The Web of Life (22-4-1) Trophic Loops

 Trophic Loops

Back to the diatom, though not the one eaten by the copepod. This different diatom had the misfortune to die of a viral infection; its contents scattered into the watery world.

This diatom DOM is at the origin of another food chain. Dead diatoms are not the only source of dissolved organic material. Alive and kicking phytoplankton philanthropically rain marine snow. Depending upon nutrient availability, phytoplankton release a substantial fraction of their photosynthetic production as DOM.

This phytoplankton largesse arises over lack of nitrogen. Light and CO2 are all they need to cobble up carbohydrates, but proteins and nucleic acids need nitrogen. Excess carbohydrates are released, rather than stored, pending sufficient nitrogen.

As long as this snow is in solution, it cannot be passed up the food chain. Up to 50% of solar energy used by phytoplankton to fix carbon is spent from the start for lack of nitrogen.

Some marine snow descends to a watery grave, to enjoy an afterlife as a carbon store, or a snack for microbes in the mud. A large fraction of these carbs on the loose are ingested by bacteria.

This microbial loop puts photosynthetic particulate product back into the trophic flow. DOM-fattened bacteria are voraciously hunted by ciliates and flagellates, themselves preyed upon by copious copepods. Here we have another marine food chain.

The average milliliter of seawater contains a million heterotrophic bacteria that play an essential role in remineralizing dissolved organic matter (DOM) by decomposing 35 to 80% of net primary production and converting it into particulate form, available for consumption by larger organisms. ~ English physicist John Taylor & Roman Stocker

Bacterial recovery of DOM is an important aspect of marine ecology. The trophic significance of this microbial loop is augmented by the efficiencies involved.

In contrast to the meager trophic efficiencies of multicellular animals, typically 10%, bacteria eat at 50% trophic efficiency, sometimes as high as 90%. Their tiny predators – flagellates, and ciliates – may consume at a 70% efficiency. As a result of the microbial loop, high efficiencies afford support for a vast population of consumers.

Other organisms chow down on DOM, such as marine invertebrate larvae, but bacteria have an inherent advantage: size.

For little ones in the deep sea, DOM is absorbed through cellular membranes. The rate at which food can be absorbed is proportional to surface area. But a microorganism’s need for energy is a function of its volume, not its surface area. The bigger, the hungrier.

The ratio of surface area to volume is nonlinear. Surface area to volume progressively grows as the size of an organism shrinks. A 2 µm long bacterium has 50 times the surface area per volume of a ciliate 100 µm long.

Hence, a bacterium is better at dining on DOM. Bacteria are also more trophically efficient in other ways, by virtue of evolutionary adaptation to better access the energy from absorbed nutrients.

Then there is a viral loop within the microbial loop. Half of the bacteria at sea are infected by bacteriophages: viruses that specialize in preying on bacteria. 20–40% of infected bacteria die each day via lysis: cell wall rupture. A bacterium bursts, releasing DOM – the same fate as the diseased diatom.

The microbial loop reintroduces DOM into the food chain. The viral loop short-circuits that, dumping marine snow once again.

The viral loop is significant. There are 4×1030 viruses in the sea – 800 tonnes – equal to 20 days’ worth of carbon fixed by the entire ocean in 20 days.