Internal Nitrogen Pools Shape the Infection of Aureococcus anophagefferens CCMP 1984 by a Giant Virus

Abstract

The pelagophyte Aureococcus anophagefferens blooms annually in shallow bays around the world, where it is hypothesized to outcompete other phytoplankton in part by using alternative nitrogen sources. The high proportion of natural populations that are infected during the late stages of the bloom suggest viruses cause bloom collapse. We hypothesized that the Aureococcus anophagefferens Virus (AaV) infection cycle would be negatively influenced in cultures acclimated to decreasing external nitrogen conditions, but that the real-time external nitrogen concentration would not influence the infection cycle. Cultures acclimated in NO − 3 concentrations (0.0147 mM; N:P = 0.1225) that showed reduced end point cell abundances, forward scatter (a proxy for size) and red fluorescence (a proxy for chlorophyll a), also produced fewer viruses per cell at a slower rate. Decreasing the external concentration of nitrogen post infection did not alter burst size or time to lysis. These data suggest that the nitrogen used for new viral progeny is present within host cells at the time of infection. Flow cytometric data of an infection cycle showed a reduction in red fluorescence around twelve hours post infection, consistent with degradation of nitrogen-rich chloroplasts during the infection cycle. Using cell and virus quota estimates, we determined that A. anophagefferens cells had sufficient nitrogen and carbon for the lower ranges of burst sizes determined but did not contain enough phosphorous. Consistent with this observation, expression of nitrate and sugar transporters did not increase in the publicly available transcriptome data of the infection cycle, while several phosphorus transporters were. Our data demonstrate that dynamics of viruses infecting Aureococcus over the course of a bloom is dictated by the host cell state upon infection, which is set a priori by external nutrient supplies.

The diel disconnect between cell growth and division inAureococcusis interrupted by giant virus infection

“…While we may define this period as nutritional stockpiling by an uninfected host to prepare for DNA synthesis and cell division, in the case of viral infection the same stockpiling must occur, only to be used in production of viral particles (Gann et al, 2020b). In considering what metabolic processes the virus alters for its own benefit, we also must consider which processes are left unaltered and how these too may serve a purpose in viral infection.…”

Kratosvirus quantuckense: the history and novelty of an algal bloom disrupting virus and a model for giant virus research

“…This suggests a method for the virocell to continue photosynthesizing after infection, though light harvesting complexes are globally down-regulated ( Moniruzzaman et al, 2018 ). Furthermore, a decrease of in situ fluorescence (effectively chlorophyll per cell) of late stage infected A. anophagefferens cells has been noted ( Gann et al, 2020b ). While these factors may indicate a decrease in photosynthetic efficiency of the infected cell, the retention of the chloroplast may be a sign of the importance of light to the infection cycle.…”

“…Eventually, virus-like particles fill the entirety of the virocell in a uniform fashion, forming a lattice, leaving only space enough for the undegraded chloroplast ( Gastrich et al, 2002 ; Gobler et al, 2007 ; Figures 5C, D ). The infection cycle is completed after approximately 24 h, at which point host cells lyse and release virus particles, with burst sizes estimated to be between 400 and 500 virions per cell ( Rowe et al, 2008 ; Brown and Bidle, 2014 ; Gann et al, 2020b ). Free viral particles have been identified at the 21 h time point, though these particles may be present as a result of host cells beginning to lyse and not egress from a still intact host cell ( Brown and Bidle, 2014 ).…”

“…The efficiency of K. quantuckense infection is shaped by the factors constraining propagation of the host cell i.e., the intracellular and extracellular environments of the virocell ( Gann et al, 2020b ). Ecologically, a higher natural availability of macronutrients (inorganic phosphorus and nitrogen) and micronutrients (silicates) correlated to an increase in viral species richness, showing that propagation of specific Nucleocytoviricota families depends on external environmental conditions ( Clerissi et al, 2014 ).…”

“…Reduced growth of A. anophagefferens during nutrient stress could theoretically limit the viral burst size as production of viral particles relies on the availability of nutrients in a cell ( Berg et al, 1997 , 2008 ; Gobler et al, 2004 ). It has been found that a 100-fold reduction in inorganic nitrogen concentration decreased burst size by ∼50% ( Gann et al, 2020b ). Such a phenomenon was observed in other host-virus systems, with a decreased burst size and increased length of infection cycle ( Cheng et al, 2015 ; Maat and Brussaard, 2016 ).…”