Microbial Seas: Sulfur Compounds, Climate, and Food Webs

Marine microbes are the most abundant life form, comprising over 90\% of ocean biomass.
Dimethylsulfide (DMS) is produced from Dimethylsulfoniopropionate (DMSP) by marine phytoplankton and macroalgae.
DMS is volatile and the primary biogenic source of atmospheric sulfur (ca. 1.5 \times 10^7 tonnes S/year).
DMSP serves as an osmoprotectant and cryoprotectant for many phytoplankton, notably dinoflagellates and coccolithophores (e.g., Emiliania huxleyi).

The CLAW hypothesis (Charlson et al., 1987) proposed DMS-driven climate regulation, where DMS leads to cloud formation, increasing Earth's albedo and cooling.
Subsequent research revealed bacterioplankton divert significant DMSP into the microbial food web, impacting carbon cycling.
DMSP supports coral reef microbial communities, being released in coral mucus and utilized by bacteria like Spongiobacter.
Corals possess the enzymatic machinery for DMSP production, contributing to local thermal stress alleviation.
The Anti-CLAW hypothesis suggested a positive feedback where ocean warming reduces DMS, leading to less cloud cover and further warming.
The CLAW hypothesis has been largely retired due to the recognized complexity of biogeochemistry and climate physics.

Elevated DMS indicates high productivity and zooplankton grazing, serving as a foraging cue for many marine animals.
Procellariiformes (e.g., albatrosses, petrels) detect picomolar DMS concentrations to locate patchy prey.
Savoca & Nevitt (2014) described a tritrophic mutualism: DMS-tracking seabirds specialize on primary consumers (e.g., krill).
Smaller, more agile birds demonstrate better DMS-tracking abilities.
Seabird faeces are iron-rich; their feeding on iron-rich krill and subsequent excretion stimulate phytoplankton growth and DMS production, contributing to climate regulation. This estimated 230,000 tonnes of seabird biomass in the Southern Ocean has a significant impact.