Bacteria inhabiting particulate matter along a temporal series in the Northwestern Mediterranean

Abstract

Marine microorganisms can be classified into 2 lifestyles depending on their relation with the particulate matter present in the environment: Free-living or Attached. Attribution to one of these categories is generally done by differential filtration, i.e. passing seawater through a filter. We tested whether instead of using just one filter, a serial filtration through various filter sizes would provide a better description of the bacterial communities attached to particles. We performed a 2-year study in a Mediterranean Sea coastal site (the Blanes Bay Microbial Observatory). Monthly, 10 L of surface water were filtered sequentially through a total of 6 polycarbonate filters of pore sizes: 0.2, 0.8, 3, 5, 10 and 20 µm. We 454 pyrosequenced the 16S rDNA and we processed the data with QIIME and Mothur. We found large variation in the total number of OTUs (richness) between the six fractions and also between the same fractions when comparing different months. However, two trends were clear: an increase in the richness when increasing the size fraction, and an annual oscillation in richness, with a minimum in late spring and a maximum in winter. More than 50% of the OTUs were not shared between fractions, suggesting that the six size fractions contain different types of particles that offer distinct microniches for the microorganisms and contribute highly in the differentiation of community composition between filters. Our results demonstrate that particle sizes play a key role in driving changes in bacterial community composition throughout a seasonal cycle