Abstract
Abstract. 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 Elphidium excavatum, 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 15N and 13C algal uptake) in this kleptoplast-bearing foraminifer. In our study E. excavatum was cultured in the lab at three salinity levels (15, 20 and 25) and uptake of C and N from the food source Dunaliella tertiolecta (Chlorophyceae) and Leyanella arenaria (Bacillariophyceae) were measured over time (after 3, 5 and 7 d). 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 E. excavatum coped better with lower than with higher salinities. The amount of absorbed C from the green algae D. tertiolecta 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 L. arenaria 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 16:8 h 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 d. In summary, it can be concluded that E. excavatum 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 E. excavatum.
Highlights
1.1 General informationForaminifera are unicellular, highly diverse marine organisms known since the early Cambrian (e.g., Scott et al, 2003; Pawlowski et al, 2003)
The uptake of C and N from green algal food sources by E. excavatum was slightly affected by salinity (Fig. 2)
The low level of ingested D. tertiolecta in comparison to other studies with Ammonia tepida (0.4–1.2 μg C mg−1 and 0.2– 0.4 μg N mg−1 at a salinity of 24) and Haynesina germanica (0.05–0.35 μg C mg−1 and 0.03–0.13 μg N mg−1 at a salinity of 24) (Lintner et al, 2020; Wukovits et al, 2017) suggests that this green algae was not a preferred dietary source of this foraminifer species. This observation can be compared with experiments by Correia and Lee (2000) which demonstrated an increased absorption of chloroplasts by E. excavatum, which corresponds to a dietary preference for diatoms
Summary
1.1 General informationForaminifera are unicellular, highly diverse marine organisms known since the early Cambrian (e.g., Scott et al, 2003; Pawlowski et al, 2003). M. Lintner et al.: The effect of the salinity, light regime and food source on C and N uptake worldwide distribution and occurs from tropical to Arctic waters (Murray, 1991). Lintner et al.: The effect of the salinity, light regime and food source on C and N uptake worldwide distribution and occurs from tropical to Arctic waters (Murray, 1991) This genus makes up a high proportion of the total foraminiferal population in the shallow water of the Mediterranean, the English Channel, the North Sea and the Baltic Sea (Murray, 1991). Elphidium excavatum shows a large morphological intraspecific variability (Miller et al, 1982) Schweizer et al (2011) showed that these species exhibit large genetic differences with respect to each other and can be regarded as subspecies rather than as ecophenotypes
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