A bloomin’ good study about dinoflagellates and vitamins

Despite many conditions being conducive in initiating a bloom, nutrients are inextricably linked with bloom dynamics. Phytoplankton utilize many vitamin forms and have different strategies to acquire and remodel vitamin precursors. Dinoflagellates in particular, have the ability to vary modes of energy acquisition. Supporting this interaction, bacteria can facilitate the growth of phytoplankton either through remineralising or producing vitamins. Gong et al., aimed to characterize the molecular underpinnings of these interactions. Whilst the paper detailed many interesting findings, I will focus on the findings regarding the interaction between dinoflagellates, vitamins and bacteria.

Briefly, Gong et al., used transcriptomic sequencing data collated throughout the year at sites along an estuary to compare to an opportunistically sampled dinoflagellate bloom. The occurrence of the bloom along a well characterized coast with comparable non bloom sites clearly provides an excellent model study system. Gong et al., thoroughly defined the bloom through analysis of accessory pigments, cell density of dinoflagellates and transcript data. They also used multiannual data of the physiochemical parameters in order to assess the conditions conducive to a bloom. All transcript sequences were annotated with taxonomic and metabolic functions and dinoflagellate gene expression were compared between sites to identify the cellular response of blooming dinoflagellates. The results of the study revealed expected differences in growth related metabolic pathways representative of increased energy production, metabolism and synthesis of cellular machinery.

However, more significantly the authors were able to infer the different roles of dinoflagellates, vitamins and bacteria. Dinoflagellates require B vitamins, including; B12 (cobalamin), B1 (thiamine) and B7 (biotin). For auxotrophic dinoflagellates, bacteria can act as necessary intermediates in their external acquisition of vitamins in contrast to autotrophic species which have the ability to biosynthesize vitamins. Dinoflagellates are known to biosynthesize biotin and thiamine. Indeed, three out of the four enzymes needed as the precursor for the biosynthesis of biotin were overrepresented in the bloom site corresponding to higher metabolic rates. Similarly, the genes for thiamine synthesis were overrepresented in the bloom sample. In both cases, genes encoding for essential enzymes involved in vitamin synthesis were not found in the samples suggesting an incomplete understanding of these pathways. Nevertheless, the results of this study point to an increased production of biotin and thiamine in the bloom sites. An unexpected result was the presence of cobalamin synthesis related genes in the bloom, as it was previously thought that dinoflagellates were unable to synthesize cobalamin. These findings lead the authors to conclude that dinoflagellates can partially biosynthesize B12 depending on the acquisition of B12 intermediates produced by bacteria. This is a significant finding as it provides evidence for the importance of remodeling strategies in phytoplankton blooms.

When vitamins are limited, auxotrophic phytoplankton secrete polysaccharides forming sticky aggregates. Supporting this, polysaccharide synthesis genes were overrepresented in the bloom sample. Bacteria use these polysaccharides as a carbon source to proliferate, and this was confirmed as the production and transportation of simple sugars was overrepresented in the bloom site. This demonstrates the use mechanisms in phytoplankton to facilitate bacterial interaction. This is advantageous for a bloom; as bacterial aggregates are known to be better at remineralizing organic materials required for the synthesis of vitamins. Additionally, the increased production of thiamine and biotin, would further stimulate bacterial growth, as microbes cannot produce these vitamins.

This study provides an extensive mechanistic analysis of a bloom, highlighting the flexible nature of vitamin metabolism in dinoflagellates. The consistent and clear biological inferences incorporated within these molecular findings makes it a very coherent paper. Some uncertainties in some of the findings demonstrates there are still significant gaps in our understanding of vitamin metabolism. The next step is to integrate the use of protonomics, to study specifically the protein response in dinoflagellates during a bloom, and perhaps, although I am aware of the caveats involved in measuring ambient nutrient levels, possibly testing how the levels of vitamins regulates the interaction between bacteria and dinoflagellates.

Reviewed paper:

Gong, W., Browne, J., Hall, N., Schruth, D., Paerl, H., & Marchetti, A. (2016). Molecular insights into a dinoflagellate bloom. The ISME Journal, 11(2), 439-452. http://dx.doi.org/10.1038/ismej.2016.129