Similarities in trends between the Arctic Shelf and Reef; the viral story

My previous blog concentrated on two papers addressing whether abiotic or biotic variables drove viral communities shifts in in the highly productive Arctic waters. To contrast, this review looks at trends in viral abundance and lytic activity in an oligotrophic coral reef system to see whether any similarities can be drawn.

The author examined the spatial and short-term temporal changes in viral abundance and lytic activity, in relation to changes in the microbial community and environmental conditions.

To understand the spatial trend, seawater was collected from four locations (North, East, South, West) from the Pacific Island, Moorea, as well as from five reef habitats (crest, back reef, lagoon, fringing reef, fore reef; furthest from land to closest).

A short-term temporal comparison took place in Opunohu Bay over 21 days, where samples were collected every 2-3days.

Water samples were also analysed for abiotic variables: dissolved inorganic nitrogen (DIN) and soluble reactive phosphate (SRP). Seawater was collected for nutrient, microbe and viral enumeration (0.5L) and for lytic viral production assays (4L).

Flow cytometry was used to enumerate viruses, bacteria and phytoplankton. They found there was three subgroups of viruses (V1, V2, V3), two subsets of bacteria high and low nucleic acid (HNA, LNA) and four phytoplankton subsets (Prochlorococcus, Synechoccus, autotrophic picoplankton, autotrophic nanoplankton).

The lytic production assay used a dilution technique, allowing for the measurement of only new viral particle production. This is a relatively new technique based on Wilhem et al (2002). Developed because prior techniques on viral production were subject to large errors and a number of conversion factors. The dilution technique reduces the background viruses so that the small changes in viral concentration can easily be picked up.

The author found that there was large spatial variation viral abundance and lytic activity, but low temporal variation. Previous research by this author (my previous post) found that viral abundance was closely linked to the host abundance rather than environmental conditions. Therefore my thoughts are that over the short-term temporal study perhaps the host community was constant.

The viral abundance was lowest in the back reef and lagoon, showing 2.2 and 2.4 fold higher abundance in the fringing and crest, respectively.  The viral production and turn over was also highest in the fringing and lowest in the back reef.  They suggest that viral lysis might be an important contributor to dissolve organic carbon, providing 2-15% for reef waters. This would be particularly important for the back reef, which is exceptionally nutrient depleted, and this dissolved organic carbon may be what sustains the small bacterial community.

Viral abundance wasn’t significantly different between time points, and temporal changes in viral production and viral turn over were explained by both biotic and abiotic factors (82%; 15% respectively). This shows the strong relationship between lytic production and changes in primary production.

There was no significant fluctuation in the nutrient concentration between space or time. This could explain the limited viral variation temporally, as similar nutrient concentration would support a similar host community (bearing in mind it is the same environment). The similarity between sites, I would have expected, as it is all an oligotrophic coral reef, and I would assume they have avoided particular upwelling areas (if present) and sampled at similar depths.

Bacterial abundance did significantly differ between reef habitats. The abundance decreased 3-fold from the fringing reef to the crest. The author suggests the highest abundance might be due to a micro-gradient in nutrient concentration, released by benthic organisms, as DIN was 1.2-fold higher at the fringing reef. The trend in LNA and HNA was similar to bacterial abundance. This is consistent with previous research that indicates HNA are closely associated with corals and LNA (such as SAR11) are more abundant in nutrient depleted water.

Phytoplankton abundance was not significantly different between habitats, but showed some spatial variation. The abundance also decreased from fringing to the crest (although a 5-fold increase between the fore reef and fringing), which the author suggests it’s likely due to higher benthic filter-feeder encounter rate the further you get from land.

Bacterial, phytoplankton, and viral abundance was highest in the Opunohu Bay (where the temporal study took place) the author suggest this is due to more suspended organic matter, caused by near-bottom currents, and terrestrial inputs from the river.

I think this study could have benefited from having more abiotic variables such as temperature or salinity to clearly show abiotic trends rather than have an abiotic factor, which is nested within biotic, as nutrient substantially influences the host (phytoplankton and bacterial) communities. I think this paper further supports my understanding, that viral abundance and activity is correlated to their hosts rather than environmental conditions.

Reference:

Payet, J. P., McMinds, R., Burkepile, D. E., & Thurber, R. L. V. (2014). Unprecedented evidence for high viral abundance and lytic activity in coral reef waters of the South Pacific Ocean. Frontiers in microbiology, 5.