Marine snow
In the deep ocean, marine snow (also known as "ocean dandruff") is a continuous shower of mostly organic
Composition
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Carbon cycle |
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Marine snow is made up of a variety of mostly organic matter, including dead or dying animals and
These aggregates grow over time and may reach several centimeters in diameter, traveling for weeks before reaching the ocean floor.Marine snow often forms during
Marine snow aggregates exhibit characteristics that fit Goldman's "aggregate spinning wheel hypothesis". This hypothesis states that phytoplankton, microorganisms and bacteria live attached to aggregate surfaces and are involved in rapid nutrient recycling. Phytoplankton have been shown to be able to take up nutrients from small local concentrations of organic material (e.g. fecal matter from an individual zooplankton cell, regenerated nutrients from organic decomposition by bacteria).[5] As the aggregates slowly sink to the bottom of the ocean, the many microorganisms residing on them are constantly respiring and contribute greatly to the microbial loop.
Aggregate dynamics
Aggregates begin as the colloidal fraction, which typically contains particles sized between one
Ballasting effect
Aggregates that sink more quickly to the bottom of the ocean have a greater chance of exporting carbon to the deep sea floor. The longer the residence time in the water column the greater the chance of being grazed upon. Aggregates formed in high dust areas are able to increase their densities faster and in more superficial layers compared to aggregates formed without dust particles present and these aggregates with increased lithogenic material have also been correlated with particulate organic carbon fluxes, however when they become heavily ballasted with lithogenic material they cannot scavenge any additional minerals during their descent, which suggests that carbon export to the deep ocean in regions with high dust deposition is strongly controlled by dust input to the surface ocean while suspended dust particles in deeper water layers do not significantly interact with sinking aggregates.[6]
Fragmentation
Once particles have aggregated to several micrometers in diameter, they begin to accumulate bacteria, since there is sufficient site space for feeding and reproduction. At this size, it is large enough to undergo sinking. It also has the components necessary to fit the "aggregate spinning wheel hypothesis". Evidence for this has been found by Alldredge and Cohen (1987) who found evidence of both respiration and photosynthesis within aggregates, suggesting the presence of both autotrophic and heterotrophic organisms.[7] During zooplankton's vertical migration, the abundances of aggregates increased while size distributions decreased. Aggregates were found in the abdomen in zooplankton indicating their grazing will fragment larger aggregates.[8]
Surface coagulation
Aggregates may also form from colloids trapped on the surface of rising
Filtration
Particles and small organisms floating through the water column can become trapped within aggregates. Marine snow aggregates are porous, however, and some particles are able to pass through them.
Particle-associated microorganisms
Planktonic
As illustrated in the diagram,
Export flux is defined as the sedimentation out of the surface layer (at approximately 100 m depth) and sequestration flux is the sedimentation out of the mesopelagic zone (at approximately 1000 m depth). A portion of the particulate organic carbon is respired back to CO2 in the oceanic
The largest component of biomass are marine protists (eukaryotic microorganisms). Marine snow aggregates collected from the bathypelagic zone were found to consist largely of fungi and labyrinthulomycetes. Smaller aggregates do not harbor as many eukaryotic organisms which is similar to what is found in the deep ocean. The bathypelagic aggregates mostly resembled those found in the surface ocean.[14] It implies higher rates of remineralization in the bathypelagic zone.
Numerically, the largest component of marine snow are the prokaryotes that colonize the aggregates. Bacteria are largely responsible for the remineralisation and fragmentation of aggregates. Remineralization occurs typically below 200 m depth.[15]
Microbial communities that form on the aggregates vary from the communities in the water column. The concentration of attached microbes are typically orders of magnitude larger than free-living microbes.[16] Isolated bacterial cultures have up to 20 times more enzymatic activity within 2 hours of aggregate attachment.[10] The dark ocean harbors around 65% of all pelagic Bacteria and Archaea.(Whitman et al., 1998)
It was previously thought that due to fragmentation, bacterial communities would shift as they travel down the water column. As seen in experiments, it now appears that the communities that form during aggregation remain associated with the aggregate and any community changes are due to grazing or fragmentation rather than new bacterial colony formation.[17]
Carbon cycling
The deep ocean harbors more than 98% of the dissolved inorganic carbon pool,[18] along with a rapid sedimentation rate that results in low particulate organic carbon inputs. It is yet to be resolved what effect microbes have on the global carbon cycle. Studies show that microbes in the deep ocean are not dormant, but are metabolically active and must be participating in nutrient cycling by not only heterotrophs but by autotrophs as well. There is a mismatch from the microbial carbon demand in the deep ocean and the carbon export from the surface ocean.[18] Dissolved inorganic carbon fixation is on similar orders of magnitude as heterotrophic microbes in the surface ocean. Model-based data reveal that dissolved inorganic carbon fixation ranges from 1 mmol C m−2 d−1 to 2.5 mmol C m−2 d−1.[18]
Microenvironments
Large aggregates can become anoxic which gives rise to anaerobic metabolisms. Typically anaerobic metabolisms are confined to areas where it is more energetically favorable. Given the abundance of denitrifying and sulfate-reducing bacteria, it is thought that these metabolisms are able to thrive within marine snow aggregates. In a model developed by Bianchi et al., it shows the various redox potentials within an aggregate.[19]
Implications
Because of the relatively long residence time of the ocean's
Increases in ocean temperatures, a projected indicator of climate change, may result in a decrease in the production of marine snow due to the enhanced stratification of the water column. Increasing stratification decreases the availability of phytoplankton nutrients such as nitrate, phosphate and silicic acid, and could lead to a decrease in primary production and, thus, marine snow.
The microbial communities associated with marine snow are also interesting to
See also
- Biological pump
- Detritivore
- Diffusion-limited aggregation
- f-ratio
- Martin curve
- Particulate organic matter
- Sea snot
- Sediment trap
- Whale fall
- Vampire squid
- Seston
References
- ^ What is marine snow? NOAA National Ocean Service. Updated:06/25/18.
- PMID 28603518.
- ^ Miller CB (2004). Biological Oceanography. Blackwell Science Ltd. pp. 94–95, 266–267.
- ^ Mann KH, Lazier JR (2006). Dynamics of Marine Ecosystems. Blackwell Publishing. p. 35.
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- ISSN 0024-3590.
- S2CID 46033413.
- .
- .
- ^ PMID 30299466.
- ^ doi:10.3390/su10030869. Material was copied from this source, which is available under a Creative Commons Attribution 4.0 International License.
- ^ .
- ^
- PMID 27648811.
- S2CID 4392859.
- S2CID 247706757.
- PMID 25527538.
- ^ .
- S2CID 134801363.
- PMID 18757282.
Further reading
- Mary Wilcox Silver (2015). "Marine Snow: A Brief Historical Sketch". Limnology and Oceanography Bulletin, 24:5-10. https://doi.org/10.1002/lob.10005
- Brakstad OG, Lewis A, Beegle-Krause CJ (2018). "A critical review of marine snow in the context of oil spills and oil spill dispersant treatment with focus on the Deepwater Horizon oil spill". Marine Pollution Bulletin. 135: 346–356. S2CID 52948259.