Marine snow

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Marine snow is a shower of organic material falling from upper waters to the deep ocean. Marine snow.jpg
Marine snow is a shower of organic material falling from upper waters to the deep ocean.

In the deep ocean, marine snow (also known as "ocean dandruff") is a continuous shower of mostly organic detritus falling from the upper layers of the water column. It is a significant means of exporting energy from the light-rich photic zone to the aphotic zone below, which is referred to as the biological pump . Export production is the amount of organic matter produced in the ocean by primary production that is not recycled (remineralised) before it sinks into the aphotic zone. Because of the role of export production in the ocean's biological pump, it is typically measured in units of carbon (e.g. mg C m−2 d−1). The term was coined by explorer William Beebe as observed 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 activity and ocean currents. Marine snow can be an important food source for organisms living in the aphotic zone, particularly for organisms that live very deep in the water column.

Contents

Composition

Marine snow is made up of a variety of mostly organic matter, including dead or dying animals and phytoplankton, protists, 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). [2] These are natural polymers exuded as waste products mostly by phytoplankton and bacteria. Mucus secreted by zooplankton (mostly salps, appendicularians, and pteropods) also contribute to the constituents of marine snow aggregates. [3] 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. [4] Most organic components of marine snow are consumed by microbes, zooplankton 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 ecosystems: 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). [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 nanometer and several micrometers. 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 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 bubbles. For example, Kepkay et al. found that bubble coagulation leads to an increase in bacterial respiration since more food is available to them. [9]

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

Central role of marine snow in the ocean carbon pump Schematic of the biological carbon pump.png
Central role of marine snow in the ocean carbon pump

Planktonic prokaryotes 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, often rendering sampling efforts difficult. These aggregates are hotspots for microbial activity. Marine bacteria are the most abundant organisms in aggregates followed by cyanobacteria and then nanoflagellates. [10] 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. [10]

As illustrated in the diagram, phytoplankton 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), zooplankton and their consumers into organic aggregates (marine snow), which is then exported to the mesopelagic (200–1000 m depth) and bathypelagic zones by sinking and vertical migration by zooplankton and fish. [11] [12] [13]

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 water column at depth, mostly by heterotrophic 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. [11] [12] [13]

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 thermohaline circulation, carbon transported as marine snow into the deep ocean by the biological pump 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, 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 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. [20] 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, 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 microbiologists. Recent research indicates transported bacteria may exchange genes 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 and pharmacy.

See also

Related Research Articles

<span class="mw-page-title-main">Plankton</span> Organisms living in water or air that are drifters on the current or wind

Plankton are the diverse collection of organisms that drift in water but are unable to actively propel themselves against currents. 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 baleen whales.

<span class="mw-page-title-main">Zooplankton</span> Heterotrophic protistan or metazoan members of the plankton ecosystem

Zooplankton are the animal component of the planktonic community, having to consume other organisms to thrive. Plankton are aquatic organisms that are unable to swim effectively against currents. Consequently, they drift or are carried along by currents in the ocean, or by currents in seas, lakes or rivers.

<span class="mw-page-title-main">Biological pump</span> Carbon capture process in oceans

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. In other words, it is a biologically mediated process which results in the sequestering of carbon in the deep ocean away from the atmosphere and the land. The biological pump is the biological component of the "marine carbon pump" which contains both a physical and biological component. It is the part of the broader oceanic carbon cycle responsible for the cycling of organic matter formed mainly by phytoplankton during photosynthesis (soft-tissue pump), as well as the cycling of calcium carbonate (CaCO3) formed into shells by certain organisms such as plankton and mollusks (carbonate pump).

The mesopelagiczone, also known as the middle pelagic or twilight zone, is the part of the pelagic zone that lies between the photic epipelagic and the aphotic bathypelagic zones. It is defined by light, and begins at the depth where only 1% of incident light reaches and ends where there is no light; the depths of this zone are between approximately 200 to 1,000 meters below the ocean surface.

The bathypelagic zone or bathyal zone is the part of the open ocean that extends from a depth of 1,000 to 4,000 m below the ocean surface. It lies between the mesopelagic above and the abyssopelagic below. The bathypelagic is also known as the midnight zone because of the lack of sunlight; this feature does not allow for photosynthesis-driven primary production, preventing growth of phytoplankton or aquatic plants. Although larger by volume than the photic zone, human knowledge of the bathypelagic zone remains limited by ability to explore the deep ocean.

The oxygen minimum zone (OMZ), sometimes referred to as the shadow zone, is the zone in which oxygen saturation in seawater in the ocean is at its lowest. This zone occurs at depths of about 200 to 1,500 m (700–4,900 ft), depending on local circumstances. OMZs are found worldwide, typically along the western coast of continents, in areas where an interplay of physical and biological processes concurrently lower the oxygen concentration and restrict the water from mixing with surrounding waters, creating a "pool" of water where oxygen concentrations fall from the normal range of 4–6 mg/L to below 2 mg/L.

f-ratio (oceanography) In oceanic biogeochemistry, the fraction of total primary production fuelled by nitrate

In oceanic biogeochemistry, the f-ratio is the fraction of total primary production fuelled by nitrate. The ratio was originally defined by Richard Eppley and Bruce Peterson in one of the first papers estimating global oceanic production. This fraction was originally believed significant because it appeared to directly relate to the sinking (export) flux of organic marine snow from the surface ocean by the biological pump. However, this interpretation relied on the assumption of a strong depth-partitioning of a parallel process, nitrification, that more recent measurements has questioned.

<span class="mw-page-title-main">Dissolved organic carbon</span> Organic carbon classification

Dissolved organic carbon (DOC) is the fraction of organic carbon operationally defined as that which can pass through a filter with a pore size typically between 0.22 and 0.7 micrometers. The fraction remaining on the filter is called particulate organic carbon (POC).

<span class="mw-page-title-main">Diel vertical migration</span> A pattern of daily vertical movement characteristic of many aquatic species

Diel vertical migration (DVM), also known as diurnal vertical migration, is a pattern of movement used by some organisms, such as copepods, living in the ocean and in lakes. The adjective "diel" comes from Latin: diēs, lit. 'day', and refers to a 24-hour period. The migration occurs when organisms move up to the uppermost layer of the water at night and return to the bottom of the daylight zone of the oceans or to the dense, bottom layer of lakes during the day. DVM is important to the functioning of deep-sea food webs and the biologically-driven sequestration of carbon.

<span class="mw-page-title-main">Microbial loop</span> Trophic pathway in marine microbial ecosystems

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 phytoplankton-zooplankton-nekton. In soil systems, the microbial loop refers to soil carbon. The term microbial loop was coined by Farooq Azam, Tom Fenchel et al. in 1983 to include the role played by bacteria in the carbon and nutrient cycles of the marine environment.

The microbial food web refers to the combined trophic interactions among microbes in aquatic environments. These microbes include viruses, bacteria, algae, heterotrophic protists. In aquatic ecosystems, microbial food webs are essential because they form the basis for the cycling of nutrients and energy. These webs are vital to the stability and production of ecosystems in a variety of aquatic environments, including lakes, rivers, and oceans. By converting dissolved organic carbon (DOC) and other nutrients into biomass that larger organisms may eat, microbial food webs maintain higher trophic levels. Thus, these webs are crucial for energy flow and nutrient cycling in both freshwater and marine ecosystems.

<span class="mw-page-title-main">Gelatinous zooplankton</span> Fragile and often translucent animals that live in the water column

Gelatinous zooplankton are fragile animals that live in the water column in the ocean. Their delicate bodies have no hard parts and are easily damaged or destroyed. Gelatinous zooplankton are often transparent. All jellyfish are gelatinous zooplankton, but not all gelatinous zooplankton are jellyfish. The most commonly encountered organisms include ctenophores, medusae, salps, and Chaetognatha in coastal waters. However, almost all marine phyla, including Annelida, Mollusca and Arthropoda, contain gelatinous species, but many of those odd species live in the open ocean and the deep sea and are less available to the casual ocean observer. Many gelatinous plankters utilize mucous structures in order to filter feed. Gelatinous zooplankton have also been called Gelata.

<span class="mw-page-title-main">Bacterioplankton</span> Bacterial component of the plankton that drifts in the water column

Bacterioplankton refers to the bacterial component of the plankton that drifts in the water column. The name comes from the Ancient Greek word πλανκτος, meaning "wanderer" or "drifter", and bacterium, a Latin term coined in the 19th century by Christian Gottfried Ehrenberg. They are found in both seawater and freshwater.

<span class="mw-page-title-main">Ecosystem of the North Pacific Subtropical Gyre</span> Major circulating ecosystem of ocean currents

The North Pacific Subtropical Gyre (NPSG) is the largest contiguous ecosystem on earth. In oceanography, a subtropical gyre is a ring-like system of ocean currents rotating clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere caused by the Coriolis Effect. They generally form in large open ocean areas that lie between land masses.

<span class="mw-page-title-main">Mycoplankton</span> Fungal members of the plankton communities of aquatic ecosystems

Mycoplankton are saprotrophic members of the plankton communities of marine and freshwater ecosystems. They are composed of filamentous free-living fungi and yeasts that are associated with planktonic particles or phytoplankton. Similar to bacterioplankton, these aquatic fungi play a significant role in heterotrophicmineralization and nutrient cycling. Mycoplankton can be up to 20 mm in diameter and over 50 mm in length.

<span class="mw-page-title-main">Particulate organic matter</span>

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<span class="mw-page-title-main">Jelly-falls</span> Marine carbon cycling events whereby gelatinous zooplankton sink to the seafloor

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<span class="mw-page-title-main">Viral shunt</span>

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<span class="mw-page-title-main">Benthic-pelagic coupling</span> Processes that connect the benthic and pelagic zones of a body of water

Benthic-pelagic coupling are processes that connect the benthic zone and the pelagic zone through the exchange of energy, mass, or nutrients. These processes play a prominent role in both freshwater and marine ecosystems and are influenced by a number of chemical, biological, and physical forces that are crucial to functions from nutrient cycling to energy transfer in food webs.

<span class="mw-page-title-main">Martin curve</span> Mathematical representation of particulate organic carbon export to ocean floor

The Martin curve is a power law used by oceanographers to describe the export to the ocean floor of particulate organic carbon (POC). The curve is controlled with two parameters: the reference depth in the water column, and a remineralisation parameter which is a measure of the rate at which the vertical flux of POC attenuates. It is named after the American oceanographer John Martin.

References

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