I don't think normal algae is harmful, with the exception of blue-green algae, which isn't a true algae but a bacteria, and that red-tide stuff that poisons shellfish, etc. To get regular algae, I'm pretty sure that most kinds have to be exposed to light, not warmth. And a lot of algae is caused by excess nutrients in the water.

From what I understand, water in a container isn't too much of a likely source of botulism.

Some excerpts from the Center for Disease Control (CDC) regarding botulism:

"Clostridium botulinum is... a group of bacteria commonly found in soil. These... organisms grow best in low oxygen conditions.

"In the United States an average of 110 cases of botulism are reported each year. Of these, approximately 25% are foodborne, 72% are infant botulism, and the rest are wound botulism... wound botulism has increased because of the use of black-tar heroin, especially in California.

"Foodborne botulism has often been from home-canned foods with low acid content, such as asparagus, green beans, beets and corn. However, outbreaks of botulism from more unusual sources such as chopped garlic in oil, chile peppers, tomatoes, improperly handled baked potatoes wrapped in aluminum foil, and home-canned or fermented fish.

For more info, see http://www.cdc.gov/ncidod/dbmd/diseaseinfo/botulism_g.htm