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 release and settlement of the reproductive cells must be just right.

Biphasic lifecycle

In seaweeds with biphasic lifecycles, the organism alternates between a growth phase with one set of genetic material (chromosomes, n) and a growth phase with two sets of genetic material (2n). In some seaweed species, the two growth phases have different appearances which is called heteromorphic alteration of generations. Alternatively, when seaweeds with biphasic lifecycles have similar morphologies in their two growth phases, they are said to have isomorphic alternation of generations. The diagram below showing the life cycle of Ulva is an example of an isomorphic biphasic lifecycle.

Image credit: Bast 2014

Lifecycle and substrate

Sugar kelp, winged kelp, and skinny kelp have biphasic life cycles with heteromorphic alternation of generations. Juvenile kelp grows from the spots where female gametophytes attach after zoospore settlement, so an appropriate substrate must be available early on in the kelp’s lifecycle. This is why kelp nurseries use seed spools to as a substrate for kelp spores to settle on.

Image credit: Atlantic Sea Farms

Between ensuring that a seaweed is able to reproduce successfully and that the subsequent growth of the juvenile organisms happen on a substrate that can be used for farming, it is necessary to have a lot of control over 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.
Spore release

The nursery process for kelp begins with inducing spore release from reproductive kelp tissue.

Video credit: Jaclyn Robidoux, Maine Sea Grant

Seed spool innoculation

PVC tubes wrapped with nylon twine are used as a substrate for kelp spore settlement in a process called innoculation.

Image credit: Ocean’s Balance

Grow to size

Kelp spores settle, become gametophytes, then grow into sporophytes. Sporophytes are grown in the lab until they are sufficiently sized to move to the farm.

Image credit: National Fisherman

Notes on procuring seed

For seaweed farmers in Maine, there are a 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 contract to farmers.

For established farmers considering starting their own 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 own seed. They should also be aware of the significant learning curve that goes with producing reliable, high quality seed. 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 reproduction of a couple 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.

Where to buy kelp seed in Maine

These are a few of the options for purchasing kelp seed in the state of Maine. Additional options may be available.

Atlantic Sea Farms

Atlantic Sea Farms provides all kelp seeds to partner farmers for free, but also makes organic kelp seed spools availabe for purchase to those outside their farmer network.


Atlantic Sea Farms kelp seed

Ocean's Balance

Ocean’s Balance offers seaweed farmers kelp seed spools grown in their state-of-the-art nursery.


Ocean’s Balance kelp seed

Springtide Seaweed

Springtide seaweed takes orders for organic seeded seaweed spools.


Springtide Seaweed kelp seed

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.

In this schematic, the growth phases are indicated by the label ‘mitosis’ while the reproductive phases are indicated by the label ‘meiosis.’ In both the haploid and the diploid life cycles, there is one growth phase and one reproductive phase that alternate (R1, G1, R1, G1, etc.). In the biphasic life cylce, there are two separate growth and reproductive phases (R1, G1, R2, G2, R1, G1, etc.). 

Image credit: Libre Texts Biology

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 a 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 chromosomes– and a sporophyte phase– in which the seaweed has the complete two sets of chromosomes.

Mitosis (diploid and haploid cells)

Mitosis is the process of cell replication that creates two identical daughter cells from the mother cell. In mitosis, both daughter cells possess the same quanity of genetic material (in the form of chromosomes) as the mother cell. Mitosis can occur in cells with two sets of choromosomes (diploid cells) and in cells with one set of chromosomes (haploid cells). Mitosis is key in any growth phase and any form of asexual reproduction. 

Image credit: EarthHow

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 cells
  2. Second, in addition to being haploid, reproductive cells are able to combine with each other in a process known as fertilization 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 cells, 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.

Meiosis (forming haploid cells)

Meiosis is the process of cell replication that creates four distinct daughter cells from a single mother cell. In meiosis, all of the daughter cells have one set of genetic material in the form of chromosomes (haploid cells) while the mother cell has two sets of genetic material (diploid cells). Meiosis is the process that creates haploid cells from diploid cells, and is key in any form of sexual reproduction. 

Image credit: EarthHow

For seaweeds, there are three patterns of growth and reproductive phases defined in terms of meiosis: zygotic meiosis, gametic meiosis, and sporic meiosis. In this section, sporic meiosis will be of greatest interest to us because Maine’s commercially valuable kelps as well as other farmed red seaweeds like dulse (Palmaria palmata) reproduce via sporic meiosis.

Zygotic Meiosis

One haploid growth phase and followed by a diploid reproductive phase. The purpose of the diploid zygote is to undergo meiosis that starts a new haploid growth phase. 

Image credit: Algae 3rd Edition

Gametic Meiosis

One diploid growth phase followed by production of haploid reproductive cells. The purpose of the haploid reproductive cells is to combine via fertilization to start a new diploid growth phase. This type of meiosis includes animals.

Image credit: Algae 3rd Edition

Gametic Meiosis

A diploid growth phase followed by production of haploid cells that undergo a second growth phase followed by a second reproductive phase which ends with fertilization and a return to the diploid growth phase. This type of meiosis includes sugar kelp, skinny kelp, and winged kelp.

Image credit: Algae 3rd Edition

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.

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

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. In fact, 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.

Kelp sorus tisse.

Image credit: David Leith, Maine Family Seafarm Coop

Near the end of the adult sporophyte phase when the organism reaches maturity, the alga produces a reproductive tissue (in kelp, this is called the sorus tissue (sori plural)). When the environmental conditions are appropriate and the reproductive tissue is fully developed, reproductive cells known as spores are released. Spore release is the beginning of the first reproductive phase which lasts until the spores settle, germinate, and begin to divide. At that point, cell division starts in the spores and they become gametophytes, marking the beginning of the second growth phase.

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 sugar kelp

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In the mature adult kelp, sorus tissue develops. Sorus tissue contains the reproductive cells.

Haploid reproductive zoospores are released into the watercolumn and are mobile for about 24 hours until settlement.

Male and female gametophytes grow from the settled zoospores in a haploid vegetative phase.

Male and female gametophytes produce reproductive cells. The motile male reproductive cell fertilizes the female reproductive cell positioned on the female gametophyte.

The newly formed kelp zygote begins to grow via mitosis and becomes the juvenile sporophyte.

Image credit: Visch et al. 2019

Connecting the dots: the full life cycle of dulse

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The male and female gametophytes begin to grow.

The mature adult dulse produces male and female reproductive cells that will settle and become male and female gametophytes.

The male gametophyte takes a year to grow vegetatively before it is mature and able to release the male gamete cells.

The female gametophyte is mature and produces the female gamete cell in the same year as the reproductive release from the adult sporophyte.

The female gametophyte is fertilized by gametes from the male gametophyte from the previous year and developes into a juvenile sporophyte.

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.