To be clear, the pronunciations of "alga", "algae", and "algal" are, respectively, /al-guh/, /al-jee/, and /al-gull/. The alternation of the sound of the g is due to a strict adherence to the rule that words taken from Latin and Greek always soften c's and g's before e, i, y, ae and oe (the last two because they are treated as e for pronunciation as well).

There is no situational allowance for a c or g to be softened in the absence of those vowels, but 2 exceptions have arisen: "suggest" and "exaggerate", in which the double g must have come to be pronounced single before the sound evolved to a /j/.

There is a general allowance on a word-by-word basis elsewhere in English for a g to be hard before those vowels, and there is one such exception in a Greek-derived word: the prefix "gyneco-". I don't know if it's because a soft g would make it sound too much like "vagina" or what. (Actually a second example is "giga-", but that's not fimly established--in "Back to the Future", Doc Brown pronounces "gigawatt" /jih-ga-waht/", according to the rule. Somehow computer people hadn't picked up on that when they coined "gigabyte".)

There is no situational allowance for a c to be hard before those vowels, but 2 exceptions have arisen: "soccer", derived from the word "association", which has no /k/ sound (so that's confusing); and "Celtic", which was reverted to a hard c by someone else ignorant of the evolution of European languages: http://en.wikipedia.org/wiki/Pronunciation_of_Celtic#Pronunciation