Extracellular organic carbon (EOC) in the genusCarpophyllum (Phaeophyceae): Diel release patterns and EOC lability

Release of extracellular organic carbon (EOC) by the four species of the brown alga genusCarpophyllum was investigated in light : dark incubations of 30 to 40 h duration using a¹⁴C-method. Plants were collected during 1986 in the Hauraki Gulf area, Auckland, New Zealand. InC. maschalocarpum (Turn.) Grev. andC. plumosum (A. Richard) J. Ag. rather low release rates in light (2 to 5% of the photosynthetic rates) were followed by high release rates after 4 to 5 h in darkness (10 to 15% of previous photosynthetic rates). The opposite pattern, with high release in light and low in darkness, was found in experiments withC. angustifolium J. Ag. andC. flexuosum (Esper) Grev. The pattern with substantially higher release in dark than in light is observed for the first time. EOC products were dominated by small molecules (<1000 daltons) in all four species and both in light and darkness. Decomposition experiments exhibited a preferential bacterial utilization of low molecular weight products. Thus, a relative enrichment with larger molecules took place. Bacterial conversion efficiencies of EOC in 5-d experiments averaged 0.24 and 0.09 for products released in light and dark, respectively. The rate constants for decomposition of products released in light were lower than for those released in darkness.