Co-feeding of microalgae and bacteria may result in increased N assimilation in Artemia as compared to mono-diets, as demonstrated by a 15N isotope uptake laboratory study

• Using 15N stable isotope to study on food assimilation in Artemia.
• The N assimilation from bacteria was improved when co-feeding with microalgae.
• The N assimilation from microalgae was improved when co-feeding with bacteria.

This study investigated the effect of the co-feeding of bacteria and microalgae on nitrogen (N) assimilation in an Artemia franciscana gnotobiotic laboratory culture test. Two strains of bacteria were used, HT3 and HT6, isolated from previous Artemia laboratory cultures. These were fed in combination with microalga Dunaliella tertiolecta strains, either the high quality DT 19/6B or the low quality DT 19/27 strain. Each combination of algae and bacteria was offered in different proportions, i.e. 10/90, 50/50 and 90/10% on a dry weight basis, while the total amount of food supplied, which was based on a reference algae mono-diet, was kept constant. Mono-diets consisting of 100% algae and 100% bacteria were added as controls. N assimilation from either food source was determined by feeding in separate tests 15N labeled microalgae with non-labeled bacteria and vice versa. Axenically hatched Artemia nauplii were fed these diets for 24 h, after which they were analyzed for 15N content. The results of 15N analysis showed that the N assimilation from bacteria in Artemia was improved when the bacteria were 10% replaced by microalgae, and for the combination of DT 19/6B and HT6 this increase was significant (P < 0.05). Also 50% replacement of HT6 by DT 19/6B resulted in higher, but non-significant, N assimilation from the bacteria as compared to the 100% bacteria mono-diet. A similar non-significant increase of N assimilation from DT 19/6B was observed as compared to the 100% algae diet when these algae were up to 50% replaced by HT6 bacteria. Other combinations of bacterial and microalgal strains generally produced similar or lower N assimilation as compared to the labeled mono-diet, when the labeled food component (bacteria or algae) was gradually replaced by the other (algae or bacteria). This study shows that, provided suitable strains are used, offering microalgae and bacteria together as food for Artemia may result in a synergistic effect in how this filter-feeding organism takes advantage of the respective food sources. This Artemia study can also be considered as a model for future research on the feeding biology of other filter-feeding aquaculture organisms.