Interpretation of nutrient-enrichment bioassays by looking at sub-populations in a marine bacterial community

Abstract

Nutrient-enrichment bioassays represent one frequently applied way of assessing the nutrient status of natural bacterial communities. Interpretation of these is normally based on gross community responses like total bacterial abundance or production. To add more resolution, we supplemented these gross community measures with bacterial community analysis using DGGE and FCM. Sub-samples from a fjord on the Norwegian west coast were incubated in triplicate assay bottles spiked with organic C (glucose), mineral N (NH 4Cl) and P (KH2PO4) in all 8 possible combinations. Both 24 and 72 h after nutrient addition, 3H-leucine incorporation in all cultures enriched with organic C (alone or in combinations with N and P) was higher than for all non-C treatments, consistent with an interpretation of a C-limited bacterial community in the original sample. FCM demonstrated an increase in the high-DNA fraction of the bacterial community, indicating either a growth of these as a subpopulation, or a shift from low to high DNA of bacteria activated by the added glucose. DGGE analysis revealed different band patterns in C-enriched cultures compared to non-C treatments, indicating that only one or a few sub-populations responded with a higher growth rate to the C substrate given. The conclusion from gross community responses should therefore be interpreted only with care as indicating C-limitation for all sub-populations in the bacterial community.