Regulation of acquired metabolic potential by the marine ciliate Mesodinium rubrum
The evolutionary fitness of organisms depends upon their ability to successfully acquire or synthesize metabolic resources for growth and reproduction. These abilities depend first upon the metabolic potential (e.g., photosynthesis, nitrogen fixation, etc.) encoded within each organism’s genome. However, some organisms are able to extend their inherent metabolic potential within their lifetimes through the acquisition of genes and organelles from other species. One example of this acquired metabolic potential (AMP) is acquired photosynthesis, in which otherwise heterotrophic organisms retain ingested chloroplasts as an active photosynthetic source of fixed carbon. Retention time and regulation of these acquired plastids vary among organisms, and may be an indication of their relative importance to organism life history and/or of co-evolutionary history. Here, we present studies of a model case of acquired photosynthesis in the bloom-forming marine ciliate Mesodinium rubrum, an obligate phototroph that depends upon the ingestion of cryptophyte algal prey to maintain its pool of acquired chloroplasts. We tested the extent of the ciliate’s regulatory control of its plastids by quantifying its acclimation to different light levels, and comparing its response to environmental perturbations (i.e., large changes in light intensity) with that of its prey.
Our results indicate that M. rubrum, despite its need to periodically feed to maintain its chloroplasts, nevertheless resembles pure phototrophs in its ability to fine-tune its photosystems by adjusting pigment content and packaging to ambient light levels. Furthermore, we find correspondence between M. rubrum’s light response, and the seasonal environmental conditions it experiences in its natural environment. For example, the minimum light requirement for growth matches in situ measurements of winter low-light conditions. However, differences between M. rubrum and its algal prey in the ability to adjust to abrupt changes in light intensity suggest that M. rubrum’s regulatory control of the plastid is reduced compared to the native algal host. Still, M. rubrum relies on photosynthesis for the majority of its carbon supply, indicating that AMP plays a key role in expanding the niche of this planktonic species.