Seagrass, the secret producer of TEPs

Transparent exopolymer particles (TEPs: Polysaccharides enriched in deoxy sugars and galactose) form an important constituent of particulate organic carbon (POC) in the ocean. Phytoplankton (predominantly diatoms) are thought to be a major source of TEPs, as well as some benthic organisms (Heinonen et al., 2007) and macroalgae detritus (Thornton, 2004). TEPs are thought to be phytoplankton orientated, although in the Mediterranean littoral zone TEP concentrations remain relatively high despite the low levels of planktonic primary production, this suggests a non-planktonic source. Seagrass beds are important sources of DOC in marine systems and therefore could provide the precursors needed for TEP formation. The authors of this study investigate the role of seagrass in TEP production by comparing two sites in the oligotrophic NW Mediterranean Sea; one characterised by high levels of seagrass (Posidonia oceanica) leaf litter retention and the other site rapidly flushed by open-sea water.

Sea surface water samples were collected fortnightly from both sites. A total of 76 samples were taken from the open-water site (Faro Cap Ses Salines) between January 2012 and March 2015, while 45 samples were taken from the accumulation site (Es Caragol beach) from the August 2012 to September 2014. TEP concentrations were determined following the colorimetric method (of Passow and Alldredge, 1995b) where by TEP is stained with alcian blue and counted.

TEP concentrations ranged from 4.6 to 90.6 μg XG Eq L−1 in Faro Cap Ses Salines and from 26.8 to 1878.4 μg XG Eq L−1 in Es Caragol, with significantly higher mean TEP concentrations recorded at Es Caragol. Seasonal fluctuations were observed; at Faro Cap Ses Salines the highest concentrations of TEP were recorded in February, June and July which likely coincides with the phytoplankton bloom at that time. In Es Caragol fluctuations of TEP were erratic with no clear seasonal patterns and remained consistently higher than the other site. In the lab, the relationship between leaf-litter and TEP was quantified and used to support the correlation between leaf litter and TEP shown by field results e.g. water in the presence of leaf litter (in the lab) yielded TEP concentrations of 1551 μg XG Eq L−1 after 264h, which is similar to the maximum concentrations recorded in nature at the accumulation site (Es Caragol).

By showing a >10-fold difference in TEP concentrations between the two sites, this study provides the first evidence that seagrass leaf litter is a source of TEP. These findings have bio-geochemical implications and provide a new pathway to be considered with regard to fluxes of DOM and POM in the marine environment. The Authors estimate that P. oceanica releases 76 Gg C as TEPs annually to the Mediterranean Sea, however this is based on values from one meadow in autumn and highlights the need for further study if an accurate value for seagrass contribution is to be described. Using lab data to support field recordings strengthened this study and I feel this method could be replicated in temperate waters to aid in better understanding of the local and relative importance of seagrass TEP production.

Reference of this study:

Iuculano, F., Duarte, C.M., Marbà, N. and Agustí, S., 2017. Seagrass as major source of transparent exopolymer particles in the oligotrophic Mediterranean coast. Biogeosciences, 14(22), p.5069.

Additional references:

Heinonen, K.B., Ward, J.E. and Holohan, B.A., 2007. Production of transparent exopolymer particles (TEP) by benthic suspension feeders in coastal systems. Journal of Experimental Marine Biology and Ecology, 341(2), pp.184-195.

Passow, U. and Alldredge, A.L., 1995. A dye‐binding assay for the spectrophotometric measurement of transparent exopolymer particles (TEP). Limnology and Oceanography, 40(7), pp.1326-1335.

Thornton, D.C., 2004. Formation of transparent exopolymeric particles (TEP) from macroalgal detritus. Marine Ecology Progress Series, 282, pp.1-12.