PNNL

Abstract

Fayetteville Green Lake (FGL) is a recognized, extensively studied present-day model of the stratified Proterozoic Ocean. Nonetheless, biomass sedimentation in FGL remains hard to explain: Whilst virtually all sediment pigments belong to photosynthetic sulfur bacteria (PSB) from a chemocline, the isotopic carbon signature of the bulk biomass suggests its epilimnetic phytoplankton origin. Here we present experimental evidence that FGL Synechococci produce copious extracellular polysacchrides (EPS), which become impregnated with Ca, Mg and Na cations and are release to the environment as ballasted cell coverings. We propose that sedimentation of these cell-free EPS can constitute the bulk of pigment-less organic material in FGL sediment. Because increased availability of inorganic carbon (Ci) stimulates accumulation of ballasted EPS, we propose the universal role of cyanobacterial EPS in biomass sedimentation in the high-Ci Paleoproterozoic Ocean as well as in modern aquatic systems like FGL.