Microbial heterotrophs within Codium bursa: a naturally isolated microbial food web

Codlurn bursa, a balloon-llke chloroph~cean macroalga, a c h ~e v e s cons~derable biomass (300 g DW m-') In the NW Mediterranean httoral C bursa grows as a hollow sphere ranging in slze up to 40 cm In d~ameter, where the t h ~c k algal thallus (-0 5 cm) encloses a lumen fllled w ~t h seawater that receives substantial amounts of organlc carbon from the thallus The water enclosed within C bursa supports an active mcroheterotroph~c (bactena flagellates and clllates) community, about 2-fold denser than that in the ambient water where metazoans are absent Protlstan grazing removed the entlre bactenal populabon dally (121 7 % d-l) Oxygen concentratlon withln the enclosed water showed large die1 fluctuations, reaching concentrations close to saturat~on (-90%) dunng day time, and concentratlon close to anoxia (after 12 h) In the dark as a result of the comblned n ~e t a b o l ~s m of the alga and the microheterotrophs The relat~ve actlvlty and abundance of mlcroheterotrophs decreased as C bursa slze Increased, consistent wlth the reduct~on In the ratlo of macroalgal w e ~g h t Internal water volume That the carbon supplied by the algal enclosure becomes increasingly diluted as C bursa grows was also indicated by reduced exoproteolytic actlv~ty w t h dechne of mlcloheterotrophs abundance dunng C bursa growth The hlgh heterotroph~c actlvity w ~t h i n C bursa balloons resulted in nutnent concentrabon 5to 15-fold hlgher than those in the surround~ng seawater, w h ~c h could partially support algal production