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Aggregation and collisions within "marine snow"

The "marine snow" made of sinking aggregates of organic matter plays an important ecological role. Mineral particles of planktonic origin can make them sink more rapidly, but also more slowly…

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In the most productive oceanic regions, a "marine snow" is falling from the upper layers of the water column. Its "flakes", generally a few millimeters wide, aggregate organic matter mainly produced by diatoms (microscopic algae enclosed within a siliceous shell called a frustule) at the end of the growing season, when nutrient availability becomes a limiting factor. Their sinking plays an important ecological role as this is how part of the carbon fixed in the sunlit surface waters escapes biological recycling to be exported towards deep layers where it will be excluded from the exchanges with the atmosphere (thence unavailable for the climate regulation mechanisms) for centuries or millennia. In addition, this organic matter provides food to organisms living down to a thousand meters deep and also on the oceanic bottom.

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An aggregate about 3 mm wide in the graduated glass cylinder used to measure sinking speed

This is why it is important to understand how aggregates form and which factors determine their sinking speed, particularly the possible ballasting role of mineral particles. Diatoms and their siliceous frustule, but also other microalgae with calcareous exoskeleton, can be incorporated into aggregates, thereby increasing their density. This study dealt with the role of the main two calcareous particles deriving from phytoplankton: coccoliths (small plates –less than 5 µm- covering the surface of some species called coccolithophores) and the larger (> 250 µm) tests protecting the cells of another group, foraminifers.

Diatom cells were thus cultivated in the presence of foraminiferan tests ("Foram" tanks) or coccoliths ("Cocco" tanks) or neither ("Phyto" tanks). This experiment was conducted in a set of cylindrical tanks held in rotation on motorized roller tables so that aggregates could sink unimpeded by container walls. After 48 hours of rotation (3 rotations per minute) in the dark and at 15°C, aggregates were sampled to measure their size, composition and sinking speed.

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The experimental device: cylindrical tanks, motorized roller, opaque cover

Aggregates formed in all tanks but with different properties. In "Phyto" tanks, they were few and made mainly of organic matter and silica (from diatom frustules). Almost twice as many aggregates formed in the "Cocco" tanks; they contained a large proportion of mineral carbon provided by coccoliths. In "Foram" tanks, they were even more numerous but two kinds were found: a small fraction (about 10%) consisted of a single foraminiferan test plus a small amount of particulate organic matter ("Foram with test") whereas the great majority paradoxically lacked a calcareous particle ("Foram without test").

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"Phyto" aggregates settled at the bottom of the container after the experiment

Among tanks, the aggregates differed less by their composition (apart from the presence or absence of mineral carbon) than by their size, especially when related to their sinking speed. Both types lacking a calcareous particle ("Phyto" and "Foram without test") sank more slowly, because of their small size and the absence of ballast; their size/speed relation was similar. For a given size, the "Cocco" aggregates sank more rapidly because of the weight of the coccoliths. The "Foram with test" type sank more rapidly than any other type, both in absolute values (up to 700 meters per day) and in relation with size.

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Sinking speed (m/day) versus size (mm) of the different types of aggregates: "Phyto" (blue dots), "Cocco" (green dots), "Foram without test" (yellow triangles), "Foram with test" (red triangles)

Contrary to expectations, "Cocco" aggregates were not significantly more compact than those lacking a calcareous particle. A high concentration of mineral particle enhances aggregation through the increase the frequency of collisions, but the results suggest that it does not increase the amount of organic matter aggregated: in all tanks, 37 to 38 % of the particulate organic carbon in suspension at the beginning of the experiment was incorporated. This proportion is likely more dependent on the abundance and properties of the TEP, detrital organic particles produced by diatoms, whose "stickiness" plays a role in the aggregation process.

Even if some organic matter remained associated with foraminiferan test in spite of their high sinking speed, it was quite unsurprising that this speed inhibited most aggregates of tank "Foram" to form around a test. The difference of sinking speed between test and aggregate prevents the incorporation of the former into the latter. On the contrary, it causes a collision causing the expulsion of some organic matter from the aggregate, thereby giving birth to a smaller one, both of them sinking more slowly than the initial aggregate. Therefore the presence of foraminiferan tests not only does not ballast the aggregates but it reduces their sinking speed.

The aggregates sink all the more rapidly that they are large and ballasted by a calcareous particle. The results however show that the incorporation of coccoliths did not modify their porosity, thence their size. The sinking speed of the aggregates lacking a mineral particle ("Phyto" and "Foram without test") depends only on their size as they have the same composition.

In the ocean, the effect of these particles on the deep export of organic carbon is not uniformly distributed. Coccolithophores are more abundant in some areas like the North Atlantic; this is where they play the greatest role to ballast the aggregates, even though the efficiency of this transport is low. Planktonic foraminifers, in contrast, are present in all oceans but tend to be more abundant in the most productive regions like continental shelves and the Antarctic Circumpolar Current. Paradoxically, the intense growth of phytoplankton which favors the export of organic matter in aggregates occurs in the same areas where the production of the foraminiferan tests slowing this process is the highest..

 

The paper

Schmidt K., De La Rocha C. L., Gallinari M., Cortese G., 2014. Not all calcite ballast is created equal: differing effects of foraminiferan and coccolith calcite on the formation and sinking of aggregates. Biogeosciences, 11, 135–145.

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The authors

This work was done in collaboration by researchers of the Laboratoire des sciences de l'environnement marin (Lemar, IUEM), of the University of Groningen (the Netherlands) and of the GNS Science Institute (New-Zealand)

 

The journal

Biogeosciences is an international journal devoted to interactions between biological, chemical and physical processes involved into life in geosphere, hydrosphere and atmosphere. The journal is published by a private publisher on behalf of a scientific professional association (the European Geosciences Union); it is original in two ways. It is published under the open access model (free of charge and without subscription) and the review of manuscripts is open to anybody. After a rapid access-review by one of the editors, manuscripts are subject to interactive public discussion within the scientific community, while the classical peer-review process is completed. Comments, referees' evaluation and answers by authors are published on Biogeosciences Discussions website.

 

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