Both. You can use it as singular or plural.
The root word of Unicellular already tells you the answer. Uni means only one cell. So the answer is one cell.
The bare soil surface type affects the amount of runoff because it has
no other stopping mechanism that prevents runoff to happen. Just like in the
use of pesticide, it is
harmful to human agriculture because it contains an antifreezing chemical
(present in transportation vehicles). When the chemical seeps into the soil, it
will create a runoff and can seep into the groundwater causing pollution.
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