More mixotrophy in the marine microbial mix.

Since the turn of the century, the paradigm of the marine microbial assemblage being composed primarily of photoautotrophs and chemoheterotrophs is shifting and challenging our understanding of how the microbial diversity contributes to the flow of energy, carbon, and other nutrients in the oceans. In PNAS, Muñoz-Marín et al. (1) add another piece to the puzzle by demonstrating that natural populations of Prochlorococcus, the smallest and most abundant oxygenic phototroph in the oceans, can take up glucose in nanomolar amounts and has a gene (Pro1404) that codes for a sugar transporter. Mixotrophy, a combination of modes by which an organism can obtain its energy and carbon, such as the ability to switch between photoautotrophy and chemoheterotrophy, is now being recognized as a larger component of the open ocean microbial assemblage (2, 3). One important discovery was the existence of planktonic marine bacteria containing proteorhodopsin photoreceptors that are used for their lightharvesting pigments but take up organic carbon (C) for their C needs (4). These mixotrophic bacteria make up a significant percentage (up to 11%) of the microbial community in the euphotic zone (5, 6). A second discovery was that aerobic anoxygenic phototrophic bacteria (AAnPB) that can grow photoheterotrophically are not limited to benthic or estuarine ecosystems but can account for up to 20% of the bacterial assemblage in the euphotic zone (7). Thirdly, Prochlorococcus was found to assimilate amino acids and dimethylsulfoniopropionate to supplement their nitrogen (N) and sulfur (S) needs, as well as save energy from not biosynthesizing amino acids (8–10). This uptake of Nand S-containing organic compounds has been referred to as facultative photoheterotrophy, i.e., the ability to carry out chemoheterotrophy instead of strict photoautotrophy (11). However, there was no evidence that Prochlorococcus was actually obtaining energy or C from the organic compounds, like many freshwater and filamentous cyanobacteria (12), primarily because no one had tested Prochlorococcus for the uptake of organic C compounds that were lacking other essential nutrients like N or S. Therefore, it was very exciting when Gómez-Baena et al. (13) demonstrated that Prochlorococcus cells in culture can actively take up radioactively labeled [C]glucose and that this uptake was not attributable