Hemoglobin concentration in Daphnia (D. galeata-hyalina) from the epilimnion is related to the state of nutrition and the degree of protein homeostasis

The function of hemoglobin (Hb), which is a key protein of many species, was analyzed in Daphnia from the field. Temperature and oxygen content; abundance and composition of phytoplankton; and abundance, body, and clutch sizes of the Daphnia assemblage were measured in the epilimnion and around the thermocline of a reservoir for 2 yr. In addition, carbohydrate, lactate, and hemoglobin concentrations were determined in whole Daphnia. In Daphnia from the thermocline, hypoxia-induced Hb expression was responsible for an increase in Hb concentration in parallel with a decrease in oxygen content. In Daphnia from oxygen-rich water (epilimnion), however, Hb concentration unexpectedly showed a negative relationship to temperature and to the animals' temperature-dependent metabolic rate. Furthermore, the seasonal minima of Hb concentration coincided with low food (phytoplankton) availability and maxima of Daphnia's reproductive activity. These data suggest that Hb serves a dual function as respiratory protein and protein store. Laboratory experiments supported this hypothesis: starvation caused the concentration of Hb and other hemolymph proteins to decrease during normoxia, whereas the hemolymph protein store remained stable during hypoxia. Hemolymph proteins obviously function as a buffer between food (protein) availability and protein demand, with the latter increasing with temperature mainly because of elevated growth and reproduction rates. Accordingly, the size of the hemolymph protein store can indicate the state of nutrition of Daphnia (and possibly other species) from oxygen-rich water, which can easily be assessed by spectroscopic measurement of Hb in whole animals.