MadSci Network: Environment & Ecology

Re: How does Acid Rain affect algae?

Date: Mon Jan 11 11:05:47 1999
Posted By: Karen Culver-Rymsza, Biological Oceanographer
Area of science: Environment & Ecology
ID: 915396304.En


The answer to your question is yes and no. Land plants such as trees are 
affected by acid rain in two main ways. The increase in acidity changes the 
soil chemistry so that plants are less able to absorb minerals (such as 
magnesium). The increase in soil acidity also can kill a type of fungus 
that lives around the roots of many plants (called mycorrhizae). These 
fungi live in a mutualistic relationship, that is, they have a kind of 
symbiosis in which both the plant and the fungus benefit. The plant may 
benefit from the fungi's better ability to absorb nutrients of the fungus 
may help the plant in resistance to insects. So, losing mycorrhizae can 
leave plants vulnerable to insects and both loss of the fungus and 
increased soil acidity can cause plants to starve for minerals.

As for algae, these organisms are similar to land plants because increased 
acidity changes their ability to obtain certain minerals. They are quite 
different, though, in the main way that acid rain affects algae. All 
photosynthetic organisms require carbon dioxide. Land plants get it from 
the atmosphere. Aquatic plants get it from the water and acidity affects 
both the amount and chemical form of the oxidized carbon present. 

When CO2 dissolves in water it forms carbonic acid which dissociates into 
carbon dioxide (CO2), bicarbonate ion and carbonate ion. How much of each 
ion is present depends on pH or acidity. At low pHs which indicate acid 
conditions, most of the carbon is in the form of CO2. This is the only form 
that can be used directly for photosynthesis. (The other forms can be used 
by many algae, but must be converted to CO2 before entering the carbon 
synthesis cycle in the cell). In addition to changing the carbon into the 
CO2 form, it also decreases the total amount of CO2 that can dissolve in 
the water. So the result is that the carbon is available in the right form, 
but there is less of it.  Less carbon means less photosynthesis and that 
means less growth. 

So in an acid lake, you would probably find less total amount of algae. 
Since all algae can use straight CO2, you would likely find a lot of 
diversity at least until the lake becomes very acid, then you would find 
only acidophilic (or "acid loving") algae.

By the way, acid rain has little effect on marine algae. That is because 
seawater contains and can hold far more dissolved CO2 and carbonates. The 
carbonates act as a chemical buffer against changes in pH.

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