Role of viruses in a changing climate

As professor William Wilson highlighted in his lecture, the role of viruses within marine systems is commonly overlooked. Phytoplankton are known key drivers of global processes such as primary production and biogeochemical cycling. Marine coccolithophores, such as Emiliania huxleyi, are enclosed by CaCO₃ plates called coccoliths. The sinking and dissolution of these plates results in the sequestration of dissolved inorganic carbon, known as the carbonate pump. Coccolithophores form mesoscale blooms which are characterized by an exponential increase in abundance followed by a crash, initiated by lytic viruses. Despite being well studied, there are considerable discrepancies within the literature regarding the effect of ocean acidification on algal bloom dynamics. Some studies have found that reduced pH levels result in increased calcite production whereas others have reported reduced calcification, and some have even suggested no effect. The effects of decreased pH on the viruses of these algae and their interaction with their host are yet not known and may indeed contribute to this varied response of coccolithophores to ocean acidification. Accordingly, Highfield et al. (2017) investigated the impacts of elevated pCO₂ levels (760 pmv) on the structure and diversity of both the algal host, E. huxleyi and its viruses (EhVs), through a mesocosm experiment conducted in a Norwegian fjord.

The progression of the bloom was monitored through chlorophyll a concentration (flurorometrically determined), pigment analysis (High Pressure Liquid Chromatography) and cell counts of E. huxleyi and EhVs (flow cytometry). Carbon fixation was used as a proxy for primary production and was determined using ¹⁴C measurements of surface water samples. To determine genetic diversity, Polymerase Chain Reaction (PCR) and Denaturing Gradient Gel Electrophoresis (DGGE) analyses were carried out using primers specific to the calcium binding protein gene (gpa) for E. huxleyi and the major capsid protein gene (mcp) for EhV.

The results of the study suggest a negative effect of ocean acidification in this instance as a lower pH was found to significantly reduce phytoplankton biomass and primary production of an E. huxleyi dominated bloom. Within the elevated treatment, they found high variability between replicates, with significant differences in the abundance of coccolithophores. Despite this, the genetic diversity of the E. huxleyi remained stable across the replicates, treatments and the duration of the experiment. The authors therefore posit that viral infection by EhV may be driving these differences in abundances. Notably, they found that elevated pCO₂ levels can affect both the composition and diversity of EhV as in contrast to their algal host they did not stabilise through the succession of the bloom and were not consistent across replicates. Environmental stress may affect EhVs host ranges and infection characteristics (burst size and latent period) and may have caused a change in virus genotype resulting in a change in viral community composition.

This study really demonstrates the importance of including viruses when studying marine systems, particularly in the context of climate change. Indeed, the authors provide evidence that suggests a potential knock-down effect of increased pCO2 on E. huxleyi driven by viral responses to acidified conditions. Despite this, it is still not clear whether the observed response is due to the environmental conditions affecting viruses directly or their interaction with the host. Future experiments should aim to better dissect the host and viral responses, in order to accurately predict their ecological interactions in future ocean acidification scenarios.

Referenced paper:

Highfield, A., Joint, I., Gilbert, J., Crawfurd, K., & Schroeder, D. (2017). Change in Emiliania huxleyi Virus Assemblage Diversity but Not in Host Genetic Composition during an Ocean Acidification Mesocosm Experiment. Viruses, 9(3), 41. http://dx.doi.org/10.3390/v9030041