Questions: Marine Microbial Community Structure and Function

Question 1

A bloom of a single dominant bacterial species is observed in a coastal water sample. Based on microbial community dynamics, what is the most likely ecological mechanism that will return the community to higher diversity?

• A. Nutrient depletion will starve the dominant species, allowing weaker competitors to recover
• B. Viral lysis will preferentially target the most abundant species, reducing its dominance through density-dependent predation
• C. Protist grazing will consume the dominant species uniformly until competitors recover
• D. The dominant species will self-limit through quorum sensing once it reaches high density

Answer: (B)

Viral predation is density-dependent — the most abundant host species is infected most frequently. This prevents competitive exclusion by keeping dominant populations in check while less-abundant species escape infection pressure. Nutrient depletion is also a factor, but it suppresses all species, not selectively the dominant one. This viral diversity-maintenance function is one of the reasons the ocean supports staggering microbial diversity.

Question 2

What is the ecological role of the microbial loop, and what would happen to the food web without it?

• A. It transfers dissolved organic carbon directly to top predators, bypassing intermediate trophic levels
• B. It converts dissolved organic carbon (which would otherwise be lost from the food chain) back into particulate biomass accessible to higher trophic levels
• C. It mineralizes organic matter into inorganic nutrients that fuel phytoplankton growth, with no pathway to higher trophic levels
• D. It produces dissolved organic carbon from CO₂ through bacterial chemosynthesis, supplementing phytoplankton production

Answer: (B)

Without the microbial loop, dissolved organic carbon released by dying phytoplankton or exudation would effectively disappear from the food web — it's too small for zooplankton to eat. Heterotrophic bacteria consume this dissolved material and convert it into bacterial biomass (particulate carbon), which protists can then eat, and those protists are in turn consumed by zooplankton. The microbial loop rescues a major fraction of primary production that would otherwise be lost.

Question 3

Viral lysis (the 'viral shunt') reduces the transfer of carbon to higher trophic levels, meaning viruses are overall harmful to ocean productivity.

• T. True
• F. False

Answer: False

The viral shunt does redirect carbon away from higher trophic levels (fish, zooplankton) by lysing cells and releasing their contents back into the dissolved pool. But this is not simply harmful — it accelerates nutrient regeneration (nitrogen, phosphorus back to inorganic forms), fueling further phytoplankton and bacterial growth. The viral shunt also maintains microbial diversity through density-dependent control. Whether it increases or decreases overall ecosystem productivity depends on context; the claim that it is 'overall harmful' is an oversimplification.

Question 4

The majority of marine bacterial diversity can now be characterized by growing representative strains in laboratory cultures with modern media formulations.

• T. True
• F. False

Answer: False

Estimates suggest that over 99% of marine microbial species cannot be cultured under standard laboratory conditions — this is the 'great plate count anomaly.' Our understanding of true marine microbial diversity comes almost entirely from molecular methods: 16S rRNA gene sequencing identifies who is present, and metagenomics reconstructs metabolic capabilities from environmental DNA. Culture-based methods, despite improvements, capture only a tiny fraction of actual diversity.

Question 5

Explain why viruses are considered keystone components of marine microbial communities rather than simply parasites that reduce microbial biomass.

Model answer: Viruses exert density-dependent control — they preferentially infect the most abundant species, preventing any single species from monopolizing resources and maintaining community diversity. Viral lysis also releases cell contents (carbon, nitrogen, phosphorus) back into the dissolved pool, accelerating nutrient regeneration and fueling further microbial growth. These 'viral shunt' pathways alter how carbon and energy move through the food web. Far from being purely destructive, viruses are structural regulators of community composition and key drivers of biogeochemical cycling.

The keystone framing captures the fact that viruses have outsized ecological effects relative to their own biomass. By selectively lysing dominant species, they maintain the 'rare biosphere' — diverse low-abundance populations that can bloom when conditions change. By short-circuiting carbon transfer to higher trophic levels and returning nutrients to dissolved pools, they alter ecosystem energetics. Removing viruses from ocean models substantially changes predicted nutrient cycles and community structure.