe it is necessary to closely observe the lives of kelps to understand these mechanisms.
・Kelps have microgeneration. What ability does microgeneration in the natural environment have for survival in the ever-changing environment and for returning to kelp after maturation and fertilization, which we see in our daily life (sporophyte generation)? It is necessary to answer such questions one by one to understand the potential resilience of seaweed beds (the quantity and quality of seed banks).