Laboratory-made particles as a useful approach to analyse microbial processes in marine macroaggregates

Two kinds of particles were created in the laboratory in rolling tanks to simulate natural macroaggregates. Laboratory-made particles type 1 were formed under sterile conditions from a mixture of cells and extracellular products of four diatom species in artificial seawater, and inoculated with a marine microbial assemblage. Laboratory-made particles type 2 were created directly from natural seawater. These particles were characterised in terms of maximum length, volume and abundance. Chemical composition (carbohydrates, amino acids and total organic carbon) and bacterial and flagellate abundances were measured in the particles rind ambient water. We found that both kinds of laboratory-made particles were similar in terms of size, chemical composition and microbial abundance. Moreover, they resembled natural marine aggregates in size and volume. However, laboratory-made particles showed higher concentrations of carbohydrates, amino acids and total organic carbon as well as higher microbial abundance when compared to natural macroaggregates. This difference can be explained by the sampling method, since natural aggregates are frequently collected in the sea with syringes including ambient water, and consequently diluted, whereas in this study the laboratory-made particles were carefully collected without ambient water. Thus, both kinds of laboratory-made particles might be a good alternative for the analysis of microbial processes in marine macroaggregates. Advantages and disadvantages of these two types of laboratory-made particles are discussed.