Transparent exopolymer particles (TEP) link phytoplankton and bacterial production in the Gulf of Aqaba

Variations in transparent exopolymer particles (TEP), bacterial biomass production (BP) and primary productivity (PP) were followed over 52 h at a deep water station in the Gulf of Aqaba (Eilat, Israel) during the spring, in April 2008. About 20 h after the start of the study, there was a short (similar to 15 h) but intense storm event that probably caused a nutrient pulse and, subsequently, a brief outgrowth of diatoms in the euphotic layer. Concentrations of TEP and BP ranged from 23 to 228 mu g gum xanthan equivalents l(-1) and from 0.2 to 0.6 mu g C l(-1) h(-1), respectively. Concentrations of TEP and BP were measured in unfiltered and in GF/C (1.2 mu m)-prefiltered samples. Most of the TEP (59 +/- 21% of total TEP, mean +/- SD) were in the smaller (GF/C-filtered) size fraction (0.4-1.2 mu m); however, after the crash of the diatom bloom, the majority of TEP were in the >1.2 mu m size fraction. 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 d of our study, PP and BP in the euphotic zone averaged 480 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 suggest that TEP acted to link carbon 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 of Aquaba. Sinking TEP could provide not only organic carbon substrates for associated bacteria but also form `hot spots' of elevated microbial metabolism and nutrient cycling throughout the water column.