Are zeolites capable of modifying the yield of marine micro-algae cultures? A case study with Emiliania huxleyi and a product of zeolitic nature

Growth of the marine alga, Emiliania huxleyi, and chemical changes in the culture medium were assessed in response to the addition of differing concentrations of ZESTEC-56, a product of zeolitic nature (PZN). E. huxleyi was inoculated in natural seawater containing 0.025, 0.050 and 0.10 g l-1 PZN. In addition, algae were also inoculated in a PZN elutriate (1.0 g l-1 PZN in seawater stirred for 30 min). Total concentrations of dissolved trace metals and organic ligands (and respective conditional stability constants) were determined, in all media including the controls, by anodic and cathodic striping voltammetry in the initial medium and after 7-day algal growth. When compared with the control culture, 0.025 and 0.050 g l-1 PZN in situ with E. huxleyi did not significantly change the micro-algae growth yield. The highest PZN amount (0.10 g l-1) caused a decrease both in growth rate and yield. In the PZN elutriate culture cell yield was significantly greater than the controls. The PZN caused changes in the total concentrations of dissolved metallic micro-nutrients, adsorbing some (e.g. Zn) and releasing others (e.g. Mn). The ion-exchange processes seem to be relatively slow, as the extent of changes in the elutriate culture, where the contact between PZN and medium was only 30 min, was much lower than that caused by in situ PZN for 7 days. In addition, the amount of exudates released per cell was also influenced by the PZN probably due to the changes in the available trace elements. This work indicated that the changes caused by zeolites in a culture medium, in terms of micro-nutrient available levels can influence the micro-algae yield in natural seawater cultures. Hence, it follows that the characteristics and concentration of the zeolite should be considered when one wishes to use it in a given aquacultural medium, for instance for stimulation of the bacterium-mediated decomposition of non-consumed fish feed at reduced levels of ammonia. Depending on the seawater trace element composition, characteristics and concentration of the zeolite and algal species, the presence of a suitable zeolite may improve the yield of the cultures, reducing the production costs. Otherwise, an inhibition of growth may occur, which is economically disadvantageous.