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发表于 2014-7-15 15:41:49
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African and Asian Dust: From Desert Soils to Coral Reefs
Coral reefs throughout the world are in decline. Increased incidence of coral disease, decreases in reef-building corals, increases in macroalgae, and lack of recovery on damaged reefs have been widely reported. The causes and processes driving the declines remain elusive. In the past 25 years, mass mortalities of a major reef-building coral (90% to 95% of Acropora palmata) and of a key herbivore (97% of Diadema antillarum) have significantly altered coral reefs in the Caribbean region. Microbial pathogens are suspected in both mortality events. Twenty years later, the abundance of both species remains low. Eleven years after a cruise ship anchor severely damaged a coral reef in the Virgin Islands, macroalgae dominate the damaged area, and stony coral abundance remains low.
Although natural disturbances and human activities are known to have caused much of the damage occurring on coral reefs, they do not explain why increasing numbers of reefs are not recovering from damage but are remaining in alternate states that appear to be stable. Nor do they adequately explain the apparent increases in disease incidence and mortality from disease or the widespread geographical distribution of coral diseases on reefs remote from as well as near to human influence. For example, sea fan disease, caused by a soil-associated fungus (Smith et al. 1996), occurs throughout the Caribbean region, including reefs off remote carbonate islands devoid of soil. Many hypotheses have been proposed, 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, and multiple low-level stressors). Here we put forward a hypothesis that addresses the widespread distribution of coral diseases and lack of recovery on coral reefs. We will (a) present an overview of the atmospheric transport of African and Asian dust; (b) review relevant background information and current research on airborne microorganisms, coral diseases, and atmospheric transport of chemical contaminants; and (c) suggest causal mechanisms and strategic avenues of investigation.
We propose that the hundreds of millions of tons of dust transported annually from Africa and Asia to the Americas may be a significant factor in coral reef decline and may be adversely affecting other downstream ecosystems as well (Shinn et al. 2000). Why would these changes occur now, after millennia of African and Asian dust transport? We suggest that the quantities of dust transported have increased and that the composition of the dust has changed. Synthetic organic chemicals and anthropogenic pollutants, viable microorganisms, macro- and micronutrients, and trace metals are likely to be carried in the dust air masses and deposited in the oceans and on land. Singly or in combination, they may play important roles in the complex changes occurring on coral reefs worldwide.
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some scientists question the importance of atmospheric dust as a source of iron in the ocean's photic zone, arguing that the most stable and abundant form of iron (Fe [III]) in dust is relatively insoluble in seawater and not readily available biologically. However, little is known of the chemistry occurring on dust particles of small volume and high surface area during atmospheric transport, and biogeochemical oceanographers are just beginning to understand the ocean's complex biogeochemical pathways. Dust from Africa and Asia is transported long distances in chemically and physically extreme environments where the small particles are exposed to high levels of solar radiation, multiple freeze–thaw cycles, relatively acidic conditions, and predominantly inorganic salts (Jickells 1999). On its deposition to the ocean, the dust enters a radically different environment. The thin surface layer of the ocean is characterized by concentrations of phytoplankton and zooplankton and by a steep concentration gradient of organic compounds and inorganic salts. It has been suggested that during atmospheric transport, photoreduction of Fe (III), which is stable and relatively insoluble, produces Fe (II), a biologically available and soluble species (e.g., Graedel et al. 1986, Duce and Tindale 1991). Saydam and Senyuva (2002) propose that oxalate released by fungi in desert dust facilitates photoreduction of Fe (III). Complexation of Fe (II and III) with organic ligands (Butler 1998) or with clay minerals in the aerosol acts to stabilize the iron in a bioavailable form. On their deposition to the ocean surface layer, insoluble Fe (III) and some of the more soluble first-row transition metals form stable complexes with siderophores, low-molecular-weight organic ligands produced by some species of oceanic bacteria, facilitating uptake by microorganisms and phytoplankton (Butler 1998 and references therein). Young and colleagues (1991) suggested that the smaller the dust particle, the longer the residence time in the photic zone and therefore the greater the amount of iron that could be released and made available for phytoplankton and microbes. In a more direct mechanism, Fe (III) deposited in reducing environments (e.g., carbonate muds in Florida Bay) can be reduced to Fe (II). Iron limitation is known to keep many microbial pathogens at low concentrations that directly counteract the expression of pathogenicity (Weinberg 1996). Hayes and colleagues (2001) proposed that iron in dust may play a similar role in promoting microbial diseases on coral reefs. |
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发表于 2014-7-15 15:38:48
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