Viral lysis of Micromonas pusilla: Impacts on dissolved organic matter production and composition

Viral lysis of Micromonas pusilla: Impacts on dissolved organic matter production and composition

Dissolved Organic Carbon (DOC) is a fundamental source of nutrients in the open ocean. There are several suggested methods of release or production of DOC (mentioned in Nianzhi Jiao’s paper [Colin’s lecture], Microbial production of recalcitrant dissolved organic matter). DOC has 3 categories, labile (<1%, minutes-days to break down), semi-labile (~50%, days to years) and refractory (~50%, years to 1000’s). This paper uses Micromonas pusilla to assess gaps in knowledge of how viral lysis of phytoplankton affects composition, lability and cycling of DOM.

The authors tested the organic matter dynamics and came up with 3 main conclusions:

Viral lysis enhances production of both labile and refractory DOC, impacting microbial web and perhaps decreasing the efficiency of the biological pump. Tests showed that in viral infected cultures, DOC concentration increased 4.5x faster and has an end point 2.6x higher than in uninfected cultures. Measuring labile amino acids and refractory humic-like substances, it was shown that viral infection increases labile DOC by 4.1x and refractory DOC by 2.8x.

Viral lysis changes the optical signature (composition) of DOM. Viral infection by itself was shown to change the cell composition of the infected phytoplankton host, shaping the signature of the released/lysed DOM. The viral presence influences host fatty acid and pigment concentration, DNA content and cell levels of DMS and DMSP (among other things).

Viral lysis increases the production of TEP, which is critical in the formation of biofilms, and possibly influences particle aggregation, affecting Marine Snow formation. Tests showed that in Viral infected cultures, Transparent Exopolymer Particles were produced 1.8x faster and had an end point 1.5x higher than uninfected cultures.

Separately, viral infection and then subsequent lysis are believed to more than double the pool of labile DOC. It was shown that 98% of decrease in POC was down to viral lysis, suggesting an efficient transformation pathway of cells into DOC. Lysis did not only increase DOC, but other important factors, such as organic nutrients and trace metals. In comparison to literature derived estimates, viral production of DOC was 1.2-1.9x higher in the observed experiment.

In summation, this paper provides an answer to the lack of knowledge highlighted by Jiao in terms of the production of DOC by viral lysis. This is a very important process, as 10²⁹ infections are believed to occur daily, with a daily total of 10⁹ tonnes of organic carbon being released to DOM, changing everything from composition to lability and affecting the cycling of the biological pump and microbial web.

Christian Lonborg, Mathias Middelboe and Corina P.D. Brussaard. (2013). Viral lysis of Micromonas pusilla: Impacts on dissolved organic matter production and composition. Biogeochemistry.