Agents of death

Marine microbes are subject to attack by a range of organisms with voracious appetites and a variety of feeding strategies, but many are also consumers.

Viruses

Microscopic viruses are the most abundant biological agents in antarctic waters, with numbers up to 4 million per millilitre (ml). They inject their DNA or RNA and take over the cell metabolism of the host resulting in viral multiplication and eventual cell rupture. In temperate marine food webs, viruses have been shown to be major agents of death for bacteria and phytoplankton.

Protozoa

1. Puncturers - Many dinoflagellates and ciliates pierce the prey cell with a peduncle, digesting and absorbing its contents.
2. Engulfers - cover prey cells with their cytoplasm and digest them. This is the prime strategy for amoeboid cells but some other cells, notably dinoflagellates, can project a veil of cytoplasm (pallium) externally through their cell wall to enclose prey. They can sometimes successfully engulf cells larger than themselves.
3. Ingesters - feeding currents created by cilia or flagella draw prey cells to the predator where they are captured and ingested e.g. ciliates and choanoflagellates.

Mesoplankton

1. Biters - copepods
2. Swallowers - salps
3. Crushers - krill

Microbes have evolved various strategies for avoiding or resisting these modes of attack, giving them a competitive advantage.

[based on a talk by V. Smetacek]

Ciliates feeding

Click on the images below to view the QuickTime movie

A ciliate draws prey cells to it. Image: Harvey Marchant

Ciliate attempts to catch particles. Image: Harvey Marchant

Ciliate draws particles toward it. Image: Harvey Marchant

Ciliate feeding inside a dead diatom frustule. Image: Harvey Marchant

A ciliate feeding on the surface of mucilage from Phaeocystis antarctica. Image: Harvey Marchant

Ciliate: Looking into the gullet opening (depression at centre of frame). Image: Harvey Marchant

A ciliate beats its cilia to draw in food particles. Image: Harvey Marchant

A tintinnid ciliate beats its cilia to draw in food particles. Image: Harvey Marchant

A large ciliate, with "head" end at top right. Image: Harvey Marchant

Ciliate feeding on cells and detritus in marine snow particle. Image: Harvey Marchant

A choanoflagellate draws particles onto its feeding basket by beating its flagellum. Image: Harvey Marchant

Dinoflagellate containing ingested diatoms. Image: Harvey Marchant

Two protoazoa, Anisonema sp., containing ingested diatoms. Image: Harvey Marchant