Increased temperature causes different carbon and nitrogen processing patterns in two common intertidal foraminifera (<i>Ammonia tepida</i> and <i>Haynesina germanica</i>)

Abstract

Abstract. Benthic foraminifera are highly abundant heterotrophic protists in marine sediments, but future environmental changes will challenge the tolerance limits of intertidal species. Metabolic rates and physiological processes in foraminifera are strongly dependent on environmental temperatures. Temperature-related stress could therefore impact foraminiferal food source processing efficiency and might result in altered nutrient fluxes through the intertidal food web. In this study, we performed a laboratory feeding experiment on Ammonia tepida and Haynesina germanica, two dominant foraminiferal species of the German Wadden Sea/Friedrichskoog, to test the effect of temperature on phytodetritus retention. The specimens were fed with 13 C and 15 N labelled freeze-dried Dunaliella tertiolecta (green algae) at the start of the experiment and were incubated at 20, 25 and 30 • C respectively. Dual labelling was applied to observe potential temperature effects on the relation of phytodetrital carbon and nitrogen retention. Samples were taken over a period of 2 weeks. Foraminiferal cytoplasm was isotopically analysed to investigate differences in carbon and nitrogen uptake derived from the food source. Both species showed a positive response to the provided food source, but carbon uptake rates of A. tepida were 10-fold higher compared to those of H. germanica. Increased temperatures had a far stronger impact on the carbon uptake of H. germanica than on A. tepida. A distinct increase in the levels of phytodetrital-derived nitrogen (compared to more steady carbon levels) could be observed over the course of the experiment in both species. The results suggest that higher temperatures have a significant negative effect on the carbon exploitation of H. germanica. For A. tepida, higher carbon uptake rates and the enhanced tolerance range for higher temperatures could outline an advantage in warmer periods if the main food source consists of chlorophyte phytodetritus. These conditions are likely to impact nutrient fluxes in A. tepida/H. germanica associations.

Food supply and size class depending variations in phytodetritus intake in the benthic foraminifer Ammonia tepida

“…There is not much known about foraminiferal nitrogen regeneration from organic matter, but isotope tracer studies in the Arabian Sea have shown a high contribution to the processing of phytodetrital nitrogen by benthic foraminifera ( Enge et al, 2016 ). Feeding experiments with A. tepida have also shown a coupling of carbon and nitrogen intake from D. tertiolecta detritus ( Wukovits et al, 2017 ), which is also apparent in the present study (see ).…”

“…Thermal response curves revealed a steep rise of oxygen consumption in A. tepida between 20°C and 30°C ( Bradshaw, 1961 ) and in respiration of A. tepida which was substantially higher at 23°C compared to 7°C ( Cesbron et al, 2016 ). This effect of temperature has also been observed on the level of carbon processing in A. tepida in a former feeding experiment ( Wukovits et al, 2017 ). There, a single low food pulse (∼220 µg C m −2 ) at three different temperatures (20°C, 25°C, 30°C) resulted in the highest levels of pC in A. tepida at 25°C.…”

“…In the current study, the constant food supply caused a similar rapid rise in pC at 25°C, but only until the fourth day, and it dropped on the following days despite constant food supply. The former study was carried out in vials containing solely phytodetritus ( Wukovits et al, 2017 ), while in this study a heat sterilized sediment layer was present. Sediment composition reportedly influences locomotion speed in A. tepida ( Jauffrais et al, 2016 ), while live prey and inorganic matter slow foraminiferal movement and high organic matter content increases locomotion speed.…”

“…These diets do not correspond with the natural variety of food sources for A. tepida in the German Wadden Sea. However, A. tepida individuals kept in the laboratory have previously shown a good response to Dunaliella -derived diets ( Bradshaw, 1961 ; Linshy et al, 2014 ; Wukovits et al, 2017 ). Considering the abundance of several sources of OM as potential food with different nutritional value and processing rates in nature, results of this experiment might reflect a good estimate of the mean carbon and nitrogen processing of A. tepida .…”

“…Microphytobenthos is additionally present throughout the year, with a strong reduction of abundant species in winter ( Scholz and Liebezeit, 2012a ). In such high productivity environments, foraminifera play an important role in sediment nutrient fluxes and phytodetrital carbon and nitrogen processing ( Moodley et al, 2000 ; Wukovits et al, 2017 ). For example, Moodley et al (2000) have evaluated that the genus Ammonia contributes with 1–7% to the processing of newly introduced, fresh phytodetrital carbon in an estuarine environment.…”