Transparent exopolymer particles: Effects on carbon cycling in the ocean

Abstract

International audienceTransparent Exopolymer Particles (TEP) have received considerable attention since they were first described in the ocean more than 20 years ago. This is because of their carbon-rich composition, their high concentrations in ocean’s surface waters, and especially because of their ability to promote aggregation due to their high stickiness (i.e. biological glue). As large aggregates contribute significantly to vertical carbon flux, TEP are commonly seen as a key factor that drives the downward flux of particulate organic carbon (POC). However, the density of TEP is lower than that of seawater, which causes them to remain in surface waters and even move upwards if not ballasted by other particles, which often leads to their accumulation in the sea surface microlayer. Hence we question here the generally accepted view that TEP always increase the downward flux of POC via gravitational settling. In the present reassessment of the role of TEP, we examine how the presence of a pool of non-sinking carbon-rich particulate organic matter in surface waters influences the cycling of organic carbon in the upper ocean at daily to decadal time scales. In particular, we focus on the role of TEP in the retention of organic carbon in surface waters versus downward export, and discuss the potential consequences of climate change on this process and on the efficiency of the biological carbon pump. We show that TEP sink only when ballasted with enough high-density particles to compensate their low density, and hence that their role in vertical POC export is not solely linked to their ability to promote aggregation, but also to their contribution to the buoyancy of POC. It follows that the TEP fraction of POC determines the degree of retention and remineralization of POC in surface waters versus its downward export. A high TEP concentration may temporally decouple primary production and downward export. We identify two main parameters that affect the contribution of TEP to POC cycling; TEP stickiness, and the balance between TEP production and degradation rates. Because stickiness, production and degradation of TEP vary with environmental conditions, the role of TEP in controlling the balance between retention versus export, and hence the drawdown of atmospheric CO2 by the biological carbon pump, can be highly variable, and is likely to be affected by climate change

Spatial and Seasonal Variation of Biomineral Suspended Particulate Matter Properties in High-Turbid Nearshore and Low-Turbid Offshore Zones

“…Besides the quantity of biomass, the quality, such as stickiness, is important for controlling flocculation kinetics, as reported in previous research [50,55]. Specifically, extracellular polymeric substances (EPSs) or transparent extracellular polymers (TEPs) are sticky and increase flocculation [19,20,22,23]. Long polymeric chain structures of EPSs and TEPs, which are produced by aquatic microorganisms (e.g., algae), can bind biomass and sediment particles to large mineral, biomineral, and biological aggregates.…”

Effect of wind speed on the size distribution of gel particles in the sea surface microlayer: insights from a wind–wave channel experiment

“…A major source of TEP and CSP in the ocean are phyto-and bacterioplankton (Alldredge et al, 1993;Long and Azam, 1996;Stoderegger and Herndl, 1999). Previous studies highlighted the importance of gels for increasing gelatinous biofilm formation in the sea surface microlayer (SML) (Wurl and Holmes, 2008;Cunliffe et al, 2013) and mediating vertical organic matter transport, either up to the atmosphere or down to the deep ocean (AzetsuScott and Niven, 2005;Ebling and Landing, 2015;Guasco et al, 2014;Mari et al, 2017). In addition, it has been suggested that gels play an important role in air-sea exchange processes.…”

Composition and Vertical Flux of Particulate Organic Matter to the Oxygen Minimum Zone of the Central Baltic Sea: Impact of a sporadic North Sea inflow

“…CC BY 4.0 License. Scott and Passow, 2004); therefore they could also reduce the density of marine aggregates and 123 decrease their sinking velocity if the ratio of dense particles to TEP is too small (Azetsu-Scott and 124 Engel and Schartau, 1999;Mari et al, 2017). Mixed aggregates containing MnOx 125 and TEP have reported before for the GB and LD (Dellwig et al 2010;Glockzin et al 2014).…”