In nature diatoms as well as other planktonic algal species exist as mixed communities. This makes it difficult to examine interactions between algae cells and bacterial assemblages that surround them. The extent of these associations has also not been established between algae and bacteria at the species level. Previous studies have observed that these algal-bacterial interactions appear to be symbiotic. They can be either commensal or parasitic depending on the environmental conditions and the individuals involved. For example, while dissolved organic matter may be present in the environment for use by heterotrophic bacteria, they are also able to use algal derived carbon sources to meet their need for carbon. Algal derived DOM may differ by species and might also be provided more consistently than what is available in the environment. Therefore, there may be a selective advantage for bacteria to use these algal derived carbon sources with preference to carbon exudates from certain algal species as opposed to others. At the same time bacterial cells may be able to provide certain nutrients that are required for algal growth and development. In this way algae and bacteria are able to coexist in away where one is able to provide certain things to the other. However, when resources become limiting bacteria may outcompete algae for limiting nutrients in the environment, while algae continue to provide carbon to bacterial cells.
This investigation aimed to find whether algal cells could serve as a niche for specific bacterial species. It was done using uni-algal diatom cultures and commensal bacteria they may have. The idea is that these diatoms can harbor distinct bacterial assemblages that have been selected to grow in association with specific algal hosts. As the forms of labile DOM released by algal cells differ between diatom species and would allow for growth of bacteria assemblages better adapted using it. This would also provide direct evidence of a form of co evolution where the identity of bacterial community composition is dependent on the identity of the algal cells that they surround.
All diatom cultures contained at least one type of CFB or α-Proteobacteria. As individuals from these groups may be better adapted to live in association with diatoms. Schafer was able to show that each of the non-axenic uni-algal diatom cultures had unique satellite bacteria that were high in species richness. This diversity was reproducible in duplicate cultures and across time (remaining fairly stable), with small changes only observed in the abundance of bacterial species. However, he was unable to conclude whether the bacterial assemblages observed for each diatom species would persist with influx of bacterial cells from other algal cell cultures. This would have given a greater insight into the extent of these algal-bacterial linkages, as bacterial species not adapted efficiently use the carbon released from algal cells would have a hard time changing the composition of the better adapted bacteria already associated with that algal species.
The discussion itself focused around whether this algal-bacterial association observed in this study in uni-algal cultures was enough to suggest that these two have co evolved. Part of the problem to answering this question is that this relationship involves a single algal cell and a diverse group of bacterial cells (rather than one individual). It may suggest a certain flexibilily in the bacterial cells themselves, where they may be able to adapt to different algal derived DOM and their composition may be more dependent on the ability of bacteria to compete with each other for these resources than the kinds of exudates released from algal cells. While in nature we have examples where certain associations can be very specific and the survival success of one individual is dependent on the other, such as humming birds that have co evolved with a specific set of flower types that they pollinate. This may not be the case of the association of diatom species with distinct bacterial assemblages. How strong do these linkages have to be in order to consider these associations a product of co-evolution? What more needs to be learned before the assumption of co-evolution could be accepted?