Trade-offs of lipid remodeling in a marine predator-prey interaction in response to phosphorus limitation.

Abstract

Phosphorus (P) is a key nutrient limiting bacterial growth and primary production in the oceans. Unsurprisingly, marine microbes have evolved sophisticated strategies to adapt to P limitation, one of which involves the remodeling of membrane lipids by replacing phospholipids with non-P-containing surrogate lipids. This strategy is adopted by both cosmopolitan marine phytoplankton and heterotrophic bacteria and serves to reduce the cellular P quota. However, little, if anything, is known of the biological consequences of lipid remodeling. Here, using the marine bacterium <i>Phaeobacter</i> sp. MED193 and the ciliate <i>Uronema marinum</i> as a model, we sought to assess the effect of remodeling on bacteria-protist interactions. We discovered an important trade-off between either escape from ingestion or resistance to digestion. Thus, <i>Phaeobacter</i> grown under P-replete conditions was readily ingested by <i>Uronema</i>, but not easily digested, supporting only limited predator growth. In contrast, following membrane lipid remodeling in response to P depletion, <i>Phaeobacter</i> was less likely to be captured by <i>Uronema</i>, thanks to the reduced expression of mannosylated glycoconjugates. However, once ingested, membrane-remodeled cells were unable to prevent phagosome acidification, became more susceptible to digestion, and, as such, allowed rapid growth of the ciliate predator. This trade-off between adapting to a P-limited environment and susceptibility to protist grazing suggests the more efficient removal of low-P prey that potentially has important implications for the functioning of the marine microbial food web in terms of trophic energy transfer and nutrient export efficiency.