Although natural disturbances and human activities are knownto have caused much of the damage occurring on coral reefs, they do not explainwhy increasing numbers of reefs are not recovering from damage but areremaining in alternatestates that appear to be stable.Nor do they adequatelyexplainthe apparent increases in disease incidence and mortality from diseaseor the widespread geographical distribution of coral diseases on reefs remotefrom as well as near to human influence. For example, sea fan disease, causedby a soilassociated fungus occurs throughout the Caribbean region, includingreefs off remote carbonate islands devoid of soil.Many hypotheses have beenproposed, but they do not adequately explain the declines observed on reefs worldwide(e.g., global warming, pathogen introduction via ballast water or oceanic currents,chronic low levels of nutrients, long-term intense fishing pressure, andmultiple low-level stressors).
Here we put forward a hypothesis that addresses thewidespread distribution of coral diseases and lack of recovery on coral reefs. Wepropose that the hundreds ofmillions of tons of dust transported annually fromAfrica and Asia to the Americas may be a significant factor in coral reefdecline and may be adversely affecting other downstream ecosystems as well. Whywould these changes occur now, after millennia of African and Asian dust transport?We suggest that the quantities of dust transported have increased and that thecomposition of the dust has changed. African and Asian dust consists primarily ofclay soil minerals such as illite, quartz, kaolinite, chlorite, microcline,plagioclase, and calcite, which may undergo chemical change during aerosoltransport. Some elements (e.g., manganese, iron, scandium, cobalt) occur onAfrican dust particles in concentrations similar to average crustal abundance,whereas other elements (e.g., mercury, selenium,lead) accumulate, via scavenging,at concentrations three orders of magnitude greater than mean crustal abundance.
Atmospheric deposition is thought to be thedominant source of iron in the ocean’s photic zoneIron, a micronutrient, canlimit phytoplankton productivity in oligotrophic waters; newly deposited ironis quickly depleted by phytoplankton and bacteria. Turner and colleagues (1996)showed that iron flux to the oceans leads to the biotic production ofdimethylsulfide (DMS) and its release into the atmosphere. Subsequent oxidationof DMS and formation of sulfate in turn produces sulfuric acid, which withatmospheric mixing could increase the solubility of iron (in the form Fe [III])in the mineral aerosols.Walsh and Steidinger (2001) and Lenes and colleagues(2001) linked African dust to the development of extensive red tides in the Gulfof Mexico. Lenes and colleagues (2001) also showed that iron in African dustdeposited by rain fuels blooms of Trichodesmium, an iron-limited cyanobacterium. The nitrogen-fixing Trichodesmium produces nutrients (e.g., nitrates,nitrites) that, in combination with selective predation by zooplankton andmeteorological conditions, fuel blooms of the red tide dinoflagellate Karenia brevis. Recently,Bishop and colleagues (2002)documented an increase in carbon biomass in the North Pacific in response to ironinflux from a strong Asian dust event.
Some scientists question the importance of atmospheric dustas a source of iron in the ocean’s photic zone, arguing that the most stableand abundant form of iron (Fe [III]) in dust is relatively insoluble inseawater and not readily available biologically. However, little is known ofthe chemistry occurring on dust particles of small volume and high surface areaduring atmospheric transport, and biogeochemical oceanographers are justbeginning to understand the ocean’s complex biogeochemical pathways. Dust fromAfrica and
Asia is transported long distances in chemically andphysically extreme environments where the small particles are exposed to highlevels of solar radiation,multiple freeze–thaw cycles, relatively acidicconditions, and predominantly inorganic salts (Jickells 1999). On itsdeposition to the ocean, the dust enters a radically different environment. Thethin surface layer of the ocean is characterized by concentrations ofphytoplankton and zooplankton and by a steep concentration gradient of organiccompounds and inorganic salts. It has been suggested that during atmospherictransport, photoreduction of Fe (III), which is stable and relativelyinsoluble, produces Fe (II), a biologically available and soluble species.Saydam and
Senyuva (2002) propose that oxalate released by fungi in desertdust facilitates photoreduction of Fe (III). Complexation of Fe (II and III) withorganic ligands (Butler 1998) orwith clay minerals in the aerosol acts tostabilize the iron in a bioavailable form. On their deposition to the oceansurface layer, insoluble Fe (III) and some of the more soluble first-row transitionmetals form stable complexes with siderophores, low-molecular-weight organicligands produced by some species of oceanic bacteria, facilitating uptake bymicroorganisms and phytoplankton. Young and colleagues (1991) suggested thatthe smaller the dust particle, the longer the residence time in the photic zoneand therefore the greater the amount of iron that could be released and madeavailable for phytoplankton and microbes. In a more direct mechanism, Fe (III)deposited in reducing environments (e.g., carbonate muds in Florida Bay) can bereduced to Fe (II). Iron limitation is known to keep many microbial pathogensat low concentrations that directly counteract the expression of pathogenicity.Hayes and colleagues (2001) proposed that iron in dust may play a similar rolein promoting microbial diseases on coral reefs. | 
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发表于 2012-11-26 23:24:16
