Differential impact of lytic viruses on prokaryotic morphopopulations in a tropical estuarine system (Cochin estuary, India)

Abstract

Our understanding on the importance of viral lysis in the functioning of tropical estuarine ecosystem is limited. This study examines viral infection of prokaryotes and subsequent lysis of cells belonging to different morphotypes across a salinity gradient in monsoon driven estuarine ecosystem (Cochin estuary, India). High standing stock of viruses and prokaryotes accompanied by lytic infection rates in the euryhaline/mesohaline region of the estuary suggests salinity to have an influential role in driving interactions between prokaryotes and viruses. High prokaryotic mortality rates, up to 42% of prokaryote population in the pre-monsoon season is further substantiated by a high virus to prokaryote ratio (VPR), suggesting that maintenance of a high number of viruses is dependent on the most active fraction of bacterioplankton. Although myoviruses were the dominant viral morphotype (mean = 43%) throughout the study period, there was significant variation among prokaryotic morphotypes susceptible to viral infection. Among them, the viral infected short rod prokaryote morphotype with lower burst estimates (mean = 18 viruses prokaryote-1) was dominant (35%) in the dry seasons whereas a substantial increase in cocci forms (30%) infected by viruses with high burst size (mean = 31 viruses prokaryote-1) was evident during the monsoon season. Such preferential infections of prokaryotic morphopopulations with respect to seasons can have a strong and variable impact on the carbon and energy flow in this tropical ecosystem.

Fig 1

Seasonal variability in (a) Viral abundance (VA), (b) Prokaryote abundance (PA), (c) Total viable prokaryotes (TVC) and (d) Virus to prokaryote ratio (VPR). The stations are represented in the X axis and the red dotted box represents different salinity zones.

Fig 2

Seasonal variability in (a) the percentage of viral infected prokaryote cells (FIC) and (b) Burst size (BS) estimates. The stations are represented in the X axis and the red dotted box represents different salinity zones.

Fig 3

Seasonal variations in (a) Viral morphotypes, (b) Viral infected prokaryotic morphotypes and (c) Average burst size estimates.

Fig 4. Principal component analysis (PCA) biplot representing the distribution of microbial parameters and their interrelationship of physicochemical variables.

Temp: Temperature, DO: Dissolved oxygen, NO₂: Nitrite, NO₃: Nitrate, PO₄: Phosphate, SiO₄: Silicate, PA: Prokaryotic abundance, TVC: Total viable prokaryotic count, VA: Viral abundance, VPR: Virus to prokaryote ratio, FVIC: Frequency of visibly infected cells, FIC: Frequency of infected prokaryotic cells, VIPM: Viral mediated prokaryotic mortality, BS: Burst size estimates.

Fig 5

Transmission electron micrographs of viral morphotypes belonging to Myoviridae (a, b), Podoviridae (c, d), Siphoviridae (e, f) and non-tailed viruses (g, h); scale bar = 50 nm, and viral infected prokaryotes belonging to different morphotypes such as fat rods (i, j), elongated rods (k, l), short rods (m, n) and cocci (o, p), scale bar = 100nm.