The effect of the salinity, light regime and food source on carbon and nitrogen uptake in a benthic foraminifer

Abstract

p Foraminifera are unicellular organisms that play an important role in marine organic matter cycles. Some species are able to isolate chloroplasts from their algal food source and incorporate them as kleptoplasts into their own metabolic pathways, a phenomenon known as kleptoplastidy. One species showing this ability is i Elphidium excavatum /i , a common foraminifer in the Kiel Fjord, Germany. The Kiel Fjord is fed by several rivers and thus forms a habitat with strongly fluctuating salinity. Here, we tested the effects of the food source, salinity and light regime on the food uptake (via span classCombining double low line"inline-formula" 15N /span and span classCombining double low line"inline-formula" 13C /span algal uptake) in this kleptoplast-bearing foraminifer. In our study i E. excavatum /i was cultured in the lab at three salinity levels (15, 20 and 25) and uptake of C and N from the food source i Dunaliella tertiolecta /i (Chlorophyceae) and i Leyanella arenaria /i (Bacillariophyceae) were measured over time (after 3, 5 and 7 span classCombining double low line"inline-formula" d /span ). The species was very well adapted to the current salinity of the sampling region, as both algal N and C uptake was highest at a salinity of 20. It seems that i E. excavatum /i coped better with lower than with higher salinities. The amount of absorbed C from the green algae i D. tertiolecta /i showed a tendency effect of salinity, peaking at a salinity of 20. Nitrogen uptake was also highest at a salinity of 20 and steadily increased with time. In contrast, C uptake from the diatom i L. arenaria /i was highest at a salinity of 15 and decreased at higher salinities. We found no overall significant differences in C and N uptake from green algae vs. diatoms. Furthermore, the food uptake at a light-dark rhythm of span classCombining double low line"inline-formula" 16:8 /span span classCombining double low line"inline-formula" h /span was compared to continuous darkness. Darkness had a negative influence on algal C and N uptake, and this effect increased with incubation time. Starving experiments showed a stimulation of food uptake after 7 span classCombining double low line"inline-formula" d /span . In summary, it can be concluded that i E. excavatum /i copes well with changes of salinity to a lower level. For changes in light regime, we showed that light reduction caused a decrease of C and N uptake by i E. excavatum /i . /p .