Chemotactic and Growth Responses of Marine Bacteria to Algal Extracellular Products

The possibility that planktonic algae possess a "phycosphere," a zone surrounding them created by the production of extracellular products which may serve as bacterial nutrients, is examined. 2. Bacterial growth in algal cultures to which no additional organic material is added is greatest only as the cultures age and algal cell lysis becomes obvious. 3. Marine bacterial isolates are chemotactic to filtrates from algal cultures, but the response is significant only to filtrates from old cultures, again where cell lysis is evident. 4. Specific compounds known to occur as algal extracellular products attract bacteria, but the threshold concentrations for attraction are unexpectedly high when compared with the generally very low concentrations of organic compounds in natural sea water. 5. The validity of the phycosphere concept and its potential importance to marine microorganisms is discussed.