Abstract
The combination of lower diet quality and increased metabolic rates is assumed to cause cascading effects on organismic C cycling. Future changes in CO2 levels or terrestrial nutrient discharges in marine ecosystems can lead to increased phytoplankton C:N ratios relative to consumer C:N ratios, lowering the quality of the food source. In this study, we compared the single and interactive effects of diet quality and temperature on the feeding behavior and C and N intake and release of a common and abundant intertidal mixotrophic protist, the foraminifer Haynesina germanica. Two batches of artificially produced and dual isotope-labeled (13C/15N) chlorophyte detritus with different C:N ratios (5.6 and 7.1) were fed to the foraminifer at 3 different temperatures (15, 20, 25°C). We observed a strong interactive effect of temperature and diet. A very strong increase in feeding rates was observed at 20°C for the low-quality food source. Respiration rates of carbon derived from the low-quality diet (C:N ratio of 7.1) were lower than those of the high-quality diets and increased at 25°C. This indicates that a high C content of the diet might be of advantage in calcifying mixotrophs, since respired excess C could be advantageous for test calcification. Additionally, respired excess C could be a useful resource of CO2 for kleptoplast photosynthesis and functionality in the mixotrophic lifestyle of H. germanica. Further, the observed effects of diet and temperature could impact nutrient fluxes in the habitat of H. germanica, possibly leading to food-web shifts in the future.
Highlights
Foraminifera are key players in marine benthic food webs (e.g. Lipps & Valentine 1970, Moodley et al 2000, Nomaki et al 2008)
H. germanica sequesters chloroplasts exclusively from pennate diatoms, including chloroplasts derived from the diatom Phaeodactylum tricornutum (Pillet et al 2011), which is considered to be a suitable food source for H. germanica cultured under laboratory conditions (Ward et al 2003)
There was no effect of the temperature treatments for the low-quality food source (7.1 D, Table 2), but the Phytodetritus-derived nitrogen intake (phytoN) intake was significantly elevated at 15°C compared to 20 and 25°C for the high-quality food source (5.7 D, Table 2)
Summary
Foraminifera are key players in marine benthic food webs (e.g. Lipps & Valentine 1970, Moodley et al 2000, Nomaki et al 2008). Lipps & Valentine 1970, Moodley et al 2000, Nomaki et al 2008) These highly abundant protists are ubiquitous primary consumers and detritivores that play a critical role in the processing of marine particulate organic matter (POM). Benthic foraminifera adapt a wide range of strategies for nutrient supply, including herbivory, detrivory, Publisher: Inter-Research · www.int-res.com. The feeding strategy of H. germanica was investigated earlier in laboratory observations (Austin et al 2005). H. germanica sequesters chloroplasts exclusively from pennate diatoms, including chloroplasts derived from the diatom Phaeodactylum tricornutum (Pillet et al 2011), which is considered to be a suitable food source for H. germanica cultured under laboratory conditions (Ward et al 2003)
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