MICROBIAL HETEROTROPHS WITHIN CODIUM-BURSA - A NATURALLY ISOLATED MICROBIAL FOOD-WEB

Codium bursa, a balloon-like chlorophicean macroalga, achieves conside rable biomass (300 g DW M-2) in the NW Mediterranean littoral. C. burs a grows as a hollow sphere ranging in size up to 40 cm in diameter, wh ere the thick algal thallus (approximately 0.5 cm) encloses a lumen fi lled with seawater that receives substantial amounts of organic carbon from the thallus. The water enclosed within C. bursa supports an acti ve microheterotrophic (bacteria, flagellates and ciliates) community, about 2-fold denser than that in the ambient water, where metazoans ar e absent. Protistan grazing removed the entire bacterial population da ily (121.7% d-1). Oxygen concentration within the enclosed water showe d large diel fluctuations, reaching concentrations close to saturation (approximately 90%) during day time, and concentration close to anoxi a (after 12 h) in the dark, as a result of the combined metabolism of the alga and the microheterotrophs. The relative activity and abundanc e of microheterotrophs decreased as C. bursa size increased, consisten t with the reduction in the ratio of macroalgal weight : internal wate r volume. That the carbon supplied by the algal enclosure becomes incr easingly diluted as C bursa grows was also indicated by reduced exopro teolytic activity with decline of microheterotrophs abundance during C bursa growth. The high heterotrophic activity within C bursa balloons resulted in nutrient concentration 5- to 15-fold higher than those in the surrounding seawater, which could partially support algal product ion.