Abstract. Benthic foraminifera are abundant marine protists which play an important role in the transfer of energy in the form of organic matter and nutrients to higher trophic levels. Due to their aquatic lifestyle, factors such as water temperature, salinity and pH are key drivers controlling biomass turnover through foraminifera. In this study the influence of salinity on the feeding activity of foraminifera was tested. Two species, Ammonia tepida and Haynesina germanica, were collected from a mudflat in northern Germany (Friedrichskoog) and cultured in the laboratory at 20 ∘C and a light–dark cycle of 16:08 h. A lyophilized algal powder from Dunaliella tertiolecta, which was isotopically enriched with 13C and 15N, was used as a food source. The feeding experiments were carried out at salinity levels of 11, 24 and 37 practical salinity units (PSU) and were terminated after 1, 5 and 14 d. The quantification of isotope incorporation was carried out by isotope ratio mass spectrometry. Ammonia tepida exhibited a 10-fold higher food uptake compared to H. germanica. Furthermore, in A. tepida the food uptake increased with increasing salinity but not in H. germanica. Over time (from 1–5 to 14 d) food C retention increased relative to food N in A. tepida while the opposite was observed for H. germanica. This shows that if the salinity in the German Wadden Sea increases, A. tepida is predicted to exhibit a higher C and N uptake and turnover than H. germanica, with accompanying changes in C and N cycling through the foraminiferal community. The results of this study show how complex and differently food C and N processing of foraminiferal species respond to time and to environmental conditions such as salinity.

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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.

<p><span><span>Foraminifera are unicellular organisms which are important for marine C and N processing. Feeding experiments showed that the food uptake and thus the turnover of organic matter are influenced by changes of physical parameters (e.g., temperature, salinity). Since many areas of the Baltic Sea are strongly affected by anthropogenic activity and therefore contaminated by heavy metals from shipping in the past, this study examined the effect of heavy metal pollution on the food uptake of the most common foraminiferal species of the Baltic Sea, </span></span><span><span><em>E</em></span></span><span><span><em>lphidium</em></span></span><span><span><em> excavatum</em></span></span><span><span>. </span></span><span><span>In 2019, we collected water and sediment containing living </span></span><span><span><em>E</em></span></span><span><span><em>.</em></span></span><span><span><em> excavatum</em></span></span><span><span> in </span></span><span><span>the Kiel Fjord</span></span><span><span>. In laboratory experiments,</span></span><span><span> Baltic Sea seawater was enriched with metals at various levels above normal seawater: Zn (9.2-, 144- and 1044-fold), Pb (2.4-, 48.5- and 557-fold) and Cu (5.6- and 24.3-fold), and the </span></span><span><span>foraminiferal </span></span><span><span>uptake of </span></span><span><sup><span>13</span></sup></span><span><span>C- and </span></span><span><sup><span>15</span></sup></span><span><span>N-labelled phytodetritus was measured by isotope ratio mass spectrometry. Significant differences in food uptake were observable at different types and levels of heavy metals in sea water. An increase in the Pb concentration did not affect food uptake, whereas strong negative effects were found for high levels of Zn and especially for Cu. Interestingly, experiments with short incubation periods (1 and 5 days) showed greater differences in food uptake </span></span><span><span>from undisturbed conditions </span></span><span><span>than those of longer incubation times (10 and 15 days). In summary, an increase in the heavy metal pollution in the Kiel Fjord will likely lead to a significant reduction in the turnover of organic matter by foraminifera such as </span></span><span><span><em>E. excavatum</em></span></span><span><span>.</span></span></p>

Foraminifera are abundant unicellular organisms that play an important role in marine element cycles. A large benthic foraminifer obligatory bearing photosymbionts is Heterostegina depressa. We studied potential impacts of sunscreens available on the market on the activity of photosymbionts on H. depressa by means of pulse-amplitude modulated (PAM) fluorescence microscopy. We included four different sunscreens, with two of them sold as “conventional” and two more stated as “eco-friendly”. Further, the impact of pure Ensulizole (phenylbenzimidazole sulfonic acid) was tested, which is a common agent of sunscreens. Foraminifera were incubated at varying concentrations (10, 50 and 200 mgL−1) of different sunscreens and the pure Ensulizole for 14 days. The photosynthetic performance was measured after 1,3, 7 and 14 days. Pure Ensulizole had a strong negative impact on the photobionts, which was reflected by a significant reduction of the areal fluorescence signal. “Eco-friendly” sunscreens affected the health of foraminifera more severely compared to “conventional” ones. We assume that metal nanoparticles like titanium dioxide or zinc oxide of “eco-friendly” sunscreens are causing this impact, because these substances were already classified as toxic for several microorganisms.