Are foraminifera what they eat?

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In the oceans microscopic plants called phytoplankton use carbon dioxide, light and nutrients to produce organic matter, which provides the basis of the marine food-chain. The process where carbon dioxide is converted into organic matter is termed photosynthesis. The element carbon occurs on earth in three forms, with the same amount of protons, but increasing neutrons: 99% of carbon exists like carbon-12 (6 protons, 6 neutrons), ~1% as carbon-13 (6 protons, 7 neutrons), and a minute fraction of carbon-14 (6 protons, 8 neutrons). Plants actively discriminate against the heavier carbon-13 during photosynthesis (it is easier to incorporate the smaller atom) causing relative enrichment in carbon-12. Comparison of the ratio of carbon-13 over carbon-12 (=carbon isotope) of the carbon in carbon dioxide and that of organic matter provides a measure of fractionation. Within the Atlantic Ocean the carbon isotope ratio of phytoplankton generally tracks productivity, with phytoplankton from high productivity regions having considerably higher carbon isotopic ratios compared with low productivity regions. If we could measure phytoplankton organic carbon isotopes on geological samples we might be able to obtain information of past productivity changes in the area. The problem is that (1) many of the proteins that form the organic material may degrade after burial in sediments, (2) the organic material extracted from bulk sediments (it is not possible to just extract organic material from phytoplankton material) may be sourced from several other sources (including terrestrial plants, or old material carried by a deep sea current from somewhere else), and so may not represent actual surface water conditions. This research proposal will test whether planktonic foraminifera (zooplankton that live near the sea surface and eats phytoplankton) organic carbon isotopes also track productivity, to assess it's application for historical reconstructions of marine productivity.