Sunday, 20 May 2012

These Phototrophs, I love them so much...

Polished Stromatolite made
by cyanobacteria [**]
In the oceans, ubiquitous microscopic organisms,  the phytoplankton,  account for approximately half the production of organic matter on Earth. These organisms -  phototrophs [*], initially a kind of bacteria and later then algae -  are key for producing oxygen at earth. Since more than 3 Billion years they  are blubbering away -  causing around 2.4 billion years ago the Oxygen Catastrophe. The Oxygen Catastrophe, also called the  Great Oxygenation Event,  marks the transition to an atmosphere with abundant free oxygen to breath.

Phototrophs were producing oxygen already a little of 600 Million years before causing the  Oxygen Catastrophe. However, initially organic matter and dissolved iron captured any free oxygen then these became saturated [%]. The excess free oxygen started to accumulate in the atmosphere. This rising oxygen levels have wiped out a huge portion of the Earth's anaerobic inhabitants at the time. Thus Cyanobacteria, by producing oxygen that was toxic to anaerobic organisms, were essentially responsible for what was likely the largest extinction event in Earth's history, but opening the path to live as we know it. And still today we relying today on these ubiquitous microscopic organisms,  the phytoplankton. 

Analyses of satellite-derived phytoplankton concentration, which are available since 1979, have indicated that phytoplankton concentrations fluctuate over decades and may be linked to climate forcing. Historical records of ocean transparency measurements and direct chlorophyll observations show time dependence of phytoplankton biomass at local, regional and global scales since 1899 Phytoplankton biomass seems to decline in several ocean regions  [1].  Inter-annual phytoplankton biomass fluctuations are superimposed on long-term trends. These fluctuations being correlated with basin-scale climate indices, whereas long-term declining trends are related to increasing sea surface temperatures.The global rate of decline seems to be ~1% of the global median  phytoplankton biomass per year. Such a decline of oxygen producers or food producers would need to be considered for geochemical cycling and fisheries. It is going well beyond local phenomena, such as  seasonal oxygen deficits that are observed to occur more frequently. 

Seasonal oxygen deficits in coastal ecosystems  already today represents  an acute perturbation to ecological dynamics and fishery sustainability  [2]. Its known that anthropogenic nutrient loading has increased the frequency and severity of  oxygen deficits  in  semi-enclosed seas such as the Baltic.  Also in some parts of the better mixed North Sea summer oxygen levels are declining  to critical values, probably because of ocean warming and the decay of photosynthetic blooms that form as a result of nutrient influx [3].  Historical data over the last century highlight an increase in seasonal oxygen depletion and a warming over the past 20 years.  In 2010, dissolved oxygen in central North Sea  came close to ecological critical values [#] that, if reached, would require management action under the European Union's Water Framework Directive.   

This image shows cold water up-welling near the coast of Peru
(purple) and joining the South Equatorial Current, which flows
westward across the Pacific Ocean. This MODIS SST image
from January 1-8, 2001 shows the ocean in normal conditions,
Credit: NASAaption
Oxygen deficits in open-coast up-welling systems reflects ocean conditions that control the delivery of oxygen-poor and nutrient-rich deep water onto continental shelves. Up-welling systems support a large proportion of the world's fisheries. Therefore understanding how changes in ocean climate lead to  up-welling-driven oxygen deficits  is critical; even for getting the "blame" right in a sensitive region", such as Persian Gulf.

When large numbers of fish began dying off the northern coast of Oman in the Persian Gulf in late August 2000, the local media reported that the deaths were due to the release of contaminated ballast water from a U.S. tanker visiting the area. Red tide blooms are a common phenomenon in the coastal waters of Oman. Thus Omani authorities feared that a toxic algal bloom was killing the fish, raising concerns about health and food security for their nation's fishing industry. Neither seemed true, using data from two NASA Earth Observing System (EOS) satellites, a team of researchers demonstrated that the fish kill was due to environmental changes that severely reduced the oxygen content of the surface waters   [4, ##].

Thus, these ubiquitous microscopic bubbling organisms,  the phytoplankton, play their role. They are key for producing oxygen at earth, since more than 3 Billion years.  They are key for food. Their decline or absence, nothing to like for.

Modern stromatolites off the eastern coast of Australia.
[*] Phototrophs are organisms that gain the energy to run their metabolism from sunlight; most but are fixing carbon to build their tissues

[**, adapted from Wikipedia] Stromatolites are layered structures formed in shallow water by the trapping, binding and cementation of sedimentary grains by biofilms of micro-organisms, especially Cyanobacteria. Stromatolites provide some of the most ancient records of life on Earth.

[%; from 5]  Research published recently indicates that earth's output of "...reduction in volcanic gases brought about by a drop in mantle-melt intensity was an important precursor to oxygenation."

[#; adapted from [3]] A hydrographic survey in August 2010 mapped the spatial extent of summer oxygen depletion. Typical near-bed dissolved oxygen saturations in the stratified regions of the North Sea were 75–80 % while the well-mixed regions of the southern North Sea reached 90 %. Two regions of strong thermal stratification, the area between the Dooley and Central North Sea Currents and the area known as the Oyster Grounds, had oxygen saturations as low as 65 and 70 % (200 and 180 μmol dm−3) respectively. Low dissolved oxygen was apparent in regions characterised by low advection, high stratification, elevated organic matter production from the spring bloom and a deep chlorophyll maximum.

SeaWiFS captured this image of a dust storm over the
Arabian Peninsula on May 3, 1999
[##; adapted from [4]] Omani scientists know the Gulf of Oman and Arabian Sea contain oxygen-poor water at depths of about 100 meters  below the surface. This oxygen-poor layer is due to the fact that the whole northern Arabian Sea is so highly productive. Strong winds often sweep iron-rich desert dust out over the Gulf of Oman and Arabian Sea where much of it settles into the ocean. The iron contained in the dust effectively fertilizes biological productivity in the ocean's surface waters. Phytoplankton under the right conditions have the capacity to “bloom,” into exponentially large numbers in a matter of days. Over time, these biota die at the surface and begin to sink to the bottom as detritus. As this detritus sinks it decays, thereby using up oxygen in the water column. The Arabian Sea has one of the thickest oxygen-depleted layers of ocean water found anywhere in the world. Sometimes, due to shifts in the overlying wind field, these deep oxygen-poor waters upwell to the surface. So, ironically, the very reason that Oman’s fish reserves are the largest in the world also indirectly leads to periodic mass fish kills. Satellite imagery gave an indication that there was indeed an up-welling event along the coast of Oman. Remotely-sensed sea surface temperature data showed that cool up welled water appeared at the surface along the Batinah coast as early as August 21, 2000, reaching coolest temperatures by the time of the peak of the fish kill on September 4, 2000.

[1] Daniel G. Boyce, Marlon R. Lewis & Boris Worm;  Nature 466, 591–596 (29 July 2010) 
[2] Brian A. Grantham, Francis Chan, Karina J. Nielsen, David S. Fox, John A. Barth, Adriana Huyer, Jane Lubchenco & Bruce A. Menge; Nature 429, 749-754 (17 June 2004) |
[3] Bastien Y. Queste, Liam Fernand, Timothy D. Jickells und Karen J. Heywood, Biogeochemistry, 2012, DOI: 10.1007/s10533-012-9729-9

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