Life and particulate organic matter in the ocean have fundamentally shaped the planet. On the most basic level,
particulate organic matter can be defined as both living and non-living matter of biological origin with a size of ≥0.2 μm in diameter, including anything from a small bacterium (0.2 μm in size) to blue whales (20 m in size). Organic matter plays a crucial role in regulating global
marine biogeochemical cycles and events, from the
Great Oxidation Event in Earth's early history to the sequestration of atmospheric carbon dioxide in the deep ocean. Understanding the distribution, characteristics, dynamics, and changes over time of particulate matter in the ocean is hence fundamental in understanding and predicting the marine ecosystem, from food web dynamics to global biogeochemical cycles.
Measuring POM Optical particle measurements are emerging as an important technique for understanding the ocean carbon cycle, including contributions to estimates of their downward flux, which sequesters carbon dioxide in the deep sea. Optical instruments can be used from ships or installed on autonomous platforms, delivering much greater spatial and temporal coverage of particles in the
mesopelagic zone of the ocean than traditional techniques, such as
sediment traps. Technologies to image particles have advanced greatly over the last two decades, but the quantitative translation of these immense datasets into biogeochemical properties remains a challenge. In particular, advances are needed to enable the optimal translation of imaged objects into carbon content and sinking velocities. In addition, different devices often measure different optical properties, leading to difficulties in comparing results.
Ocean primary production Marine primary production can be divided into
new production from
allochthonous nutrient inputs to the
euphotic zone, and
regenerated production from
nutrient recycling in the surface waters. The total new production in the ocean roughly equates to the sinking flux of particulate organic matter to the deep ocean, about 4 billion tons of carbon annually.
Model of sinking oceanic particles Sinking oceanic particles encompass a wide range of shape, porosity, ballast and other characteristics. The model shown in the diagram at the right attempts to capture some of the predominant features that influence the shape of the sinking flux profile (red line). to several km per day (as with salp fecal pellets) which suggest that sinking velocity increases linearly with excess density (the difference from the water density) and the square of particle diameter (i.e., linearly with the particle area). Building on these expectations, many studies have tried to relate sinking velocity primarily to size, which has been shown to be a useful predictor for particles generated in controlled environments (e.g., roller tanks. However, strong relationships were only observed when all particles were generated using the same water/plankton community. When particles were made by different plankton communities, size alone was a bad predictor (e.g., Diercks and Asper, 1997) strongly supporting notions that particle densities and shapes vary widely depending on the source material. Mucous-rich particles have been shown to float despite relatively large sizes, whereas oil- or plastic-containing aggregates have been shown to sink rapidly despite the presence of substances with an excess density smaller than seawater. In natural environments, particles are formed through different mechanisms, by different organisms, and under varying environmental conditions that affect aggregation (e.g., salinity, pH, minerals), ballasting (e.g., dust deposition, sediment load;). A universal conversion of size-to-sinking velocity is hence impracticable.
The biological carbon pump The dynamics of the particulate organic carbon (POC) pool in the ocean are central to the
marine carbon cycle. POC is the link between surface primary production, the deep ocean, and sediments. The rate at which POC is degraded in the dark ocean can impact atmospheric CO2 concentration. Therefore, a central focus of marine organic geochemistry studies is to improve the understanding of POC distribution, composition, and cycling. The last few decades have seen improvements in analytical techniques that have greatly expanded what can be measured, both in terms of organic compound structural diversity and isotopic composition, and complementary
molecular omics studies. The
biological carbon pump describes the collection of biogeochemical processes associated with the production, sinking, and
remineralization of organic carbon in the ocean. In brief, photosynthesis by microorganisms in the upper tens of meters of the water column fix inorganic carbon (any of the chemical species of dissolved carbon dioxide) into
biomass. When this biomass sinks to the deep ocean, a portion of it fuels the metabolism of the organisms living there, including deep-sea fish and benthic organisms. production of fast-sinking fecal material and active vertical migration. where some of it (~0.2–0.5 Gt C) is sequestered for several millennia. The biological carbon pump is hence of similar magnitude to current carbon emissions from fossil fuels (~10 Gt C year−1). Any changes in its magnitude caused by a warming world may have direct implications for both deep-sea organisms and atmospheric carbon dioxide concentrations. Especially particle size and composition are important parameters determining how fast a particle sinks, and which organisms can find and utilize it. In general, particles in a fluid are thought to sink once their densities are higher than the ambient fluid, i.e., when excess densities are larger than zero. Larger individual phytoplankton cells can thus contribute to sedimentary fluxes. For example, large
diatom cells and diatom chains with a diameter of >5 μm have been shown to sink at rates up to several 10 s meters per day, though this is only possible owing to the heavy ballast of a silica
frustule. Both size and density affect particle sinking velocity; for example, for sinking velocities that follow
Stokes' Law, doubling the size of the particle increases the sinking speed by a factor of 4. As such, any climate-induced change in the structure or function of phytoplankton communities is likely to alter the efficiency of the biological carbon pump, with feedbacks on the rate of climate change. In the diagram on the right, the sinking POC is moving downward followed by a chemical plume. The plain white arrows represent the carbon flow. Panel (a) represents the classical view of a non-bioluminescent particle. The length of the plume is identified by the scale on the side. Panel (b) represents the case of a glowing particle in the bioluminescence shunt hypothesis. Bioluminescent bacteria are represented aggregated onto the particle. Their light emission is shown as a bluish cloud around it. Blue dotted arrows represent the visual detection and the movement toward the particle of the consumer organisms. Increasing the visual detection allows a better detection by upper trophic levels, potentially leading to the fragmentation of sinking POC into suspended POC due to sloppy feeding. ==See also==