In the deep ocean, marine snow (also known as "ocean dandruff") is a continuous shower of mostly organic

detritus In biology, detritus () is dead particulate organic material, as distinguished from dissolved organic material. Detritus typically includes the bodies or fragments of bodies of dead organisms, and fecal material. Detritus typically hosts comm ...

falling from the upper layers of the

water column A water column is a conceptual column of water from the surface of a sea, river or lake to the bottom sediment.Munson, B.H., Axler, R., Hagley C., Host G., Merrick G., Richards C. (2004).Glossary. ''Water on the Web''. University of Minnesota-D ...

. It is a significant means of exporting

energy In physics, energy (from Ancient Greek: ἐνέργεια, ''enérgeia'', “activity”) is the quantitative property that is transferred to a body or to a physical system, recognizable in the performance of work and in the form of ...

from the

light Light or visible light is electromagnetic radiation that can be perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), corresponding to frequencies of 750–420 t ...

-rich

photic zone The photic zone, euphotic zone, epipelagic zone, or sunlight zone is the uppermost layer of a body of water that receives sunlight, allowing phytoplankton to perform photosynthesis. It undergoes a series of physical, chemical, and biological pro ...

to the aphotic zone below, which is referred to as the

biological pump The biological pump (or ocean carbon biological pump or marine biological carbon pump) is the ocean's biologically driven sequestration of carbon from the atmosphere and land runoff to the ocean interior and seafloor sediments.Sigman DM & GH ...

. Export production is the amount of

organic matter Organic matter, organic material, or natural organic matter refers to the large source of carbon-based compounds found within natural and engineered, terrestrial, and aquatic environments. It is matter composed of organic compounds that have c ...

produced in the

ocean The ocean (also the sea or the world ocean) is the body of salt water that covers approximately 70.8% of the surface of Earth and contains 97% of Earth's water. An ocean can also refer to any of the large bodies of water into which the wor ...

primary production In ecology, primary production is the synthesis of organic compounds from atmospheric or aqueous carbon dioxide. It principally occurs through the process of photosynthesis, which uses light as its source of energy, but it also occurs through ...

that is not recycled ( remineralised) before it sinks into the aphotic zone. Because of the role of export production in the ocean's

, it is typically measured in units of

carbon Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent—its atom making four electrons available to form covalent chemical bonds. It belongs to group 14 of the periodic table. Carbon ma ...

(e.g. mg C m⁻² d⁻¹). The term was first coined by the explorer

William Beebe Charles William Beebe ( ; July 29, 1877 – June 4, 1962) was an American naturalist, ornithologist, marine biologist, entomologist, explorer, and author. He is remembered for the numerous expeditions he conducted for the New York Zoological ...

as he observed it from his bathysphere. As the origin of marine snow lies in activities within the productive photic zone, the prevalence of marine snow changes with seasonal fluctuations in

photosynthetic Photosynthesis is a process used by plants and other organisms to convert light energy into chemical energy that, through cellular respiration, can later be released to fuel the organism's activities. Some of this chemical energy is stored in ...

activity and

ocean current An ocean current is a continuous, directed movement of sea water generated by a number of forces acting upon the water, including wind, the Coriolis effect, breaking waves, cabbeling, and temperature and salinity differences. Depth conto ...

s. Marine snow can be an important food source for organisms living in the aphotic zone, particularly for organisms which live very deep in the water column.

Composition

Marine snow is made up of a variety of mostly organic matter, including dead or dying animals and

phytoplankton Phytoplankton () are the autotrophic (self-feeding) components of the plankton community and a key part of ocean and freshwater ecosystems. The name comes from the Greek words (), meaning 'plant', and (), meaning 'wanderer' or 'drifter'. ...

protist A protist () is any eukaryotic organism (that is, an organism whose cells contain a cell nucleus) that is not an animal, plant, or fungus. While it is likely that protists share a common ancestor (the last eukaryotic common ancestor), the e ...

s, fecal matter, sand, and other inorganic dust. Most trapped particles are more vulnerable to grazers than they would be as free-floating individuals. Aggregates can form through abiotic processes (i.e. extrapolymeric substances). These are natural

polymer A polymer (; Greek '' poly-'', "many" + '' -mer'', "part") is a substance or material consisting of very large molecules called macromolecules, composed of many repeating subunits. Due to their broad spectrum of properties, both synthetic a ...

s exuded as waste products mostly by phytoplankton and

bacteria Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were am ...

Mucus Mucus ( ) is a slippery aqueous secretion produced by, and covering, mucous membranes. It is typically produced from cells found in mucous glands, although it may also originate from mixed glands, which contain both serous and mucous cells. It ...

secreted by

zooplankton Zooplankton are the animal component of the planktonic community ("zoo" comes from the Greek word for ''animal''). Plankton are aquatic organisms that are unable to swim effectively against currents, and consequently drift or are carried along by ...

(mostly salps, appendicularians, and

pteropods Pteropoda (common name pteropods, from the Greek meaning "wing-foot") are specialized free-swimming pelagic sea snails and sea slugs, marine opisthobranch gastropods. Most live in the top 10 m of the ocean and are less than 1 cm long. The ...

) also contribute to the constituents of marine snow aggregates. 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

algal bloom An algal bloom or algae bloom is a rapid increase or accumulation in the population of algae in freshwater or marine water systems. It is often recognized by the discoloration in the water from the algae's pigments. The term ''algae'' encompass ...

s. As phytoplankton accumulate, they aggregate or get captured in other aggregates, both of which accelerate the sinking rate. Aggregation and sinking is actually thought to be a large component of sources for algae loss from surface water. Most organic components of marine snow are consumed by

microbe A microorganism, or microbe,, ''mikros'', "small") and ''organism'' from the el, ὀργανισμός, ''organismós'', "organism"). It is usually written as a single word but is sometimes hyphenated (''micro-organism''), especially in olde ...

and other filter-feeding animals within the first 1,000 metres of their journey. In this way marine snow may be considered the foundation of deep-sea mesopelagic and

benthic The benthic zone is the ecological region at the lowest level of a body of water such as an ocean, lake, or stream, including the sediment surface and some sub-surface layers. The name comes from ancient Greek, βένθος (bénthos), meaning " ...

ecosystem An ecosystem (or ecological system) consists of all the organisms and the physical environment with which they interact. These biotic and abiotic components are linked together through nutrient cycles and energy flows. Energy enters the syst ...

s: As sunlight cannot reach them, deep-sea organisms rely heavily on marine snow as an energy source. The small percentage of material not consumed in shallower waters becomes incorporated into the muddy "ooze" blanketing the ocean floor, where it is further decomposed through biological activity. 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). 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

nanometer 330px, Different lengths as in respect to the molecular scale. The nanometre (international spelling as used by the International Bureau of Weights and Measures; SI symbol: nm) or nanometer (American and British English spelling differences#-re, ...

and several

micrometer Micrometer can mean: * Micrometer (device), used for accurate measurements by means of a calibrated screw * American spelling of micrometre The micrometre ( international spelling as used by the International Bureau of Weights and Measures; ...

s. The colloidal fraction of the ocean contains a large amount of organic matter unavailable to grazers. This fraction has a much higher total mass than either phytoplankton or bacteria but is not readily available due to size characteristics of the particles in relation to potential consumers. The colloidal fraction must aggregate in order to be more bioavailable.

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 compared to aggregates formed without dust present and these aggregates with increased lithogenic material have also been correlated with particulate organic carbon fluxes. Aggregates that are able to increase their ballast effect can only do so in the surface ocean as minerals have not been observed to accumulate as they move down the water column.

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. 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.

Surface coagulation

Aggregates may also form from colloids trapped on the surface of rising bubbles. For example, Kepkay et al. found that bubble coagulation leads to an increase in bacterial respiration since more food is available to them.

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

prokaryotes A prokaryote () is a single-celled organism that lacks a nucleus and other membrane-bound organelles. The word ''prokaryote'' comes from the Greek πρό (, 'before') and κάρυον (, 'nut' or 'kernel').Campbell, N. "Biology:Concepts & Con ...

are further defined into two categories, free-living or particle associated. The two are separated by filtration. Particle-associated bacteria are often difficult to study, because marine snow aggregates are often ranging in sizes from 0.2 to 200 μm, sampling efforts are often difficult. These aggregates are hotspots for microbial activity. Marine bacteria are the most abundant organisms in aggregates followed by

cyanobacteria Cyanobacteria (), also known as Cyanophyta, are a phylum of gram-negative bacteria that obtain energy via photosynthesis. The name ''cyanobacteria'' refers to their color (), which similarly forms the basis of cyanobacteria's common name, bl ...

and then nanoflagellates. Aggregates can be enriched about one thousand times more than the surrounding seawater. Seasonal variability can also have an effect on microbial communities of marine snow aggregates with concentrations being the highest during the summer. As illustrated in the diagram,

fix carbon dioxide in the euphotic zone using solar energy and produce particulate organic carbon. The particulate organic carbon formed in the euphotic zone is processed by marine microorganisms (microbes),

and their consumers into organic aggregates (marine snow), which is then exported to the mesopelagic (200–1000 m depth) and

bathypelagic zone The bathypelagic zone or bathyal zone (from Greek βαθύς (bathýs), deep) is the part of the open ocean that extends from a depth of below the ocean surface. It lies between the mesopelagic above, and the abyssopelagic below. The bathypelagi ...

s by sinking and vertical migration by zooplankton and fish.Basu, S. and Mackey, K.R. (2018) "Phytoplankton as key mediators of the biological carbon pump: Their responses to a changing climate". ''Sustainability'', 10(3): 869. . CC-BY icon

Material was copied from this source, which is available under
Creative Commons Attribution 4.0 International License
Passow, U. and Carlson, C.A. (2012) "The biological pump in a high CO2 world". ''Marine Ecology Progress Series'', 470: 249–271. .Turner, J.T. (2015) "Zooplankton fecal pellets, marine snow, phytodetritus and the ocean's biological pump". ''Progress in Oceanography'', 130: 205–248. ''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

at depth, mostly by

heterotrophic A heterotroph (; ) is an organism that cannot produce its own food, instead taking nutrition from other sources of organic carbon, mainly plant or animal matter. In the food chain, heterotrophs are primary, secondary and tertiary consumers, but ...

microbes and zooplankton, thus maintaining a vertical gradient in concentration of dissolved inorganic carbon (DIC). This deep-ocean DIC returns to the atmosphere on millennial timescales through thermohaline circulation. Between 1% and 40% of the primary production is exported out of the euphotic zone, which attenuates exponentially towards the base of the mesopelagic zone and only about 1% of the surface production reaches the sea floor. 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 A fungus ( : fungi or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. These organisms are classified as a kingdom, separately fr ...

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. 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. 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. Isolated bacterial cultures have up to 20-times more enzymatic activity within 2 hours of aggregate attachment. 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.

Carbon cycling

The deep ocean harbors more than 98% of the dissolved inorganic carbon pool. 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. 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.

Microenvironments

Large aggregates can become anoxic which give rise to anaerobic metabolisms. Typically anaerobic metabolisms are confined to areas where it is more energetically favorable. Give 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.

Implications

Because of the relatively long residence time of the ocean's thermohaline circulation, carbon transported as marine snow into the deep ocean by the

can remain out of contact with the atmosphere for more than 1000 years. That is, when the marine snow is finally decomposed to inorganic nutrients and dissolved

carbon dioxide Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is t ...

, these are effectively isolated from the surface ocean for relatively long time-scales related to ocean circulation. Consequently, enhancing the quantity of marine snow that reaches the deep ocean is the basis of several geoengineering schemes to enhance

carbon sequestration Carbon sequestration is the process of storing carbon in a carbon pool. Carbon dioxide () is naturally captured from the atmosphere through biological, chemical, and physical processes. These changes can be accelerated through changes in lan ...

by the ocean.

Ocean nourishment Ocean fertilization or ocean nourishment is a type of technology for carbon dioxide removal from the ocean based on the purposeful introduction of plant nutrients to the upper ocean to increase marine food production and to remove carbon dioxid ...

and iron fertilisation seek to boost the production of organic material in the surface ocean, with a concomitant rise in marine snow reaching the deep ocean. These efforts have not yet produced a sustainable fertilization that effectively transports carbon out of the system. Increases in ocean temperatures, a projected indicator of

climate change In common usage, climate change describes global warming—the ongoing increase in global average temperature—and its effects on Earth's climate system. Climate change in a broader sense also includes previous long-term changes to ...

, 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 Nitrate is a polyatomic ion with the chemical formula . Salts containing this ion are called nitrates. Nitrates are common components of fertilizers and explosives. Almost all inorganic nitrates are soluble in water. An example of an insolu ...

phosphate In chemistry, a phosphate is an anion, salt, functional group or ester derived from a phosphoric acid. It most commonly means orthophosphate, a derivative of orthophosphoric acid . The phosphate or orthophosphate ion is derived from phosph ...

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

microbiologist A microbiologist (from Greek ) is a scientist who studies microscopic life forms and processes. This includes study of the growth, interactions and characteristics of microscopic organisms such as bacteria, algae, fungi, and some types of para ...

s. Recent research indicates transported bacteria may exchange

gene In biology, the word gene (from , ; "...Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanings. The Mendelian gene is a b ...

s with previously thought to be isolated populations of bacteria inhabiting the breadth of the ocean floor. In such an immense area there may be as yet undiscovered species tolerant of high pressures and extreme cold, perhaps finding use in

bioengineering Biological engineering or bioengineering is the application of principles of biology and the tools of engineering to create usable, tangible, economically-viable products. Biological engineering employs knowledge and expertise from a number o ...

and

pharmacy Pharmacy is the science and practice of discovering, producing, preparing, dispensing, reviewing and monitoring medications, aiming to ensure the safe, effective, and affordable use of medication, medicines. It is a miscellaneous science as it ...

References

External links

SpaceRef.com, Deep sea bacteria get new genes from marine snow

NIWA, What grows up must fall down: the potential impact of climate change on plankton and carbon export

{{fishery science topics Aquatic ecology Bioindicators Biological oceanography Chemical oceanography Ecological processes Fisheries science Geochemistry Oceanographical terminology