Certain species of what are now recognized as indicators of environmental pollution

The correct answer is lichens.

Explanation:

Lichens are a symbiotic association between the fungi and algae. The lichens are bioindicators, that is, they are very subtle to atmospheric pollution like nitrogen as they obtain all their water and nutrients from dry and wet atmospheric deposition. The deposition of nitrogen can enhance the nutrients load.  

Some of the species of lichens are more forbearing to nitrogen in comparison to others. The communities of lichens are monitored by the scientists, that is, if an enhancement in nitrogen tolerant species in association with a reduction in nitrogen sensitive species takes place, it suggests that there is an elevation in atmospheric nitrogen deposition.  

the solute can move "downhill," from regions of higher to lower concentration, relying on the specificity of the protein carrier to pass through the membrane. this process is called passive transport or facilitated diffusion, and does not require energy.