Respiration rates in bacteria exceed phytoplankton production in unproductive aquatic systems

Abstract

PLANKTONIC bacteria are a fundamental component of the organic carbon cycle in aquatic systems¹. Organic carbon consumption by planktonic bacteria is the sum of bacterial production (BP) and bacterial respiration (BR). It is now estimated that 30–60% of phytoplankton production (the amount of inorganic carbon fixed by phytoplankton photosynthesis, corrected for phytoplankton respiration) in marine and freshwater systems is processed by bacteria^1–3. These estimates of carbon flow through bacteria are conservative, however, because losses due to bacterial respiration are seldom directly measured^4,5. We report here that bacterial respiration is generally high, and tends to exceed phytoplankton net production in unproductive systems (less than 70 to 120 μg carbon per litre per day). A large proportion of the world's aquatic systems have phytoplankton productivities below this value⁶. Bacterial growth efficiency (BGE) is the result of BP and BR[BGE = BP/(BR + BP)]. Comparisons of our models of bacterial respiration with published models of bacterial secondary production^1,7 show that bacterial growth efficiency must range from less than 10% to 25% in most freshwater and marine systems, well below the values commonly assumed in many current ecological models^1,2,8,9. The imbalance between bacterial respiration and phytoplankton production suggests that in unproductive aquatic systems, the biological system is a net source of CO₂.