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

Abstract

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.

A major shift in Daphnia genetic structure after the first ice‐free winter in a German reservoir

“…In addition, summer conditions with low food availability and high predation pressure seem unfavourable for D. galeata , which shows better performance under spring conditions with high food abundance and low predation pressure (Jankowski & Straile, 2004). In 2005 and 2006, the biovolume of algae smaller than 20 μm, considered suitable food for the daphnids in the Saidenbach Reservoir, amounted to between 0.25 and 0.29 mm 3 L −1 during the period of spring full circulation in April (Schwerin et al. , 2010).…”

Haemoglobin‐mediated response to hyper‐thermal stress in the keystone speciesDaphnia magna

“…Hb crude extracts were obtained from pools of ten individuals per genotype following established protocols (Schwerin, Zeis, Horn, Horn, & Paul, 2010). In brief, after removing adhering water, the pools of animals were weighed on a precision scale (BP 211D; Sartorius) and homogenized in 100 μl of M4 medium and mini complete protease inhibitor cocktail (Roche) using Teflon ® pestles (Kimble Kontes, Sigma Aldrich, Darmstadt, Germany).…”

Phenotypic changes in Daphnia pulex under oxygen deficiency, resource limitation and predation risk

“…The expression level of hemoglobin was modulated by exposure not only to oxygen deficiency but also to low food availability and kairomone. It may be considered that the different mechanisms explain the observations: the reduction in hemoglobin may be associated with decrease in total protein in the hemolymph (Schwerin et al, 2010), and the enhanced hemoglobin expression with kairomone may be caused by the activity of methyl farnesoate (Daimon & Shinoda, 2013). In addition, the fitness parameters (somatic growth, reproduction and survival) improved under low DO conditions, indicating that hemoglobin synthesis intricately affects the pattern of other phenotypic changes in Daphnia .…”

“…Food limitation reduced the hemoglobin expression level in high O 2 conditions even in the presence of kairomone (Table 2 and Figure 3). Schwerin et al (2010) found that hemoglobin concentrations in a Daphnia galeata ‐ hyalina hybrid species complex, sampled from a field population in Saidenbach Reservoir in Germany, showed the minimal value when chlorophyll a concentrations were lower. In addition, their laboratory experiments demonstrated that starvation can reduce concentrations of hemoglobin in Daphnia under high O 2 conditions.…”

“…However, the higher expression level of hemoglobin under low DO conditions showed no change even under starvation (treatment D + F, Figure 3). Hemoglobin can have two functions: as a respiratory hemolymph protein, for enhancing oxygen transport capacity; and as a protein store, for buffering food availability and protein demands (Schwerin et al, 2010). Therefore, it is assumed that the reduction in hemoglobin under starvation can be caused by a decrease in total protein, and that enhancement can result from an increase in respiratory protein.…”