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Pseudo-nitzschia can divide asexually, forming characteristic long chains of cells. These chains can fragment as they grow.

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With each division two genetically identical cells are produced. One is the same size as the original cell while the other is slightly smaller. Over time, this results in size reduction of the population.

Developed cells exit the auxospore

Cell size cannot decrease forever, so once the diatom reaches about half of its original size it must reproduce sexually. Two parent cells align and each one forms two gametes.

One parent produces active gametes and the other produces passive gametes. Active gametes migrate to meet the passive gamete and fuse.

The fused gametes form a structure called the auxospore. These auxospores elongate and protect the developing Pseudo-nitzschia cell until it is fully grown.

Pseudo-nitzschia produce a toxin called domoic acid, but toxicity of a single cell depends on its species as well as environmental conditions. These pennate diatoms are protected by a silica structure, called a frustule, which is composed of two nested halves.

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Similar to the dinoflagellate life cycle, the life cycle of diatoms (Pseudo-nitzschia, for example) includes both asexual and sexual phases. During asexual production, two genetically identical cells are produced, but one is the same size as the parent cell, while the other is slightly smaller Over time, asexual reproduction in most diatoms results in a decrease in the mean cell size of the population, which can only be restored through sexual reproduction. During sexual reproduction, two parent cells align, and one produces two “active” gametes” while the second produces two “passive” gametes. The active gametes migrate and fuse to the passive gametes, which then form a structure called an auxospore. Auxospores function to protect the cell as it develops and elongates, thus restoring cell size. If diatoms are unable to undergo sexual reproduction, cell division will continue, but the mean size of the daughter cells will continue to decrease until they die. This is why diatoms can be difficult to maintain in culture over long periods of time. Diatoms do not produce cysts, but there is some evidence that they can enter a “quiescent phase” that may allow them to withstand unfavorable conditions.

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Sexual phase

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Asexual phase

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Pseudo-nitzchia Life Cycle

gametes

auxospores

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