Organic carbon release by phytoplankton: its composition and utilization by bacterioplankton

Abstract

The present study characterized the rate of production of extracellularly released organic carbon (ROC) by phytoplankton, its molecular weight distribution, subsequent utilization and transformation by bacterioplankton in situ. Primary production rate of phytoplankton was high during the study due to continuous blooms of small dinoflagellates and ranged from 59.8 to 298.7μg CI−1 h−1. The rate of organic carbon release varied from 1.3 to 123.7 μg CI−1 h−1 and constituted from 4.0 to 68.9% of the total carbon fixed in photosynthesis. The ROC was fractionated on molecular weight (MW) basis. A low MW fraction less than 500 daltons (18.5% of ROC), a fraction of MW 10 000–30 000 daltons (30% of ROC), and high MW fraction of > 300 000 daltons (15.4% of ROC), were the most dominant in ROC. Bacterioplankton utilized a significant portion of ROC, ranging from 18 to 77%. Part of the utilized ROC incorporated by the bacterioplankton (31 – 56%), and the remainder was respired (mineralized). ROC not utilized by bacteria was composed of high MW compounds. The dynamics of the in situ utilization of ROC and its role as a link between autotrophic and heterotrophic processes in the estuary are described.