Phytoplankton, bacterioplankton and virioplankton across the Great Barrier Reef

Microbes are very important in processing carbon and organic matter in the sea. Factors regulating bacterial growth rates and productivity have been widely studied for temperate coastal areas, the open ocean and tropical convergence zones and gyres, however, little is known about what drives phytoplankton and bacterioplankton growth and productivity in tropical shelf waters. Bacterioplankton play a critical role in coral reef and reef-associated waters but it is not known if bacterioplankton dynamics in coral reef and non-coral reef waters are controlled by similar factors.

Alongi et  al. (2015) took various measurements from 14 stations across the southern GBR shelf to test the hypothesis that phytoplankton, bacterioplankton and virioplankton communities will show two peaks in abundance. One inshore due to turbulent mixing of terrestrially-derived nutrients and the other offshore within the channels of the Pompey Reef complex due to strong vertical mixing and Resuspension of benthic material.

The rates of bacterioplankton and phytoplankton productivity and the bacterioplankton community structure and diversity data indicate a difference between the shallow inshore and the remainder of the shelf. The ratio of bacterial and primary production and the percentage of pelagic N and P demands derived from the benthos show a peak in activity both inshore and in the Pompey Reef Complex.

Other patterns emerged as well, such as differences in microbial community structure and function across the shelf between dry and wet seasons. Pelagibacter communities dominate across the shelf in the wet season but during the dry season differences were observed in relative abundance of Pelagibacter at inshore and mid-lagoon sites.  This suggests that the dominance of Pelagibacter is controlled through seasonal river run-off. This could be due to an increase in nutrients and organic compounds to the shelf. NifH-based data indicate that all inshore and mid-lagoon communities were dominated by sequences affiliated with other eutrophic systems, such as the South China Sea; the communities within the reef channels possessed sequences common to other reefal diazotrophs; and the shelf-break flora dominated by oceanic cyanobacteria.

Principal component analysis shows a statistical correlation between bacterioplankton and phytoplankton as well as between bacterioplankton and virioplankton.  Virioplankton abundance was positively correlated with bacterial numbers and production, implying a significant impact on bacterial dynamics through viral infection. As it can be seen there is a tight regulatory mechanism between the different groups. Furthermore, benthic matter and some benthic organisms are suspended in the water column resulting in a tight coupling between benthic and pelagic cycles. Similarly to the across-shelf patterns, the benthic contribution peaked both inshore and in the proximity of the Pompey Reef channels.

This research shows how microbial communities can be regulated by seasonal change and the influx of terrestrial nutrients can influence the shelf community structure. The interlinked nature of different microbial communities and at different demonstrates a clear example of the microbial loop process.  Although this study allows us a further insight into coral reef dynamics, it only focuses on a small portion of the GBR. To develop a better understanding of these dynamics on a larger scale as well as the accuracy of these finding, more studies will need to be conducted across tropical reefs. Like the authors were able to like similarities in their findings to the South China Sea, this will allow more studies to be brought together to further compare the similarities and differences of coral reefs and associated waters and develop a more generalised view. 

Alongi, D. M., Patten, N. L., McKinnon, D., Köstner, N., Bourne, D. G., & Brinkman, R. (2014). Phytoplankton, bacterioplankton and virioplankton structure and function across the southern Great Barrier Reef shelf. Journal of Marine Systems 142, 25-39.

http://www.sciencedirect.com/science/article/pii/S0924796314002309