A resting spore is a resistant cell, adapted to survive adverse environmental conditions. Resting spore is a term commonly applied to both diatoms and fungi.
In fungi and oomycetes A resting spore can be a spore created by fungi which is thickly encysted (has a thick cell wall) in order to survive through stressful times, such as drought. It protects the spore from biotic (microbial, fungal viral), as well as abiotic (wind, heat, xeric conditions) factors. Resting spores of a particular oomycete species are important in causing late potato blight. They can lie dormant within the soil of a field for decades until the right conditions occur for viability (plant host present, rain, fire etc.).
In diatoms A similar resting spore life stage is also present in diatoms, and in such case, is also often referred to as the hypnospore. Importantly, the resting spore of marine diatoms is not an obligate stage of the life cycle, except in the minority of studied taxa, where spore production immediately follows the first cellular product of sexual reproduction, the auxospore. Generally, resting spore formation in diatoms is primarily considered a survival tactic for adverse conditions by producing dense spores with thick silica frustules that can sink cells out of the surface, typically high in light and temperature, into the cooler, darker and nutrient rich depths. Spores have been observed to last decades in such conditions that reduce metabolic demand, awaiting mixing events that may carry them back into favorable environmental conditions where they may germinate.
Formation, morphology and germination The formation of resting spores is considered to be the consequence of environmental stress. Spore formation has been described immediately proceeding bloom formation although there are a variety of potential causes for spore formation in blooms. Studies of resting spore formation in diatoms have found that nitrogen limitation, temperature, and light limitation are all capable drivers of spore formation. The differences in resting spore formation reflects the immense diversity in the phylogeny of diatoms. Notably, resting spores are not a feature of all diatoms. Although they can be found in freshwater varieties, and pennate diatoms, they are considered most prevalent in marine centric diatoms. Within marine centric diatoms, resting spore formation has been most commonly observed from a vegetative parent cell, but some species have been noted to require an auxospore parent cell, which is the product of sexual reproduction. The formation of resting spores is product of two acytokinetic divisions of the parent cell, wherein the cytoplasm of the daughter cells is shared. The resting spores produced may be either exogeneous (mature spore has no contact with parent cell), endogenous (completely enclosed within the parent cell), or semi-endogenous (only the hypovalve of the resting spore enclosed within the parent thecae). The common characteristic of diatom resting spores is a thick silica frustule. Generally, the frustule will be morphologically similar to the vegetative cell, but it can differ greatly. The frustule itself may be with or without a cell girdle, which dictate alternate germination processes, whereby the thecae of resting spores with girdles become the hypotheca and both valves are shed when the girdle is absent.
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