Extreme Life of the Sea - Lecture 4 The Microbial Loop

Summary

In this lecture, Steve Palumbi explores the microbial loop, highlighting the crucial role microbes play in ocean ecosystems. He delves into the complex predator-prey dynamics between bacteria and viruses, explaining how viruses affect bacterial populations and contribute to the food chain. Palumbi discusses evolutionary arms races between microbes and viruses, the ecological importance of species like Prochlorococcus, and the hypothesis of 'kill the winner,' which explains how viruses prevent microbial dominance. Through these interactions, he sheds light on the remarkable biodiversity and balance within the ocean's microbial world.

Takeaways

• 😀 Microbes in the ocean play a crucial role in the microbial loop, contributing significantly to the ocean's food chain and carbon cycle.
• 😀 The traditional ocean food chain involves fish and zooplankton consuming smaller organisms, but the microbial loop offers a hidden side where bacteria and other microbes are central.
• 😀 Bacteria, while too small for larger organisms to consume directly, are consumed by single-celled organisms like ciliates and flagellates, which are then eaten by zooplankton.
• 😀 About a quarter of Earth's organic carbon is tied up in microbes, making them key players in ocean ecosystems.
• 😀 Viruses act as predators on bacteria, attaching to them, killing them, and affecting the dynamics of microbial populations in the ocean.
• 😀 The relationship between viruses and bacteria is a co-evolutionary arms race where bacteria evolve new receptors to defend against viruses, while viruses evolve new ways to attack.
• 😀 Prochlorococcus, a common ocean microbe, can be attacked by viruses that bring photosynthetic genes, allowing the virus to keep the host’s photosynthetic machinery alive long enough to reproduce.
• 😀 Viral infections in ocean microbes can be tracked through satellite imagery, where blooms of microbes like coccolithophores disappear as they are consumed by viruses.
• 😀 Some microbes respond to viral infection by undergoing programmed cell death or turning into gametes, helping protect their clone mates from viral infection.
• 😀 Viruses may act as keystone species in the ocean, keeping dominant microbial populations in check and promoting diversity through a process known as 'kill the winner.'

Q & A

What is the microbial loop in ocean ecosystems?

The microbial loop refers to the role of microbes in the ocean's food chain, where bacteria, viruses, and single-celled organisms interact to transfer energy and nutrients through the ecosystem, influencing the flow of organic carbon in marine environments.

Why are microbes considered more abundant than stars in the universe?

Microbes, particularly in the ocean, are extremely numerous, with their population far exceeding the number of stars in the universe. This immense population plays a crucial role in the movement of organic carbon and energy in ocean ecosystems.

How do flagellates and ciliates contribute to the microbial loop?

Flagellates and ciliates serve as intermediaries in the microbial loop by consuming bacteria and being consumed by larger organisms like zooplankton. This helps transfer energy from bacteria into higher parts of the food chain, contributing to the overall energy flow in marine ecosystems.

What role do viruses play in the microbial loop?

Viruses act as predators of bacteria, attaching to their cells, injecting genetic material, and causing the bacteria to burst. This viral predation releases new viral particles into the water, which can then infect other bacteria, influencing microbial populations and the flow of organic matter in the ocean.

How do viruses co-evolve with their bacterial hosts?

Viruses and bacteria engage in a co-evolutionary arms race, where bacteria evolve new receptors to avoid viral attacks, while viruses evolve new strategies to infect bacteria. This ongoing interaction shapes the evolution and population dynamics of both viruses and bacteria.

What is the significance of prochlorococcus in the ocean?

Prochlorococcus is one of the most abundant microbes in the ocean and plays a critical role in photosynthesis, contributing significantly to oxygen production. Some viruses that infect prochlorococcus carry photosynthetic genes, which help maintain the host's photosynthetic machinery long enough for the viruses to reproduce.

How do viruses affect the dynamics of microbial blooms in the ocean?

Viruses can cause microbial blooms, like those of coccolithophores, to collapse. As viruses infect and kill the cells, they release viral particles that spread the infection, resulting in a rapid decline in microbial populations, which is observable in satellite images of ocean blooms.

What is programmed cell death, and how do microbes use it to resist viruses?

Programmed cell death is a defense strategy where an infected microbial cell undergoes metabolic processes that kill itself, preventing the virus from replicating further. This action helps protect the organism's clone mates from infection and reduces the spread of the virus.

What is the concept of 'kill the winner' in microbial populations?

'Kill the winner' is a hypothesis suggesting that viruses target the most abundant microbial species in the ocean, preventing them from dominating the ecosystem. By regulating the population size of these dominant microbes, viruses help maintain microbial diversity.

How do keystone species function in ocean ecosystems?

Keystone species are organisms that have a disproportionately large impact on their ecosystems. In ocean ecosystems, viruses might act as keystone species by controlling the population of dominant microbes, similar to how predators like sea otters or starfish regulate the populations of other species in their respective habitats.