I (American English) have only ever heard /ˈældʒi/, to the extent that this question surprised me—I hadn't really considered the possibility of pronouncing the word any other way, although I am well aware of the variation in the pronunciation of g in other words such as fungi.
I listened to the first 20 pronunciations of algae on Youglish. The 4 speakers who used [g] all sounded like they had an accent other than American English.
When I listened to the first 18 British English pronunciations, 14 of them used /giː/, 1 used /geɪ/, and 3 used /dʒiː/.
A WordReference Forums thread from 2009 seems to provide further anecdotal support for the idea of a UK/US split.
Based on this, it does seem to me that the ratio of /dʒ/ to /g/ is higher in the US than in other areas (the Oxford English Dictionary also seems to indicate this with the order in which it gives the pronunciations for British English and American English, as tchrist says in a comment below GEdgar's answer).
I don't know of any tendency for the word to be pronounced differently by biologists and non-biologists. There is no uniform, unanimously agreed-upon system of pronouncing biological terms in English—in general, there is similar variation among biologists as among other speakers in the pronunciation of words taken from Latin.