Mesodinium rubrum: the phytoplankter that wasn't

Recent reports suggest that the potential phototrophic role of plastidic ciliates in marine ecosystems may be considerable. A critical review of the literature demonstrates some confusion surrounding acceptance of the trophic position of even a well-established example of a photosynthetic c~liate, Mesodinium rubrum. Despite good evidence of obligate phototrophy from bloom studies, this species has, until recently, been omitted from the majority of routine phytoplankton counts, and has either been assigned to the microzooplankton or completely overlooked. Moreover, problems involved with sampling, enumeration and estimates of productivity for M. rubrum are also highlighted from the literature. These principally result from extremes of fragility, motility and vertical aggregation, which are commonly noted for this ciliate. Several recent studies, which have minimized some of these sampling problems and grouped the microplankton into more meaningful ecological categories, suggest that M. rubrum has an extremely widespread distribution and can be a very significant member of the phytoplankton. The combination of trophic and methodological difficulties appear to have compounded a serious underestimation of the contribution of M. rubrum to the primary productivity of coastal, estuarine and upwelling ecosystems, during both bloom, and perhaps more significantly, non-bloom conditions.