Lecture 18 - Marine Viruses

Question 1

Give and describe the two most abundant marine cyanobacteria.

Answer 1

Prochlorococcus - small cyanobacteria. Dominates in ocean gyres and nutrient-limited areas.

Syenochococcus - dominates in coastal regions, polar and higher latitude areas. Alter their pigmentation, enables them to photosynthesise at different depths.

Question 2

What is the cyanobacteria most often seen in the oceans?

Answer 2

Prochlorococcus

Question 3

What is the most dominant group of heterotrophs?

Answer 3

SAR11.
Some of smallest free-living cells - fits through a .2-micron filter.
Lives in low nutrient conditions.

Question 4

What percentage of bacteria found in oceans at any given point is SAR11?

Answer 4

30-50%

Question 5

Give the steps to the marine carbon cycle.

Answer 5

1) Phytoplankton produce carbon.
2) Phytoplankton die and sink, forming POM.
3) Phages infect phytoplankton, lyse cells open producing DOM (Dissolved Organic Matter).
4) Bacteria feed on DOM produced by phages.
5) This forms CO2 which is released back into the atmosphere.

Question 6

What is the size of the average marine virus?

Answer 6

50nm

Question 7

How do viruses meet host cells?

Answer 7

Through probability

Question 8

How many viruses are there per ml of seawater?

Answer 8

10 million

Question 9

When does abundance of viruses decrease?

Answer 9

• With depth
• With distance from the shore
  Because there is less to feed on.

Question 10

For every microbial cell on the surface, how many viruses are associated with it?

Answer 10

10

Question 11

What is the second largest component of biomass (after bacteria and archaea)?

Answer 11

Viruses.

Question 12

If all marine viruses are lined up, what would be the distance they would reach?

Answer 12

73 million light years

Question 13

What are the three most studied groups of marine phages?

Answer 13

• Podoviridae
• Siphoviridae
• Myoviridae

Question 14

Describe

a) Podoviridae
b) Siphoviridae
c) Myoviridae

Answer 14

a) Podoviridae = short tails.
b) Siphoviridae = long, non-contractile tails.
c) Myoviridae = contractile tails.

Question 15

Which marine phage resembles T4?

Answer 15

Myoviridae.

Question 16

Why are the most-studied marine phages double-stranded DNA viruses?

Answer 16

They are easiest to sample, can be sequenced easily.

Extracting RNA from seawater is also difficult.

Question 17

What might viruses have initially evolved?

Answer 17

Nuclei

Question 18

Describe the lysogenic infection of a virus.

Answer 18

Insertion of a virus into the genome of the host, into a tRNA, acting as a prophage.
As host replicates, produces a second copy of the virus.
An environmental change leads to increased abundance of the host.
Virus detects an increase in abundance and switches to lytic cycle.
Splits open host cell to meet new hosts.

Question 19

What is the viral shunt?

Answer 19

Keeping carbon within a bacterial loop, preventing it from reaching higher trophic levels.

Question 20

What percentage of microbes at any given time are infected by viruses?

Answer 20

30%

Question 21

What percentage of microbes are killed each day by viruses?

Answer 21

20%

Question 22

What percentage of carbon in the ocean goes through the viral shunt?

Answer 22

25%

Question 23

What are auxiliary metabolic genes?

Answer 23

Viral versions of host genes used to hijack host metabolism to improve infection.

Question 24

Give three examples of auxiliary metabolic genes.

Answer 24

• psbAD; increases photosynthesis.
• mazG; inhibits stringent response (alerting nearby microbes).
• pstS (increases phosphate uptake).

Question 25

Why is pstS useful for increasing phosphate uptake?

Answer 25

Because viruses need more phosphate than the host cell would normally be able to supply.

Question 26

How do phototrophs infected with psbAD respond?

Answer 26

Decrease carbon fixation.
Products from PSII increase, but never make it to light-independent reactions. Instead are funnelled into viral production.

Question 27

What is the Kill-the-Winner hypothesis?

Answer 27

That infection rates are impacted by host abundance and resistance.

Question 28

How do viruses increase in the genetic diversity of microbes?

Answer 28

With no viruses, evolution of bacteria gives it a selective advantage, allows alleles to go to fixation.
With viruses present, microbes die after reaching a certain abundance, therefore they remain very diverse.

Question 29

What percentage of marine bacteria are amenable to culturing?

Answer 29

1%

Question 30

What is metagenomics?

Answer 30

Collecting viral community DNA, sequencing it, and reconstructing their genomes.

Question 31

How much DNA is required for any sequencing to be conducted?

Answer 31

20 nanograms

Question 32

What is added to water for viral DNA to be collected?

Answer 32

Iron chloride.

Water precipitates out, viruses come together.