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Home , Dissolved organic carbon, Marine snow, Water column

The importance of ‘’ Carol Turley

Long-term removal Macroaggregates of containing a wide The deep is an range of phyto- and important sink in the held global carbon budget. About 40 % of global together in a sticky is matrix are known as carried out by marine ‘marine snow’. Carol micro-organisms <10 µm Turley describes the in size. While 10–40 % of importance of primary production may these sinking sink out of the upper 100 m particles to the of the north east Atlantic, of the most gets remineralized . during its descent so that only a small proportion arrives on the deep-sea bed. Much of the carbon fixed by the microscopic photosynthetic cells in the upper is recycled to the atmosphere within weeks through a dynamic . These single cells are just too small and to sink from the surface of the ocean and have a significant effect on long-term carbon removal. However, some of the cells aggregated into the larger sinking particles, known as marine snow, remove the carbon for centuries by transporting it to mid- and What is ‘marine snow’? Marine snow is made deep waters, or even for millions of years when it is laid LEFT: up of macroaggregates containing a wide down in sediments. Epifluorescent micrographs of recently formed marine snow range of species and sizes of living and aggregates showing the range of dead microscopic () and animals Supply of food to the deep-sea bed red autofluorescing phytoplankton (zooplankton) and their faecal pellets. It was given its The arrival of marine snow on the sea bed is the cells (top) and acridine-orange- name by divers because the aggregates, which are usually major determinant of abundance and activity of large stained bacterial cells colonizing greater than 0.5 mm across, really do look like snow and small deep-sea animals, many of which are deposit the surface (bottom). (TOP) REPRODUCED FROM TURLEY flakes when seen in the water. They are held together feeders. Many taxa respond to this seasonal influx of (2000) FEMS MICROBIOL ECOL 33, by a sticky matrix of mucopolysaccharides produced by material. For example, populations of opportunist 89–99 dying, nutrient-depleted phytoplankton cells or species may increase and its arrival may regulate the (BOTTOM) COURTESY C. TURLEY feeding webs of zooplankton. These sinking particles reproduction and growth cycles of some animals. also contain an enriched and active population of Perhaps the greatest opportunists are that bacteria, relative to free-living bacteria, which are grazed respond rapidly by increased enzyme production, DNA by bacterivorous . Marine snow is the major and protein synthesis and respiration; on occasions an component of the flux of organic material to the increase in sediment bacterial can be seen after and is generally produced in the productive upper 100 m this seasonal arrival of . Bacteria produce of the . Its production in the ocean cycles hydrolytic enzymes which cleave particulate organic also exhibits strong seasonal variation with peaks shortly matter into smaller to support their after the spring and autumn phytoplankton blooms in metabolism. There is evidence that deep-sea-adapted temperate waters such as those in the North Atlantic. bacteria are more effective at degrading the less labile

MICROBIOLOGY TODAY VOL 29/NOV 02 177 Atmosphere CO2 release Days–months Months– Centuries CO2 uptake years

Dissolved CO2 – HCO3 Respiration

Photosynthesis

Seasonal 40 m POC solubilization and respiration

Winter mixed layer – RIGHT: DOC2 CO2 HCO3 Sustenance of free-living bacteria in the 200 m Schematic diagram showing twilight zone and deep-sea the flux of carbon through the The waters of the twilight zone and deep sea are NE Atlantic water column to the Particle solubilization and respiration deep-sea sediment. probably the most under-studied oceanic environments, DEEP OCEAN DOC CO HCO– REPRODUCED FROM TURLEY (2000) 2 2 3 but make up the largest volumes of the oceans. Studies FEMS MICROBIOL ECOL 33, 89–99 have centred on the interfaces, such as the productive >2,000 m upper ocean, the sediment and coastal environments. BELOW: Resuspension Although the upper 100 m of the oceanic water A sediment core with an overlying & respiration by benthic organisms column contains higher concentrations of bacteria, the layer of marine snow. – DOC2 CO2 HCO3 COURTESY KEVIN BLACK, UNIVERSITY Detritus greatest reservoirs lie below this – about 60 % of the OF ABERDEEN POC total water column bacteria. We know that these areas DEEP-SEA SEDIMENTS Accumulation in sediment are truly nutrient-replete, so how are these bacteria FAR RIGHT TOP: material than their upper water column counterparts. surviving? Schematic diagram of processes on a marine snow aggregate and The metabolic versatility of deep-sea bacteria may, There is evidence that bacteria attached to marine interactions with the surrounding therefore, enable the breakdown of compounds that are snow may play an important role in releasing . unavailable to other organisms. Investigations indicate to the surrounding water COURTESY C. TURLEY that bacterial growth in the deep sea is limited by food and the free-living bacteria that live there through rather then the enormously high pressures and low extracellular enzymic hydrolysis of the particulate FAR RIGHT BOTTOM: often found there. organic carbon it contains. Presumably, this will be The distribution of bacteria biomass and production with depth patchy and sporadic, and in the form of microzones and integrated over sunlit zone, Life on marine snow around the descending aggregates. But is that sufficient mesopelagic and bathyl zones of Not surprisingly, bacteria and flagellates find the to sustain them? Some scientists have proposed that the oceanic water column. marine snow to be rich in organic and inorganic these bacteria also tap the substantial reservoir of old, COURTESY C. TURLEY nutrients, which produce active populations in an refractory dissolved organic carbon found in the deep sea. otherwise nutrient-replete environment. Bacteria Others have proposed that there is a downward flux of that colonize the marine snow clearly play an important the more labile dissolved organic carbon occurring in the role in the remineralization and solubilization of upper oceans. particulate organic carbon, so that many aggregates will Not surprisingly, these sinking particles are also the not only be formed in the sunlit waters of the upper main food resource for the animals that live in these ocean, but will also be recycled there. However, many – watery depths and they have evolved adaptations to most likely the larger, stronger aggregates – do escape scavenge the particles during their descent. However, to the twilight zone of the the enormous reservoir of carbon within the free-living midwaters and the dark bacteria is also a great potential food resource – if it can be abyss of the deep sea where captured. Little work has been done on the food web in decomposition rates by deep waters, but there are animals that can filter large the colonizing bacteria volumes of water or capture microscopic cells using their originating from surface mucus webs and have the potential to extract the waters may be reduced by bacteria. the cooler temperatures and higher pressures. Mechanism for genetic exchange between Protein and DNA syn- isolated populations? thesis in bacteria attached The deep ocean covers 60 % of the earth’s surface and to the aggregates in the previously scientists thought that bacteria surviving at surface waters may be these enormous pressures and depths, in total darkness, drastically influenced by were very isolated from most other bacteria occurring in the high pressures (100 the surface of the oceans. However, as many as 1×1012 atm every 1,000 m) as well bacterial cells per m2 per year, which is equivalent as the low temperatures to around 3×1013 plasmid encoded phenotypic experienced during the per m2 per year, can be transferred from the surface of the sinking of large particles. ocean to the deep-sea bed, a distance of about 4–5 km, The reduced microbial through sinking of marine snow. The big question then activity on such particles is does the formation and sinking of marine snow also act may contribute to the as a method of genetic exchange between populations delivery of relatively previously assumed to be genetically isolated from the undegraded aggregates enormous population of bacteria found in deep-sea to the deep-sea bed. sediments?

178 MICROBIOLOGYTODAY VOL 29/NOV 02 Further reading Alldredge, A.L. & Silver, The Aggregate Microniche M.W. (1988). Characteristics, dynamics and significance of marine snow. Prog Oceanogr 20, MICROZONES 41–82. Deming, J.W. & Baross, CO 2 J.A. (2000). Survival, Microbial degradation Aggregate POC dormancy and non-culturable & remineralization DOC cells in extreme deep-sea Free-living Attached micro-organisms environments. In bacteria DOC Non-Culturable Suspended or Shear & break-up In The Environment, pp. 147– slow-sinking POC 197. Edited by R.R. Colwell & G.J. Grimes. Washington, Collision & DC: American Society for adhesion Rapid . sedimentation Lampitt, R.S., Hillier, W.R. & Challenor, P.G. (1993). NE Atlantic: 47–59°N 20°W – summer Seasonal and diel variation in the open ocean concentration –2 –2 –1 Integrated bacterial carbon (g C m ) Integrated bacterial production (g C m a ) of marine snow aggregates. 01 23 050100 0 0 Nature 362, 737–739. 38% Epipelagic Zone 68% 200 200 m 200 30% 25% Turley, C.M. (2000). Bacteria 1,000 1,000 m 1,000 in the cold deep-sea and sediment- Bathypelagic Zone Depth 32% Depth 7% water interface of the NE Atlantic. FEMS Microbiol 4,000 4,000 m 4,000 107 108 109 03 6 Ecol 33, 89–99. No. of bacteria (l–1) Bacterial production (µg C l–1 d–1) Abyssopelagic Zone Turley, C.M., Lochte, K. & Lampitt, R.S. (1995). Transformations of Conclusions biogenic particles during The formation and sinking of marine snow in the sunlit sedimentation in the upper water column and its microbial degradation northeastern Atlantic Ocean. during and after its descent is of key importance to the Phil Trans R Soc Lond B 348, global as well as to the delivery of food to 179–189. organisms within the inner space of the deep-sea water column and on the deep-sea bed. The deep oceans are the largest and perhaps the oldest biosphere on Earth and hold an enormous reservoir of bacteria that is a potential resource of diverse and unique micro-organisms that may have evolved around 3.5 billion years ago. Marine snow plays a role in the sustenance of these micro- organisms and in combination with the wide range of pressures and temperatures found in the deep sea may contribute to the creation of a high metabolic and phylogenetic diversity. Obviously, there are still many unresolved questions and it is this and the fascination of the unknown that drives researchers in this ‘extreme’, at least to us, environment.

Dr Carol Turley is a senior researcher at Plymouth Marine Laboratory, Prospect Place, The Hoe, Plymouth PL7 1TP, UK. Tel. 01752 633464; Fax 01752 633101 email [email protected]

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