Implications for the mesopelagic microbial gardening hypothesis as determined by experimental fragmentation of Antarctic krill faecal pellets

1. Detritivores need to upgrade their food to increase its nutritional
value. One method is to fragment detritus promoting the colonisation­ of
nutrient-rich microbes, which consumers then ingest along with the
detritus; so-called microbial gardening. Observations and numerical models
of the detritus-dominated ocean mesopelagic zone have suggested microbial
gardening by zooplankton is a fundamental process in the ocean carbon
cycle leading to increased respiration of carbon-rich detritus. However,
no experimental evidence exists to demonstrate that microbial respiration
rates are higher on recently fragmented sinking detrital particles. 2.
Using aquaria-reared Antarctic krill faecal pellets we showed
fragmentation increased microbial particulate organic carbon (POC)
turnover by 1.8x, but only on brown faecal pellets, formed from the
consumption of other pellets. Microbial POC turnover on un-and fragmented
green faecal pellets, formed from consuming fresh phytoplankton, was
equal. Thus, POC content, fragmentation, and potentially nutritional value
together drive POC turnover rates. 3. Mesopelagic microbial gardening
could be a risky strategy, as the dominant detrital food source is
settling particles. Even though fragmentation decreases particle size and
sinking rat, it is unlikely that an organism would remain with the
particle long enough to nutritionally benefit from attached microbes. We
propose ‘communal gardening’ occurs whereby additional mesopelagic
organisms nearby or below the site of fragmentation consume the particle
and the colonised microbes. 4. To determine how fragmentation impacts the
remineralisation of sinking carbon-rich detritus, and to parameterise
microbial gardening in mesopelagic carbon models, three key metrics from
further controlled experiments and observations are needed; how particle
composition (here, pellet colour/krill diet) impacts the response of
microbes to the fragmentation of particles; the nutritional benefit to
zooplankton from ingesting microbes after fragmentation along with
identification of which essential nutrients are being targeted; how both
these factors vary between physical (shear) and biological particle
fragmentation.