In the Seaweed Nursery: Reasoning and Reproduction

Introduction: why a kelp nursery?

Similarly to land plants, the lifecycle of seaweed begins with some type of ‘seedling.’ Of course, seaweeds are not land plants, so their life cycles are different and their juvenile stages are not ‘seeds’ in the way we traditionally think of a seed. For example, the kelp life cycle begins with single-celled, free-swimming sporesA tiny, single-celled, reproductive unit that us capable of producing offspring without sexual reproduction. that are released from the kelp’s reproductive tissue. After settlementA stage in an organism's life cycle where the organism goes through a transformation between a planktonic larval stage and a benthic adult stage. and germinationThe development of a plant from a seed or spore after a period of dormancy., these grow into a gametophyte, kelp’s microscopic filamentous phase. The gametophyte produces gametesThe reproductive cell of an animal or plant. which join together to form a zygoteA fertilized egg. that grows into a juvenile sporophyte.A phase in the life cycle of plants that produces asexual spores. Juvenile sporophytes are small kelp blades that then grow to adulthood attached to a substrate via a holdfastRoot-like structure that anchors the seaweed to the seafloor., which anchors the kelp blade during its growth to full size.

The reproductive strategyThe way an organism mates and/or raises their offspring. and growth patterns of seaweeds vary by species and can be quite complex. Some seaweeds, including the kelps of commercial interest in Maine, have what is called a biphasic life cycleA lifecycle that alternates between a microscopic larval phase and a macroscopic adult phase.. This means they alternate between a microscopicMillimeter sized. and a macroscopicCentimeter sized. growth phase. Other seaweeds may have only one growth phasePhase in an organism's life, before their reproductive stage, where most of the organism's energy is put toward internal processes. while still others have as many as three. For successful reproduction and growth of seaweed, the environmental conditions during the release and settlement of the reproductive cells must be just right.

Between ensuring that seaweed is able to reproduce successfully and that the subsequent growth of the juvenile organisms happens on a substrate that can be used for farming, it is necessary to have a lot of control over the early stages of a seaweed’s life cycle. To provide this level of control, researchers and farmers have developed nurseriesA place that produces larval and juvenile, fish, shellfish and macroalgae to support the aquaculture industry. where farmed seaweeds spend the first few weeks of their development. These nurseries are essentially saltwater lab facilities that provide control over a variety of environmental factors, allowing nursery operators to raise juvenile seaweed to the point where it can be transferred to the open ocean.

Notes on procuring seed

For seaweed farmers in Maine, there are two options for obtaining seaweed ‘seed’: 

1. Purchase seed from an existing commercial nursery 
2. Start your own nursery to provide seed

New farmers will nearly always purchase seed from an existing nursery rather than growing their own seed right off the bat. For them, it is most important to master grow-outThe time period from when the cultured organism is removed from the nursery to when it is harvested. before expanding into nursery techniques. The exception to this rule would be potential farmers whose business plan is explicitly to generate juvenile seaweed seed. For farmers looking to purchase seed, Maine has multiple commercial seaweed nurseries which offer seeds for sale or as part of a contract with farmers.

For established farmers considering starting their nursery operations, it’s important to evaluate whether they have the time, funding, space, equipment, farm size, species expertise, and technical/lab skills needed to produce their seed. They should also be aware of the significant learning curve that goes with producing reliable, high-quality seeds. Based on these factors, investing in a nursery may make more or less sense to your business plan. To help the curious seaweed nerd understand what goes into producing seed for a seaweed farm, this section will describe the reproduction of a couple of key seaweed species in Maine. The following section will include an overview of the nursery process for kelp as well as links to in-depth manuals for starting nurseries for various seaweed species.

Reproduction: where does baby seaweed come from?

Multicellular organisms cycle between different phases during their life cycles depending on how they have evolved. To understand seaweed reproduction generally, we need to begin with a description of the two different phases of a multicellular organism’s life cycle: the growth phasePhase in an organism's life, before the reproductive stage, where most of the organism’s energy is put toward internal processes. and the reproductive phase.Phase in the life of an organism which starts when the organism reaches reproductive maturity and uses their energy to create reproductive cells.

During growth phases, cells in an organism take up nutrients and use energy to replicate, grow, and perform other internal functions like fighting off diseases. When an organism reaches maturity in a growth phase, energy is also used to prepare for reproduction. In seaweed, the growth phases are generally referred to as vegetative phases. Depending on what type of reproductive cycle the seaweed has, specific growth phases may include a gametophyte phase– in which the seaweed has only one set of chromosomesProtein structure found in the nucleus of most cells that contains an organism's genetic information.– and a sporophyte phase– in which the seaweed has the complete two sets of chromosomes.

Alternatively, during a reproductive phase, cells in an organism use their energy to produce and release reproductive cells that will grow to become the next generation of organisms. These reproductive cells are special for two reasons: 

1. First, instead of containing two sets of genetic material as regular cells do, reproductive cells only contain one set of chromosomes. These cells with one set of genetic material are known as haploid cellsCells that contain a single set of chromosomes.. 
2. Second, in addition to being haploid, reproductive cells are able to combine with each other in a process known as fertilizationThe process of combining the male sperm with the female egg during reproduction. to form a new organism. 

When two reproductive cells fertilize, they form a single new cell with two sets of genetic material known as a zygote. Zygotes are formed through fertilization of two haploid cells and are considered diploid cellsA cell that contains two copies of each chromosome., meaning they have two sets of chromosomes. In multicellular organisms, the creation of a zygote indicates the end of the reproductive phase and the beginning of a new growth phase.

Generally, algae with a sporic life cycle–like sugar kelp– follow the pattern: growth phase (1), reproductive phase (1), growth phase (2), reproductive phase (2), repeat. The first growth phase is known as the sporophyte phase and this is the only point at which the alga has a full two sets of chromosomes (diploid). The first growth phase begins with a single-celled, diploid individual called a zygote. When the zygote begins cell division and growth, it becomes a juvenile sporophyte and eventually an adult sporophyte.

Gametophytes are multicellular despite containing only one set of chromosomes, and their development marks the second growth phase of the alga. Gametophytes are different from sporophytes because they are haploid while sporophytes are diploid. Additionally, gametophytes may have a completely different appearance from sporophytes. Many gametophytes have historically been mistaken for entirely different species from their sporophyte counterparts. This visual difference between two or more growth phases of an organism is referred to as the heteromorphic alternation of generations.

Growth of from spore to sporophyte. (a) spores, (b) settled spores begining to germinate, (c) gametophyte, (d) male and female gametophytes, (e) microscopic juvenile sporophyte, (f) macroscopic juvvenile sporophyte.

 Image credit: Velíz et al. 2006

Kelp sorus tisse.

Image credit: David Leith, Maine Family Seafarm Coop

When gametophytes reach maturity, they enter their second reproductive phase by producing and releasing single reproductive cells known as gametes. These gametes are male or female and combine via the process of fertilization to form a zygote. In sugar kelp, the male gamete is released from the male gametophyte and fertilizes the female gamete, which stays attached to the female gametophyte. When fertilization occurs, the resulting single-celled, diploid ‘zygote’  marks the completion of the cycle and the return to the first growth phase. 

Connecting the dots: the full life cycle of dulse

Image credit: Stévant et al. 2023

If you are following a pre-established methodology to grow a given seaweed species in a nursery setting, basic knowledge of that seaweed’s reproductive cycle is necessary. If you plan to experimentally attempt nursery cultivation for a new species, extensive knowledge of that seaweed’s reproductive cycle is necessary– and even then, attempts at cultivation are often not successful.