Transparent Exopolymer Particles (TEP) link phytoplankton and bacterial production in the Gulf of Aqaba

Abstract

Variations in Transparent Exopolymer Particles (TEP), bacterial biomass production (BP) and Primary Productivity (PP) were followed over 52 hours at a deepwater station in the Gulf of Aqaba (Eilat) during the spring, in April 2008. About 20 hours after the start of this study, there was a short (~15h) but intense storm event that probably caused a nutrient pulse and subsequently a brief outgrowth of diatoms in the euphotic water column. Concentrations of TEP and BP ranged from 23 to 228 μg Gum Xanthan equivalents L-1 and from 0.2 to 0.6 μg C L-1 h-1, respectively. TEP and BP were measured in both unfiltered and GF/C (1.2 μm) filtered samples. Most of the TEP (mean 59 ± 21% of total TEP) were usually in the smaller size fraction (>0.4 <1.2 μm) but after the crash of the diatoms the majority of TEP were > 1.2 μm. In the GF/C filtered fraction, BP averaged 59 ± 12% and 93 ± 5% of total BP in the upper water column and from 300 m, respectively. Significant correlations were observed between TEP and BP suggesting that active heterotrophic bacteria may have been associated with these particles. During the 3 days of this study PP and BP in the euphotic zone averaged 480 mg C m-2 d-1 and 225 mg C m-2 d-1 respectively suggesting that about half or more of the primary produced carbon was metabolized by heterotrophic bacteria in the upper water column. Coincident with strong mixing caused by the storm, TEP concentrations decreased in the surface water and increased at depth. We conjecture that TEP acted to link C flux between the primary producers and heterotrophic bacteria and that the downward movement of TEP from the upper water layers may be an important process in transferring organic carbon to deeper waters of the Gulf. Sinking TEP could provide not only organic C substrates for associated bacteria but also form "hot spots" of elevated microbial metabolism and nutrient cycling throughout the water column.