Some bacteria are capable of adjusting the fluidity of their membranes to the temperature of the environment. For example, they use desaturases, which are enzymes that turn saturated fatty acids into desaturated ones. In which season would cells need this enzyme (summer or winter, when would it be most beneficial for bacteria)

Winter,to conserve heat in the body.

Explanation:

The adaptation of the phospholipd bilayer ensures that the  phospholipid tails face one another inwards,while the hydrophilic  polar head point outwards.These tails can either by saturated or unsaturated. The high the number unsaturated chains the more the fluidity of the cell membrane,This because the un saturated chain are usually bent -kink-and therefore affects the alignments of the chains make them not properly arranged giving room for fluidity(heat).

In addition the longer the length of these chains the higher the fluidity of the membrane.

Note the more saturated the chains ,the more packed the cell membrane the less fluidity and tendency to freeze up.

In cold environment,as the temperature drops,membrane becomes less fluid, bacteria could not control body temperature,and the membrane fluidity drops.To survive these bacteria responds by increasing the number of unsaturated  fatty acids chains(desaturated ) to increase the fluidly to keep warm  to avoid been frozen up,thus winter is the beneficial

condensation. good luck!