In the deep ocean, marine snow is a continuous shower of mostly organic

detritus In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechanis ...

falling from the upper layers of the

water column A water column is a concept Concepts are defined as abstract ideas A mental representation (or cognitive representation), in philosophy of mind Philosophy of mind is a branch of philosophy that studies the ontology and nature of the mind ...

. It is a significant means of exporting

energy In physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department of knowledge which relates to the order of nature, or, in other words, to the regula ...

energy

from the

light Light or visible light is electromagnetic radiation within the portion of the electromagnetic spectrum that is visual perception, perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nan ...

light

-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 proc ...

to the

aphotic zone The aphotic zone (aphotic from Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its populat ...

below, which is referred to as the

biological pump The biological pump, also known as the marine carbon pump, is, in its simplest form, the ocean's biologically driven sequestration of carbon Carbon (from la, carbo "coal") is a with the C and 6. It is lic and —making four s availab ...

. 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 In classical physics and general che ...

produced in the

ocean The ocean (also the sea The sea, connected as the world ocean or simply the ocean The ocean (also the sea or the world ocean) is the body of salt water which covers approximately 71% of the surface of the Earth.

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 ch ...

that is not recycled ( remineralised) before it sinks into the

. Because of the role of export production in the ocean's

, it is typically measured in units of

carbon Carbon (from la, carbo "coal") is a with the C and 6. It is lic and —making four s available to form s. It belongs to group 14 of the periodic table. Carbon makes up only about 0.025 percent of Earth's crust. Three occur naturally, ...

carbon

(e.g.

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 natural history, naturalist, ornithologist, marine biologist, entomologist, explorer, and author. He is remembered for the numerous expeditions he conducted for the New Yo ...

as he observed it from his

bathysphere The Bathysphere (Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is approx ...

. 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 Conversion or convert may refer to: Arts, entertainment, and media * Conversion (Doctor Who audio), "Conversion" (''Doctor Who'' audio), an episode of the audio drama ' ...

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 Wind is the natural movement of air or other gases relative to a planet's surface. Wind occurs on a rang ...

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 An autotroph or primary producer is an organism that produces complex organic compound , CH4; is among the simplest organic compounds. In chemistry, organic compounds are generally any chemical compoun ...

protist A protist () is any eukaryotic Eukaryotes () are organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All organisms are composed of cells (cell theory). Organisms are c ...

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- Poly, from the Greek :wikt:πολύς, πολύς meaning "many" or "much", may refer to: Businesses * China Poly Group Corporation, a Chinese business group, and its subsidiaries: ** Poly Property, a Hong Kong inc ...

polymer

s exuded as waste products mostly by phytoplankton and

bacteria Bacteria (; common noun bacteria, singular bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of prokaryotic microorganisms. Typ ...

Mucus Mucus ( ) is a slippery aqueous secretion produced by, and covering, mucous membrane A mucous membrane or mucosa is a biological membrane, membrane that lines various cavities in the body and covers the surface of internal organs. It consists o ...

secreted by

zooplankton Zooplankton (; ) are heterotroph A heterotroph (; from Ancient Greek Ancient Greek includes the forms of the Greek language used in ancient Greece and the classical antiquity, ancient world from around 1500 BC to 300 BC. It is often ro ...

(mostly

salps A salp (plural salps, also known colloquially as “sea grape”) or salpa (plural salpae or salpas) is a barrel-shaped, planktic tunicate A tunicate is a marine invertebrate Invertebrates are animals that neither possess nor develop a ...

, appendicularians, and

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

) 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 blooms. 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 In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes ...

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 The mesopelagic zone (Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is app ...

and

benthic The benthic zone is the ecological region at the lowest level of a body of water A body of water or waterbody (often spelled water body) is any significant accumulation of water, generally on a planet's surface. The term most often refers to ...

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 The microbial loop describes a trophic pathway where, in aquatic systems, dissolved organic carbon (DOC) is returned to higher trophic levels via its incorporation into bacterial biomass, and then coupled with the classic food chain formed by phy ...

Aggregate dynamics

Aggregates begin as the colloidal fraction, which typically contains particles sized between one

nanometer file:EM Spectrum Properties edit.svg, 330px, Different lengths as in respect to the Electromagnetic spectrum, measured by the Metre and its derived scales. The nanometre is often used to express dimensions on an atomic scale and mostly in the Mo ...

and several

micrometerMicrometer can mean: * Micrometer (device) A micrometer, sometimes known as a micrometer screw gauge, is a device incorporating a calibrated screw widely used for Accuracy and precision, accurate measurement of components in mechanical engineeri ...

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 In pharmacology Pharmacology is a branch of medicine and pharmaceutical sciences concerned with drug or medication action, where a drug may be defined as any artificial, natural, or endogenous (from within the body) molecule which exerts a bio ...

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 Coagulation, also known as clotting, is the process by which blood Blood is a body fluid Body fluids, bodily fluids, or biofluids are liquid A liquid is a nearly incompressible In fluid mechanics or more generally continuum mech ...

: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 Filtration is a physical separation process A separation process is a method that converts a mixture or solution of chemical substances into two or more distinct product mixtures. At least one of results of the separation is enriched in one or ...

: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 A unicellular organism, also known as a single-celled organism, is an organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual contig ...

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 Marine prokaryotes are marine bacteria and marine archaea. They are defined by their habitat as prokaryotes that live in Marine habitat, marine environments, that is, in the saline water, saltwater of seas or oceans or the brackish water of coast ...

are the most abundant organisms in aggregates followed by

cyanobacteria Cyanobacteria (), also known as Cyanophyta, are a phylum In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ), in many languages, is one of the values of the grammatical number, grammatical ...

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 The photic zone, euphotic zone, epipelagic zone, or sunlight zone is the uppermost layer of a body of water (Lysefjord) in Norway Norway, officially the Kingdom of Norway,Names in the official and recognised languages: Bokmål Bokmå ...

using solar energy and produce

particulate organic carbon Particulate organic matter (POM) is a fraction of total organic matter operationally defined as that which does not pass through a filter pore size that typically ranges in size from 0.053 and 2 milimeters. Particulate organic carbon (POC) is ...

. The particulate organic carbon formed in the euphotic zone is processed by

marine microorganisms Marine microorganisms are defined by their habitat as microorganisms living in a Marine habitat, marine environment, that is, in the saline water, saltwater of a sea or ocean or the brackish water of a coastal estuary. A microorganism (or microb ...

(microbes),

and their consumers into organic aggregates (marine snow), which is then exported to the

(200–1000 m depth) and bathypelagic zones 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 (; from Ancient Greek "other" and "nutrition") 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 prim ...

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 Thermohaline circulation (THC) is a part of the large-scale Ocean current, ocean circulation that is driven by global density gradients created by surface heat and freshwater fluxes. The adjective ''thermohaline'' derives from ''wikt:thermo-, t ...

. 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 Marine protists are defined by their habitat as protist A protist () is any eukaryotic organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual contiguous system that embo ...

(eukaryotic microorganisms). Marine snow aggregates collected from the bathypelagic zone were found to consist largely of

fungi A fungus (plural The plural (sometimes abbreviated An abbreviation (from Latin ''brevis'', meaning ''short'') is a shortened form of a word or phrase, by any method. It may consist of a group of letters, or words taken from the full ...

fungi

and

labyrinthulomycetes The Labyrinthulomycetes (ICBN) or Labyrinthulea (International Code of Zoological Nomenclature, ICZN) are a class (biology), class of protists that produce a network of wiktionary:Filaments, filaments or tubes, which serve as tracks for the cells ...

. 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 In biogeochemistry, remineralisation (or remineralization) refers to the breakdown or transformation of organic matter (those molecules derived from a biological source) into its simplest Inorganic compound, inorganic forms. These transformations ...

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

, 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 A chemical formula is a way of presenting information about the chemical proportions of s that constitute a particular or molecule, using symbols, numbers, and sometimes also other symbols, such as pare ...

, these are effectively isolated from the surface ocean for relatively long time-scales related to

ocean circulation An ocean current is a continuous, directed movement of sea water generated by a number of forces acting upon the water, including wind Wind is the natural movement of air or other gases relative to a planet's surface. Wind occurs on a rang ...

. 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 or carbon dioxide removal File:Tree planting closeup.jpg, Planting trees is a means of carbon dioxide removal. Carbon dioxide removal (CDR), also known as greenhouse gas removal, is a process in which carbon dioxide g ...

by the ocean. Ocean nourishment 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 Contemporary climate change includes both the global warming caused by humans, and its impacts on Earth's weather patterns. There have been previous periods of climate change, but the current changes are more rapid than any known even ...

, may result in a decrease in the production of marine snow due to the enhanced

stratification Stratification may refer to: In mathematics: * Stratification (mathematics), any consistent assignment of numbers to predicate symbols * Stratified sampling , Data stratification in statistics In earth sciences: * Stable and unstable stratificati ...

of the water column. Increasing stratification decreases the availability of phytoplankton nutrients such as

nitrate Nitrate is a polyatomic ion A polyatomic ion, also known as a molecular ion, is a covalently bonded A covalent bond is a chemical bond A chemical bond is a lasting attraction between atoms, ions or molecules that enables the format ...

nitrate

phosphate In chemistry, a phosphate is an anion, salt (chemistry), salt, functional group or ester derived from a phosphoric acids and phosphates, phosphoric acid. It most commonly means orthophosphate, a derivative of phosphoric acid, orthophosphoric a ...

and

silicic acid Silicon dioxide, also known as silica, is an oxide An oxide () is a chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entity, molecular entities) composed of atoms from more t ...

, 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#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is ...

s. Recent research indicates transported bacteria may exchange

gene In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mecha ...

gene

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, bioengineering, or bio-engineering is the application of principles of biology and the tools of engineering to create usable, tangible, economically-viable products. Biological engineering employs knowledge and experti ...

and

pharmacy Pharmacy is the clinical health science The following Outline (list), outline is provided as an overview of and topical guide to health sciences: Health sciences – are those sciences which focus on health, or health care, as core p ...