Salinity-dependent algae uptake and subsequent carbon and nitrogen metabolisms of two intertidal foraminifera (<i>Ammonia tepida</i> and <i>Haynesina germanica</i>)

Abstract

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.

The effect of salinity, light regime and food source on C and N uptake in a kleptoplast-bearing foraminifera

“…species Ammonia tepida increased with salinity (Lintner et al, 2020). In the same study, both species showed large differences in the retention of C relative to N, with subsequent adverse effects on the re-cycling of these elements by mineralization/respiration and excretion to the environment.…”

“…The mean C uptake recorded at 20 PSU showed a maximum five days after food addition and declined thereafter. Such a behavior is already known from H. germanica (Lintner et al, 2020), a closely related species living in the same habitat. In Lintner et al (2020) this behavior was explained by the fact that H. germanica also contained kleptoplasts, which may serve as internal C and N sources via digestion.…”

“…Similarly, pC tended to be stimulated by prolonged starvation but the variation was too high to become significant. (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.…”

“…Currently little is known about the feeding behavior and the C and N metabolism of foraminifera species exhibiting kleptoplastidy, such as Elphidium or Haynesina. Moreover, given that plastids may either supplement the nutrition of foraminifera by providing photosynthates or by being digested, kleptoplastid species may show a slower detrimental response to starvation, or a slower uptake of (pulses of) algal food (Lintner et al, 2020). Foraminiferal food uptake depends on several factors such as size of food (Murray, 1963), the type of food (e.g., Lee and Müller, 1973;Nomaki et al, 2014), the age of the foraminifera and food quality (Lee et al, 1966), water temperature (Wukovits et al, 2017;Heinz et al 2012) or salinity (Lintner et al, 2020;Dissard et al, 2009).…”

“…It is therefore likely that E. excavatum prefers the algal diet that corresponds to the source of its kleptoplasts. Moreover, generally food (C) uptake by a kleptoplastid species (H. germanica) was lower than that of a species not showing kleptoplastidy (A. tepida) (Lintner et al, 2020;Wukovits et al 2017), indicating that the chloroplasts can supplement the C nutrition of species exhibiting kleptoplastidy. A shift in food preference in terms of C uptake from diatoms at 15 PSU to green algae at 20-25 PSU is also noteworthy and has not yet been observed in this or other foraminifera species.…”