A new technique to study nutrient flow in host-parasite systems by carbon stable isotope analysis of amino acids and glucose

Abstract

Stable isotope analysis of individual compounds is emerging as a powerful tool to study nutrient origin and conversion in host-parasite systems. We measured the carbon isotope composition of amino acids and glucose in the cestode Schistocephalus solidus and in liver and muscle tissues of its second intermediate host, the three-spined stickleback (Gasterosteus aculeatus), over the course of 90 days in a controlled infection experiment. Similar linear regressions of δ¹³C values over time and low trophic fractionation of essential amino acids indicate that the parasite assimilates nutrients from sources closely connected to the liver metabolism of its host. Biosynthesis of glucose in the parasite might occur from the glucogenic precursors alanine, asparagine and glutamine and with an isotope fractionation of - 2 to - 3 ‰ from enzymatic reactions, while trophic fractionation of glycine, serine and threonine could be interpreted as extensive nutrient conversion to fuel parasitic growth through one-carbon metabolism. Trophic fractionation of amino acids between sticklebacks and their diets was slightly increased in infected compared to uninfected individuals, which could be caused by increased (immune-) metabolic activities due to parasitic infection. Our results show that compound-specific stable isotope analysis has unique opportunities to study host and parasite physiology.

Figure 1

Average trophic fractionation (Δδ¹³C ± SD in ‰, n = 15) of individual AAs between parasite (P), host liver (L) and muscle (M) tissue over 90 dpi. Asterisks (*) indicate significant differences of average Δδ¹³C values from zero (two-sided t tests, DF = 14, p < 0.01). Trophic fractionation of EAAs between parasite and host liver was very low except for Thr, whereas significant Δδ¹³C_P-M values between the parasite and muscle tissue of the EAAs Arg, His and Thr were observed. Δδ¹³C_P-L values were negative for the NEAAs Gly and Pro and positive for Ser and Glx. Note that trophic fractionation of Asx and Glx between the parasite and muscle tissue seemed low and not significant, but regression slopes of δ¹³C values from Asx and Glx over time were very different between the two tissues (compare Table S3) and are therefore not directly comparable.

Figure 2

Multivariate analysis of Δδ¹³C_Fish-Diet values shows higher trophic fractionation of most individual AAs in liver and muscle tissue of infected sticklebacks compared to uninfected control individuals on the same diet. PC1 of the first factor (tissue) from ASCA analysis separates muscle and liver tissue of the infected and control individuals, whereas PC2 shows separate clusters of infected and control liver and muscle tissues.