Resting spore

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A resting spore is a resistant cell, used to survive adverse environmental conditions. Resting spore is a term commonly applied to both diatoms and fungi.

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In fungi

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 fungus are known create the phenomenon known as 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, [1] except in the minority of studied taxa, where spore production immediately follows the first cellular product of sexual reproduction, the auxospore. [2] 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 [3] 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, [2] temperature, [1] and light limitation [4] 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. [2]

The formation of resting spores is product of two acytokinetic divisions of the parent cell, [5] 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). [6] 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. [7]

Ecological significance

Resting spore formation plays an important role in the survival of diatoms through periods of adverse environmental conditions. The formation of a spore with a thick frustule increases the density of the diatom, allowing a drop in the water column to the sediment or pycnocline where reduced light, and potentially cooler temperatures may increase the lifespan of the spore by reducing the metabolic imbalance between demand and resource availability. [2] Dropping deeper into the water column may also put the resting spore in a place where nutrient availability is greater. The thick silica frustule can also serve during period as a resting spore to enhance the resistance to zooplanktonic grazers, [8] which are known to be a frequently dominant source of mortality for many marine phytoplankton. [9]

Ultimately, the resting spore relies that it will be mixed back up to the surface at a time when conditions for growth are favorable and it can germinate. This suggests that generally, a shallow neritic habitat [10] is needed to be able to mix back into the photic zone. If the spore is an obligate part of the life cycle, shallow depth requirement can limit range [4] unless the pycnocline is sufficient to suspend the spore. Resting spores are believed to last potentially decades with germination remaining viable. Resting spores may allow diatoms to survive environmental variability from weekly and seasonal scale to decadal patterns like the NAO. This can allow diatoms to survive in areas where they cannot grow year-round or perhaps even every year. It has even been proposed that the resting spore stage of diatoms has aided survival through mass extinction events including the cretaceous extinction, which brought an estimated three quarters of all plant and animal life to extinction. [11] This extinction was also characterized by its limitation to solar radiation, necessary for diatoms which are photosynthetic. However, as laboratory experiments have shown, light limitation could trigger resting spore formation, which might have allowed many spore forming diatoms to survive a mass extinction event such as the cretaceous. [11]

In conjunction with survival through adverse environmental conditions, the resting spore is considered important for seeding with cells that can await opportune conditions and start a population. This is believed to be important in regions like the North Atlantic where deep winter mixing is then stabilized and a shallower, nutrient rich mixed layer develops. [12] In such regions, seeding by resting spores could provide competitive advantage through founder's effect. Simply, if a diatom has a competitive growth rate, and is among the first to pioneer a newly available resource, it may have a competitive advantage. For the purposes of seeding, a spore sinking to the benthos has also been hypothesized to reduce the likelihood of advective transport outside the habitable range, [13] meaning by sinking to the sediment of an area that previously had favorably growth conditions, fewer cells will be transported outside the habitable range they will ever be able to grow in.

While these are important advantages to spore formation, the mortality rate is presumably high, particularly in oceanic zones, where pycnoclines may suspend spores, [14] but presumably many resting spores are lost beyond the reach of the mixing zone. While from the standpoint of diatoms this is disadvantageous, it has been evidenced that rapid sinking and sedimentation of resting spores, particularly in large events after blooms, may represent an important export of nutrients to the deep ocean. [15] Resting spores may be particularly important, because of their rapid sinking rate, which might reduce the opportunity for being recycled back into the food web of the photic zone.

See also

Related Research Articles

Plankton Organisms that are in the water column and are incapable of swimming against a current

Plankton are the diverse collection of organisms found in water that are unable to propel themselves against a current. The individual organisms constituting plankton are called plankters. In the ocean, they provide a crucial source of food to many small and large aquatic organisms, such as bivalves, fish and whales.

Endospore Protective structure formed by bacteria

An endospore is a dormant, tough, and non-reproductive structure produced by some bacteria in the phylum Firmicutes. The name "endospore" is suggestive of a spore or seed-like form, but it is not a true spore. It is a stripped-down, dormant form to which the bacterium can reduce itself. Endospore formation is usually triggered by a lack of nutrients, and usually occurs in gram-positive bacteria. In endospore formation, the bacterium divides within its cell wall, and one side then engulfs the other. Endospores enable bacteria to lie dormant for extended periods, even centuries. There are many reports of spores remaining viable over 10,000 years, and revival of spores millions of years old has been claimed. There is one report of viable spores of Bacillus marismortui in salt crystals approximately 250 million years old. When the environment becomes more favorable, the endospore can reactivate itself to the vegetative state. Most types of bacteria cannot change to the endospore form. Examples of bacterial species that can form endospores include Bacillus cereus, Bacillus anthracis, Bacillus thuringiensis, Clostridium botulinum, and Clostridium tetani.

Diatom Class of microalgae, found in the oceans, waterways and soils of the world

Diatoms are a major group of algae, specifically microalgae, found in the oceans, waterways and soils of the world. Living diatoms make up a significant portion of the Earth's biomass: they generate about 20 to 50 percent of the oxygen produced on the planet each year, take in over 6.7 billion metric tons of silicon each year from the waters in which they live, and constitute nearly half of the organic material found in the oceans. The shells of dead diatoms can reach as much as a half-mile deep on the ocean floor, and the entire Amazon basin is fertilized annually by 27 million tons of diatom shell dust transported by transatlantic winds from the African Sahara, much of it from the Bodélé Depression, which was once made up of a system of fresh-water lakes.

Coccolithophore Unicellular algae responsible for the formation of chalk

A coccolithophore is a unicellular, eukaryotic phytoplankton (alga). They belong either to the kingdom Protista, according to Robert Whittaker's Five kingdom classification, or clade Hacrobia, according to the newer biological classification system. Within the Hacrobia, the coccolithophorids are in the phylum or division Haptophyta, class Prymnesiophyceae. Coccolithophorids are distinguished by special calcium carbonate plates of uncertain function called coccoliths, which are also important microfossils. However, there are Prymnesiophyceae species lacking coccoliths, so not every member of Prymnesiophyceae is a coccolithophorid. Coccolithophores are almost exclusively marine and are found in large numbers throughout the sunlight zone of the ocean.

Zooplankton Heterotrophic protistan or metazoan members of the plankton ecosystem

Zooplankton are heterotrophic plankton. Plankton are organisms drifting in oceans, seas, and bodies of fresh water. The word zooplankton is derived from the Greek zoon (ζῴον), meaning "animal", and planktos (πλαγκτός), meaning "wanderer" or "drifter". Individual zooplankton are usually microscopic, but some are larger and visible to the naked eye.

Germination Process by which an organism grows from a spore or seed

Germination is the process by which an organism grows from a seed or similar structure. The term is applied to the sprouting of a seedling from a seed of an angiosperm or gymnosperm, the growth of a sporeling from a spore, such as the spores of fungi, ferns, bacteria, and the growth of the pollen tube from the pollen grain of a seed plant.

Pycnocline Layer where the density gradient is greatest within a body of water

A pycnocline is the cline or layer where the density gradient is greatest within a body of water. An ocean current is generated by the forces such as breaking waves, temperature and salinity differences, wind, Coriolis effect, and tides caused by the gravitational pull of the Moon and the Sun. In addition, the physical properties in a pycnocline driven by density gradients also affect the flows and vertical profiles in the ocean. These changes can be connected to the transport of heat, salt, and nutrients through the ocean, and the pycnocline diffusion controls upwelling.

Redfield ratio

Redfield ratio or Redfield stoichiometry is the consistent atomic ratio of carbon, nitrogen and phosphorus found in marine phytoplankton and throughout the deep oceans.

Thin layers (oceanography)

Thin layers are concentrated aggregations of phytoplankton and zooplankton in coastal and offshore waters that are vertically compressed to thicknesses ranging from several centimeters up to a few meters and are horizontally extensive, sometimes for kilometers. Generally, thin layers have three basic criteria: 1) they must be horizontally and temporally persistent; 2) they must not exceed a critical threshold of vertical thickness; and 3) they must exceed a critical threshold of maximum concentration. The precise values for critical thresholds of thin layers has been debated for a long time due to the vast diversity of plankton, instrumentation, and environmental conditions. Thin layers have distinct biological, chemical, optical, and acoustical signatures which are difficult to measure with traditional sampling techniques such as nets and bottles. However, there has been a surge in studies of thin layers within the past two decades due to major advances in technology and instrumentation. Phytoplankton are often measured by optical instruments that can detect fluorescence such as LIDAR, and zooplankton are often measured by acoustic instruments that can detect acoustic backscattering such as ABS. These extraordinary concentrations of plankton have important implications for many aspects of marine ecology, as well as for ocean optics and acoustics. It is important to note that zooplankton thin layers are often found slightly under phytoplankton layers because many feed on them. Thin layers occur in a wide variety of ocean environments, including estuaries, coastal shelves, fjords, bays, and the open ocean, and they are often associated with some form of vertical structure in the water column, such as pycnoclines, and in zones of reduced flow.

Sporogenesis is the production of spores in biology. The term is also used to refer to the process of reproduction via spores. Reproductive spores were found to be formed in eukaryotic organisms, such as plants, algae and fungi, during their normal reproductive life cycle. Dormant spores are formed, for example by certain fungi and algae, primarily in response to unfavorable growing conditions. Most eukaryotic spores are haploid and form through cell division, though some types are diploid or dikaryons and form through cell fusion.

In certain species of diatoms, auxospores are specialised cells that are produced at key stages in their cell cycle or life history. Auxospores typically play a role in growth processes, sexual reproduction or dormancy.

Akinete

An akinete is an enveloped, thick-walled, non-motile, dormant cell formed by filamentous, heterocyst-forming cyanobacteria under the order Nostocales and Stigonematales. Akinetes are resistant to cold and desiccation. They also accumulate and store various essential material, both of which allows the akinete to serve as a survival structure for up to many years. However, akinetes are not resistant to heat. Akinetes usually develop in strings with each cell differentiating after another and this occurs next to heterocysts if they are present. Development usually occurs during stationary phase and is triggered by unfavorable conditions such as insufficient light or nutrients, temperature, and saline levels in the environment. Once conditions become more favorable for growth, the akinete can then germinate back into a vegetative cell. Increased light intensity, nutrients availability, oxygen availability, and changes in salinity are important triggers for germination. In comparison to vegetative cells, akinetes are generally larger. This is associated with the accumulation of nucleic acids which is important for both dormancy and germination of the akinete. Despite being a resting cell, it is still capable of some metabolic activities such as photosynthesis, protein synthesis, and carbon fixation, albeit at significantly lower levels.

The deep chlorophyll maximum (DCM), also called the subsurface chlorophyll maximum, is the region below the surface of water with the maximum concentration of chlorophyll. A DCM is not always present - sometimes there is more chlorophyll at the surface than at any greater depth - but it is a common feature of most aquatic ecosystems, especially in regions of strong thermal stratification. The depth, thickness, intensity, composition, and persistence of DCMs vary widely. The DCM generally exists at the same depth as the nutricline, the region of the ocean where the greatest change in the nutrient concentration occurs with depth.

<i>Pseudo-nitzschia</i> Genus of marine planktonic diatoms

Pseudo-nitzschia is a marine planktonic diatom genus that accounts for 4.4% of pennate diatoms found worldwide. Some species are capable of producing the neurotoxin domoic acid (DA), which is responsible for the neurological disorder in humans known as amnesic shellfish poisoning (ASP). Currently, 58 species are known, 28 of which have been shown to produced DA. It was originally hypothesized that only dinoflagellates could produce harmful algal toxins, but a deadly bloom of Pseudo-nitzschia occurred in 1987 in the bays of Prince Edward Island, Canada, and led to an outbreak of ASP. Over 100 people were affected by this outbreak after consuming contaminated mussels; three people died. Since this event, no additional deaths have been attributed to ASP, though the prevalence of toxic diatoms and DA has increased worldwide. This anomaly is likely due to increased awareness of harmful algal blooms (HABs) and their implications for human and ecosystem health.

Chaetoceros pseudocurvisetus is a marine diatom in the genus Chaetoceros. It is an important primary producer in the oceans. C. pseudocurvisetus forms resting spores and resting cells, particularly in the absence of essential nutrients.

<i>Ditylum brightwellii</i> Species of diatom

Ditylum brightwelli is a species of cosmopolitan marine centric diatoms. It is a unicellular photosynthetic autotroph that has the ability to divide rapidly and contribute to spring phytoplankton blooms.

<i>Thalassiosira</i> Genus of single-celled organisms

Thalassiosira is a genus of centric diatoms, comprising over 100 marine and freshwater species. It is a diverse group of photosynthetic eukaryotes that make up a vital part of marine and freshwater ecosystems, in which they are key primary producers and essential for carbon cycling

Viral shunt

The viral shunt is a mechanism that prevents marine microbial particulate organic matter (POM) from migrating up trophic levels by recycling them into dissolved organic matter (DOM), which can be readily taken up by microorganisms. The DOM recycled by the viral shunt pathway is comparable to the amount generated by the other main sources of marine DOM.

Marine protists

Marine protists are defined by their habitat as protists that live in marine environments, that is, in the saltwater of seas or oceans or the brackish water of coastal estuaries. Life originated as single-celled prokaryotes and later evolved into more complex eukaryotes. Eukaryotes are the more developed life forms known as plants, animals, fungi and protists. Protists are the eukaryotes that cannot be classified as plants, fungi or animals. They are usually single-celled and microscopic. The term protist came into use historically as a term of convenience for eukaryotes that cannot be strictly classified as plants, animals or fungi. They are not a part of modern cladistics, because they are paraphyletic.

Protist shell

Many protists have protective shells or tests, usually made from silica (glass) or calcium carbonate (chalk). Protists are mostly single-celled and microscopic. Their shells are often tough mineralised forms that resist degradation, and can survive the death of the protist as a microfossil. Although protists are very small, they are ubiquitous. Their numbers are such that their shells play a huge part in the formation of ocean sediments, and in the global cycling of elements and nutrients.

References

  1. 1 2 French, Fred W.; Hargraves, Paul E. (1985-09-01). "Spore formation in the life cycles of the diatoms chaetoceros diadema and Leptocylindrus danicus". Journal of Phycology. 21 (3): 477–483. doi:10.1111/j.0022-3646.1985.00477.x. ISSN   1529-8817.
  2. 1 2 3 4 Davis, Curtiss O.; Hollibaugh, James T.; Seibert, Don L. R.; Thomas, William H.; Harrison, Paul J. (1980-06-01). "Formation of Resting Spores by Leptocylindrus Danicus (bacillariophyceae) in a Controlled Experimental Ecosystem1". Journal of Phycology. 16 (2): 296–302. doi:10.1111/j.1529-8817.1980.tb03034.x. hdl: 2027.42/65696 . ISSN   1529-8817.
  3. ↑ Heiskanen, A; Kononen, K (1994). "Sedimentation of vernal and late summer phytoplankton communities in the coastal Baltic Sea". Archiv für Hydrobiologie. 131: 175.
  4. 1 2 Hargraves, P.E. (1983). "Diatom resting spores: significance and strategies". Survival Strategies of the Algae: 49–68.
  5. ↑ Drebes, G. (1974). "Marines Phytoplankton". Eine Auswahl der Helgoländer Planktonalgen (Diatomeen, Peridineen). Thieme, Stuttgart.
  6. ↑ Tomas, Carmelo (1997). Identifying Marine Phytoplankton. Academic Press.
  7. ↑ Stosch, H. A. v; Theil, G.; Kowallik, K. V. (1973-09-01). "Entwicklungsgeschichtliche Untersuchungen an zentrischen Diatomeen". Helgoländer Wissenschaftliche Meeresuntersuchungen (in German). 25 (2–3): 384–445. doi: 10.1007/BF01611205 . ISSN   0017-9957.
  8. ↑ Hargraves, Paul; French, Fred (1975). "Observations on the Survival of Diatom Resting Spores". Nova Hedwigia, Beihefte.
  9. ↑ Calbet, Albert; Landry, Michael R. (2004-01-01). "Phytoplankton growth, microzooplankton grazing, and carbon cycling in marine systems". Limnology and Oceanography. 49 (1): 51–57. doi:10.4319/lo.2004.49.1.0051. hdl: 10261/134985 . ISSN   1939-5590.
  10. ↑ Garrison, David (1984). "Planktonic Diatoms". Marine Plankton Life Cycle Strategies: 1–11.
  11. 1 2 Kitchell, Jennifer A.; Clark, David L.; Gombos, Andrew M. (1986). "Biological Selectivity of Extinction: A Link between Background and Mass Extinction". PALAIOS. 1 (5): 504–511. doi:10.2307/3514632. JSTOR   3514632.
  12. ↑ Margalef, Ramon (1978). "Life-forms of phytoplankton as survival alternatives in an unstable environment" (PDF). Oceanologica Acta. 1: 493–509.
  13. ↑ Smetacek, V. S. (1985-01-01). "Role of sinking in diatom life-history cycles: ecological, evolutionary and geological significance". Marine Biology. 84 (3): 239–251. doi:10.1007/BF00392493. ISSN   0025-3162.
  14. ↑ Guillard, R; Kilham, P (1977). The Marine Ecology of Planktonic Diatoms. The Biology of Diatoms. pp. 372–469.
  15. ↑ Salter, Ian; Kemp, Alan E. S.; Moore, C. Mark; Lampitt, Richard S.; Wolff, George A.; Holtvoeth, Jens (2012-03-01). "Diatom resting spore ecology drives enhanced carbon export from a naturally iron-fertilized bloom in the Southern Ocean". Global Biogeochemical Cycles. 26 (1): GB1014. doi: 10.1029/2010gb003977 . ISSN   1944-9224.
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