Who are the ubiquitous, biomass-significant red fluorescent picoplanktonic cells in temperate and polar surface oceans?

The aim of the proposal is to identify the cells comprising the extra red fluorescence group of large bacterioplankton and to ascertain their metabolic potential. We propose to test a hypothesis that these red fluorescent cells are aerobic anoxygenic phototrophic bacterioplankton (AAPs) containing bacteriochlorophylls. Previous studies show that AAPs lack autotrophic inorganic carbon assimilation pathways and are incapable of photoautotrophy, relying on heterotrophy for the majority of their cellular energetics. However, our provisional measurements of 3H-leucine and 35S-methionine uptake by the red fluorescent cells suggested that the uptake rates of both amino acids by these cells, although being higher than the uptake rates by Synechococcus phototrophic cyanobacteria, are considerably lower than the amino acid uptake rates by mean heterotrophic bacterioplankton cells. Therefore, the red fluorescent cells probably harvest light to photochemically oxidise organic molecules in order to sustain their sizeable ubiquitous population in the photic temperate and polar ocean. The project proposes (i) to phylogenetically affiliate cells, flow cytometrically sorted from the extra red fluorescence group of large bacterioplankton; (ii) to design and test specific oligonucleotide probes for identification of flow sorted cells; to examine the presence of bacteriochlorophyll genes in flow sorted cells; (iii) to examine latitudinal variations in phylogenetic composition of the red fluorescence group. In order to meet the above objectives we will focus on molecular analysis of samples collected in the Atlantic Ocean, where these microorganisms were first observed. Our ultimate goal is to explain their ecological role in oceanic ecosystems.