Responses of the coastal bacterial community to viral infection of the algae Phaeocystis globosa

Abstract

The release of organic material upon algal cell lyses has a key role in structuring bacterial communities and affects the cycling of biolimiting elements in the marine environment. Here we show that already before cell lysis the leakage or excretion of organic matter by infected yet intact algal cells shaped North Sea bacterial community composition and enhanced bacterial substrate assimilation. Infected algal cultures of Phaeocystis globosa grown in coastal North Sea water contained gamma- and alphaproteobacterial phylotypes that were distinct from those in the non-infected control cultures 5 h after infection. The gammaproteobacterial population at this time mainly consisted of Alteromonas sp. cells that were attached to the infected but still intact host cells. Nano-scale secondary-ion mass spectrometry (nanoSIMS) showed ∼20% transfer of organic matter derived from the infected (13)C- and (15)N-labelled P. globosa cells to Alteromonas sp. cells. Subsequent, viral lysis of P. globosa resulted in the formation of aggregates that were densely colonised by bacteria. Aggregate dissolution was observed after 2 days, which we attribute to bacteriophage-induced lysis of the attached bacteria. Isotope mass spectrometry analysis showed that 40% of the particulate (13)C-organic carbon from the infected P. globosa culture was remineralized to dissolved inorganic carbon after 7 days. These findings reveal a novel role of viruses in the leakage or excretion of algal biomass upon infection, which provides an additional ecological niche for specific bacterial populations and potentially redirects carbon availability.

Figure 1

Changes in microbial and viral abundances in response to viral infection of P. globosa. (a) algal abundance, (b) P. globosa virus (PgV-07T) abundance (c) total heterotrophic bacterial abundance and (d) bacteriophage. Closed symbols represent non-infected control and open symbols represent the virally infected cultures. Error bars indicate standard error of mean from duplicate batch cultures (s.e.m.).

Figure 2

The abundance of major bacterial groups of Alphaproteobacteria, Gammaproteobacteria and the Bacteroidetes (a, b) as determined by CARD-FISH analyses due to viral lysis of P. globosa relative to the non-infected control cultures. Within specific taxonomic groups, P. globosa viral lysis led to rapid changes in the abundance of Alteromonas (Gammaproteobacteria) followed by Roseobacter (Alphaproteobacteria) cells (c, d). Please note the different y-axis scale in the upper and bottom panels. The inlay in the bottom panel (c, d) shows the changes in the abundance of Alteromonas and Roseobacter cells for the first 12 h of the experiment. An initial doubling in the abundance of Alteromonas cells at 5 h post infection is indicated by an arrow (d). Error bars indicate s.e.m.

Figure 3

Temporal changes in the bacterial community composition of phylotypes belonging to Gamma- and Alphaproteobacteria during and after viral infection of P. globosa relative to non-infected control cultures.

Figure 4

Changes in particulate organic (a) ¹⁵N-nitrogen (¹⁵N-PON), (b) ¹³C-carbon (¹³C-POC) and (c) carbon remineralization due to viral-mediated shunt. Closed symbols represent non-infected control and open symbols represent the viral-infected cultures. Error bars indicate s.e.m.

Figure 5

NanoSIMS imaging visualising the transfer of algal biomass towards Alteromonas cells in non-infected control cultures (upper panel) and infected P. globosa cultures (lower panel) at day 2 of the experiment. First column (a, e) illustrates the CARD-FISH image taken before nanoSIMS analyses. The corresponding CARD-FISH hybridised cells were located by the ¹⁹F signal during nanoSIMS (b, f) and their respective ¹³C (c, g) and ¹⁵N enrichments (d, h). Scale bar=2 μm.

Figure 6

The ¹³C and ¹⁵N enrichments as deduced from nanoSIMS analyses indicate the transfer of isotopically labelled biomass to Alteromonas (a, c) and Roseobacter (b, d) cells in both non-infected control and infected P. globosa cultures. The ¹³C and ¹⁵N enrichments of Roseobacter cells at 5 h were not determined due to their low cell abundance.

Figure 7

Conceptual diagram illustrating the observed temporal microbial regulation and associated biogeochemical processes due to P. globosa viral lysis using Alteromonas CARD-FISH images as an example. Viral infection of P. globosa cells led to leakage or excretion of algal organic matter, stimulating substrate assimilation by Alteromonas cells and also triggered its attachment to the infected algal host (stage 1). Viral lysis of P. globosa single cells resulted in the formation of aggregates that were colonised mostly with Alteromonas cells with an efficient transfer of viral lysates (stage 2). Potential bacteriophage lysis most likely led to aggregate dissolution, leading to regeneration of dissolved inorganic carbon and also less labile organic carbon (stage 3). Scale bar=5 μm.