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Zooplankton OPAL and Iron Study (ZOOPALIS) - Investigating the effect of grazing on Si and Fe recycling

Final Activity Report Summary - ZOOPALIS (Zooplankton OPAL and Iron Study - Investigating the effect of grazing on Si and Fe recycling)

The Marie Curie intra-European fellowship ZOOPALIS, allowed Dr Sabine Schultes, from Germany, to spend 24 months at the European Institute for Marine Studies in Brittany, France.

Detailed studies were carried out to increase our understanding of ecological interactions among marine micro-organisms. Feeding of two species of microscopic zooplankton, a flagellate and a ciliate, on an important group of phytoplankton, the diatoms, was studied in laboratory cultures. Diatoms play an important role in the cycle of carbon in the world ocean. When diatoms bloom under favourable conditions, they take up large amounts of CO2 during photosynthesis and each cell builds a protective casing of biogenic silica, or opal. Once diatoms die or are eaten by zooplankton, CO2 is released and the biogenic silica redissolves. How fast this process of dissolution takes place and especially where it occurs in the ocean, either in the surface waters or several hundred meters deep, is still unknown.

The cultures of diatoms and zooplankton grazers had firstly to be grown at the host institution in France, which specialised in chemical analyses of silicon in marine waters. It was necessary to find a combination of a grazer with a diatom species that it could feed on. Best results were achieved with an easily cultivated micrograzer, namely oxyrrhis marina, a dinoflagellate of about 20 µm in size, and a small, of 5 µm size, unicellular diatom called thalassiosira pseudonana. Chaetoceros gracilis, a unicellular diatom of similar size but with long spines that was thought to protect it from grazing, was used as alternative study prey for o. marina. A successful experiment lasted approximately one month from preparation to completion. From the moment when the grazer was added to diatom prey culture - time zero of the experiment - a large range of parameters were determined on hourly, daily or weekly intervals to follow the degradation process: dissolved silicate (Si(OH)4), biogenic silica, particulate organic carbon, abundance of bacteria, abundance of diatom cells and growth of the micrograzer.

Unexpectedly, the addition of the grazer to the culture initially stimulated uptake of Si by the diatom, instead of dissolution. This uptake did not seem to be related to the feeding activity of the grazer, but to its mere presence in the diatom culture. A molecule excreted by the grazer probably initiated a metabolic pathway in the diatom and led to Si uptake. Approximately 24 to 48 h after the addition of the grazer to the diatom culture, dissolution of biogenic silica from the diatom protective casing could be observed; it was however not clearly accelerated by the grazing activity. This was probably due to the presence of bacteria in the cultures. Bacteria themselves accelerated the dissolution of biogenic silica.

Results, even from apparently simple experimental designs, were rarely conclusive on first sight. As in nature, numerous interactions complicated the picture. The best way to unravel this complexity was to repeat experiments with a single change in design and compare results among the different experiments. The series of seven experiments which were completed during the fellowship would be interpreted with the help of simple numerical ecosystem models. Such 'theoretical' experiments that were based on actual results allowed for the continuation of ecological hypotheses' testing. Ultimately, they helped to add a little piece to the large puzzle called "plankton ecology". These microscopic, living communities in the ocean are most probably based on the same principles as land ecosystems, such as animals feeding in a forest. The scientific field of plankton ecology only recently receives increased interest, since it is thought to be one of the keys to understand the role of the ocean in climate change. With the project ZOOPALIS, scientists sought to contribute to this goal.