Decomposers, such as microbes and fungi, play an important role in the carbon cycle. They break down the remains of dead plants and animals and, in doing so, release carbon dioxide through respiration.
The independent variable in the hypothesis of an observed increased in sea turtle sickness once an increase in red tide can be seen would be the increase of red tide.
The reason for this is that red tide increases or decreases will happen independently of the amount of sea turtle sickness - therefore increasing sea turtle sickness will be dependent on the first variable. (independent one).