What effect does the alkalinity of wort have on the fermentation cycle in beer brewing?
Alkalinity really doesn't apply in the discussion of wort since the pH of wort should be below 7 thus making it an acidic solution.
Most of the discussion of pH during fermentation revolves around the yeast ability to reduce the pH and the available buffering materials present such as FAN(free available nitorgen). During the fermentation process organic acids are produced from yeast byproducts causing a drop in the overall pH level. The drop is significant in ale yeast during the first 12 hours(.4-.6) and much less so with lager yeast(.5 in 48 hours). The presence of large quantities of FAN not only spurs yeast growth and the production of organic acids it also buffers the acids moderating the drop in pH level.
Basically the lowering of the pH can reduce the production of dimethyl sulfide and is also used as a measure of fermentation health and fermentation completion.