A Toxic Relationship

Phytoplankton blooms are a naturally occurring phenomenon along coastlines worldwide. Large accumulations of certain hazardous phytoplankton species can have detrimental effects on marine wildlife as well as on human health and local economies. Numerous species of dinoflagellates, diatoms and cyanobacteria are known to release harmful ‘phytotoxins’ into the environment during these events, which are commonly referred to as harmful algal blooms (HABs).

 In order to mitigate related health epidemics, coastal waters can be monitored for HAB species along with their toxic exudates. However, identifying the mechanisms which cause HABs to arise in the first place may prove to be a more effective management strategy, perhaps allowing preventative measures to be implemented. There is continuing debate amongst the scientific community as to whether HABs are increasing due to anthropogenic activity or if they are predominantly naturally occurring events that we’ve simply become better at detecting.     

 Whilst the potential causes of HABs have received considerable attention from researchers, until recently, few studies had addressed the possible contributing roles of interactions between phytoplankton and co-occurring bacterioplankton. These interactions are most likely based upon the exchange of nutrients, for example, it has emerged that specific bacteria could modulate algal growth and bloom dynamics via the provision of iron. Iron is vital to all life on Earth, playing key roles in respiration and photosynthesis. Yet, its bioavailability within the marine environment can be poor due to its low solubility in seawater. Consequently, marine bacteria have developed complex systems to generate high-affinity iron-chelating compounds called siderophores to acquire, transform and process this fundamental element. Although iron acquisition by marine bacteria is reasonably well understood, much less is known about iron acquisition from low iron concentration marine environments by phytoplankton. Accordingly, one recent study investigated whether the growth of Lingulodinium polyedrum, a HAB associated dinoflagellate, is affected by the presence of siderophores, a potential bioavailable form of iron provided by bacteria.

 Having previously studied blooms of L. polyedrum in the field, Yarimizu et al., identified that populations of L. polyedrum and bacterial siderophore producers fluctuated in synchrony with one another. Consequently, subsequent lab study endeavoured to confirm a mutualistic relationship between L. polyedrum and the vibrioferrin siderophore producing bacterium Marinobacter algicola DG893, via culture-based techniques. For comparative study, a mutant form of DG893 without siderophore biosynthesis genes was generated.

 As expected, iron was found to be growth limiting to L. polyedrum in axenic culture. Furthermore, in concordance with patterns observed in the field, L. polyedrum growth was enhanced when cultured together with DG893. Although growth was also promoted by the mutant form of DG893, algal growth was most significantly improved by non-mutant DG893, highlighting the importance of siderophore vibrioferrin as a source of iron for the dinoflagellate. Growth enhancement by the mutant DG893 suggests that the bacteria may be offering further benefits to co-occurring algae, such as the provision of vital B vitamins.

 Furthermore, the findings of this study highlight the mutually beneficial nature of the relationship between L. polyedrum and DG893. Without the addition of an artificial carbon source, DG893 was incapable of growth in axenic culture. However, when cultured alongside L. polyedrum, DG893 grew well without requiring the addition of artificial carbon, indicating that the dinoflagellate provided an adequate reservoir dissolved organic carbon.

 Ultimately, Yarimizu et al. have made a noteworthy contribution to our understanding of potential HAB facilitation through effective demonstration of this algae-bacterium mutualism. Nevertheless, more data is needed from the field to understand how such mechanisms may operate in the natural environment. Likewise, investigating how humans may be influencing the dynamics of such mutualisms through processes such as eutrophication and overfishing could yield some interesting results.           

Reviewed Paper:

Yarimizu, K., Cruz-López, R. & Carrano, C. J. (2018). Iron and Harmful Algae Blooms: Potential Algal-Bacterial Mutualism Between Linguldinium polyedrum and Marinobacter algicola. Frontiers in Marine Science, 5: 180