The effect of nitrate supplementation on the biochemical composition of benthic diatoms and the growth and survival of post-larval abalone

Elevating the nitrogen concentration in microalgal growth media can elevate the protein content of the algae and consequently increase their dietary value to molluscs. This study examined the protein content, and dietary value to abalone post-larvae, of seven benthic diatoms acclimated to high-(“H” = 49.4 mg NO₃-N l^− 1) and standard-nitrate (“S” = 16.5 mg NO₃-N l^− 1) Walne medium. In Experiment 1, the level of nitrate in the culture medium did not significantly affect the protein content of Cocconeis sp., Cylindrotheca closterium or Nitzschia ovalis, and variation in the growth of post-larval Haliotis iris was not associated with nitrate level or the protein content of the diatoms. When the same six diatom diets were added daily as cell suspensions, growth and survival were much lower than for the equivalent strains fed as established films (P < 0.0001). In Experiment 2a, the protein content of Navicula incerta was elevated by nitrate supplementation, and the high nitrate strain retained a higher protein content (P = 0.005) even after 24 days growing in unenriched, flowing seawater. Haliotis rufescens post-larvae grew significantly faster when fed N. incerta H than when fed N. incerta S (P = 0.006) and survival was also higher with this diet (67 versus 54%). In Experiment 2b, there was no significant effect of nitrate level on protein content of four diatoms or the performance of post-larval H. rufescens. There was, however, a strong positive correlation (r = 0.95, P < 0.0001) between post-larval survival and the lipid content of the diatom diets. Post-larval growth showed positive, but marginally non-significant, relationships with diatom protein and carbohydrate, but no relationship with lipid content. Overall, only one of seven benthic diatom species acclimated to high-nitrate growth medium displayed significantly elevated protein content. Diatom protein content explained relatively little of the variation in food value of diatom strains for post-larval abalone but lipid content correlated strongly with post-larval survival.