Assimilation of Particular Organic Matter and Dissolved Organic or Inorganic Compounds by Cribroelphidium selseyense (Foraminifera)

Abstract

Marine carbon and nitrogen processing through microorganisms’ metabolism is an important aspect of the global element cycles. For that purpose, we used foraminifera to analyze the element turnover with different algae food sources. In the Baltic Sea, benthic foraminifera are quite common and therefore it is important to understand their metabolism. Especially, Cribroelphidium selseyense, also occurring in the Baltic Sea, has often been used for laboratory feeding experiments to test their effect on carbon or nitrogen turnover. Therefore, foraminifera were collected from the Kiel Fjord and fed with six different algal species in two qualities (freeze-dried algae vs. fresh algae, all 13C- and 15N-labeled). Also, labeled dissolved inorganic C and N compounds and glucose were offered to the foraminifera to test direct assimilation of dissolved compounds (carbon and nitrogen) from the water column. Our experiments showed that after 15 days of incubation, there were highly significant differences in isotope labeling in foraminifera fed with fresh algae and dry algae, depending on algal species. Further, different algal species led to different 13C and 15N enrichment in the studied foraminifera, highlighting a feeding preference for one diatom species and an Eustigmatophyte. A significant carbon assimilation from HCO3– was observed after 7 days of incubation. The N assimilation from NH4+ was significantly higher than for NO3– as an inorganic N source. The uptake of glucose showed a lag phase, which was often observed during past experiments, where foraminifera were in a steady state and showed no food uptake at regular intervals. These results highlight the importance of food quality on the feeding behavior and metabolic pathways for further studies of foraminiferal nutrition and nutrient cycling.

The change in metabolic activity of a large benthic foraminifera as a function of light supply

“…During the first 7 days, they found only a marginal assimilation of carbonate and nitrate 25 , which however increased afterwards probably because of symbiotic bacterial activity. At the same time, C. selseyense showed a continuous uptake of ammonium during the whole experiment 25 . We conclude from our data, that assimilation of carbonate, nitrate and ammonium are light-dependent and triggered by the activity of the phototobionts, while glucose uptake is continued in darkness thus maintaining the holobionts metabolism.…”

“…Under light supply, additional energy and organic compounds for foraminiferal growth are generated by the photobionts. Lintner et al 25 investigated the element uptake of the obligatory heterotrophic Cribroelphidium selseyense . During the first 7 days, they found only a marginal assimilation of carbonate and nitrate 25 , which however increased afterwards probably because of symbiotic bacterial activity.…”

“…Lintner et al 25 investigated the element uptake of the obligatory heterotrophic Cribroelphidium selseyense . During the first 7 days, they found only a marginal assimilation of carbonate and nitrate 25 , which however increased afterwards probably because of symbiotic bacterial activity. At the same time, C. selseyense showed a continuous uptake of ammonium during the whole experiment 25 .…”

“…Measurements of the isotope ratios of each sample were carried out in the Stable Isotope Laboratory for Environmental Research (SILVER) of the University of Vienna. For a detailed description of the measurement and the calculation of the incorporated amount of isotopes, see Lintner et al 25 .…”