Microorganisms in the Dutch Wadden seafloor feed on left-overs

This study was recently published in Frontiers of Ecology and Evolution. Riekenberg is a marine biogeochemist at NIOZ. He explains the source of a lot of nitrogen is not the sea water, but the seafloor. "Diatoms on intertidal mudflats, for example, are supported by the nitrogen remaining from recycled detritus in the porewaters between the grains of sand," Riekenberg says. "They feed on the processed left-overs in the bottom, so to speak."

Isotopes

For the study, Riekenberg and colleagues looked at the chemical variations in the isotopes of nitrogen. There are two predominate forms of nitrogen, 'nitrogen-14' and the heavier version, 'nitrogen-15'. When an organism digests protein, for example from plankton, the resulting nitrogen is used to create its own proteins. In this process, some of the lighter nitrogen-14 is lost, so the relative concentration of the heavy nitrogen increases a little at each step in the food chain. As a result, animals higher up the food chain retain relatively more and more nitrogen-15.

Individual amino acids

In recent years, it has become possible to also analyze the different nitrogen isotopes from individual amino acids. Certain amino acids – the building blocks of protein – cannot be created by higher organisms themselves. As a result, those essential or ‘source amino acids’ need to be obtained from the diet and remain mostly unchanged throughout the food chain. Therefore, in these amino acids, the stable isotope ratio of nitrogen does not become progressively higher with each step of eating or being eaten. Other, so-called trophic amino acids, do change a lot during metabolism through each step of the food chain. Thus, the difference in nitrogen composition between trophic and essential amino acids provides a measure of how high up the food chain an organism is, independent of any variations in underlying nitrogen sources supporting the ecosystem. This technique was used to build a trophic structure from direct measurements of the Dutch Wadden Sea food web.

Detritus in the pore water

Using samples that were collected between 2011 and 2014 during the long-lasting monitoring program of NIOZ, SIBES (the Synoptic Intertidal BEnthic Survey), Riekenberg analyzed the nitrogen isotopes of amino acids from 340 different animals from across the Dutch Wadden Sea. Thus, he was able to trace back the sources of nitrogen that these animals used. Riekenberg: “We saw that quite a bit of the nitrogen did not come from the overlying water column, but from the benthic primary producers, like diatoms, using nitrogen from the pore water at the bottom of the Wadden Sea. This nitrogen remains after the breaking down of organic matter and denitrification. Because of this, it has a distinct signal, and can be tracked into a portion of the food web.”

Ecological models

Riekenberg stresses that this new piece of the puzzle adds important knowledge to the science of the Wadden Sea ecosystem. “Now that we know that detrital nitrogen in porewaters is an important direct source of nutrients, this should be included in ecological models we make of the Wadden Sea. If our models do not include all pools of nutrients supporting the food web, then how can these models accurately reflect the ecology of the Wadden Sea, when they are used to predict future impacts or changes?”

More information

• The article Compound specific stable isotope analysis of amino acid nitrogen reveals detrital support of microphytobenthos in the Dutch Wadden Sea benthic food web is published in Frontiers of Ecology and Evolution 

Text: NIOZ
Photo's: Josje Snoek (leadphoto: diatom Asterionellopsis glacialis); Oscar Franken, NIOZ; Jim van Belzen, NIOZ