Are the special life forms found in untreated natural water sources also present in grapes, which have a high water content? Are grapes an equally good source of these novel life forms compared to lemon juice, volume per volume?

Grapes are equally good and, in fact, may be superior because they are often stored for shorter intervals and because more are eaten, and with water being a higher percentage of their content, this will allow acquiring a much greater number of these life forms from a single sitting. Most people cannot eat lemons directly because of the tart flavor, so expressing the lemon juice and diluting it to make it more palatable is a workaround. So assuming lemons are available, lemonade would be an excellent source on its own. Grapes will provide equal benefit and can be eaten directly, more readily by the average person. The life forms will largely persist for the duration that such fruit sources can be maintained without spoiling. Refrigeration will preserve them as well, just as it preserves the fruit as a whole in edible form, in most cases.