Viral lysis of algae changes bacterial communities

The microbial loop is overrun with viruses which are credited as being powerful determinants of planktonic algal and bacterial dynamics. Viral lysis releases dissolved organic carbon and nitrogen into the ‘viral shunt’, which drives many microbial and biogeochemical ocean processes.

The algae Phaeocystis globosa can form great blooms of which viral lysis can cause up to 66 % mortality. Evidence from mesocosm studies indicate that such mass lysis events can shape local bacterioplankton communities. Increases in the abundances of Alteromonas and Roseobacter Bacteria have been observed during blooms.

Viral lysis is thought to provide more organic aggregates to these bacteria, enhancing their growth. However it is suspected that bacteriophage lysis increases with bacterial densities, leading to aggregate dissolution in the long term. Aggregate-associated bacteria are typically copiotrophic, with higher enzyme activity and growth rates than their planktonic counterparts.

How the viral shunt contributes to bacterial community shifts and biogeochemical processes is poorly understood. This study examines how carbon and nitrogen released from viral lysis of P. globosa enters the bacterioplankton and subsequently alters community structure. The main techniques used were isotopic labelling, catalysed reporter deposition-fluorescent in situ hybridisation (CARD-FISH) and pyrosequencing to measure bacterial community composition and diversity. Atomic force microscopy (AFM), nanometre-scale secondary ion mass spectrometry (nanoSIMS) and isotope ratio mass spectrometry (IRMS) were used to record aggregate formation and substrate assimilation.

How viral lysis shaped bacterial communities.

The pattern of change was the rapid growth of copiotrophic opportunists from Alpha and Gammaproteobacteria, namely Alteromonas and Roseobacter. Control communities with uninfected algae were instead dominated by Alphaproteobacteria and Bacteroidetes with slower trophic strategies. Alteromonas displayed a very rapid growth increase in viral lysis conditions, but Roseobacter had a slower, steadier response. One Alteromonas phylotype was dominant, whilst controls were diverse in Gammaproteobacteria phylotypes. In contrast, viral lysis increased the diversity of Alphaproteobacteria phylotypes. This shows that viral lysis can create conditions which are highly selective of a handful of bacterial types. Interestingly, though DMS concentrations were not measured, DMS metabolising Methylophaga (Rich Boden’s favourite) rose in abundance when DMS release was expected to peak. Anyway, the increased formation of aggregates by viral lysis likely drove the success of Alteromonas and Roseobacter. However their abundances began to drop quickly in later stages, in correlation with increasing bacteriophage densities and aggregate dissolution. This supports the idea that bacteriophagic lysis actually leads to less organic aggregation in the long term; I wonder if aggregate dissolution and bacteriophagic lysis have a combined effect which synergistically leads to very rapid bacterial mortality.

Carbon and nitrogen flow from viral lysis

Alteromonas cells had already assimilated labelled nitrogen and carbon from P. globosa cells before they had lysed. This leaking of organics has not been seen before and the ecological implications during blooms require further study. Viral lysis vastly increased bacterial conversion of released algal molecules into the particulate organic matter. Alteromonas is a generalist opportunist able to metabolise a wide molecular weight range of organics, whereas Roseobacter is more specialised towards amino acids and algal osmolytes, which explains why Alteromonas was better at taking advantage of viral lysis.

The identity of the bacterial phages involved should be investigated, as well as the secondary community shifts resulting from mass bacterial lysis. I imagine there is a complex lytic cascade following a bloom, knocking over huge ecological dominoes. If infected pre-lytic algae leak organic molecules, then do infected bacteria?

Sheik, A. R., Brussaard, C. P., Lavik, G., Lam, P., Musat, N., Krupke, A., ... & Kuypers, M. M. (2013). Responses of the coastal bacterial community to viral infection of the algae Phaeocystis globosa. The ISME journal.