Response of Subantarctic microbes to new versus regenerated Fe in a
cold-core eddy
Abstract
In the Subantarctic sector of the Southern Ocean, vertical entrainment
of dissolved iron (DFe) triggers the seasonal productivity cycle.
However, diminishing physical supply of new Fe during the spring to
summer transition rapidly drives epipelagic microbial communities to
rely upon recycled DFe for growth. Hence, subpolar waters evolve
seasonally from a high fe ratio system (i.e., [uptake of new
Fe]/[uptake of new+recycled Fe]) to a low fe ratio system. Here,
we tested how resident microbes within a cyclonic eddy respond to
different Fe/ligand inputs which mimic entrained new DFe (Fe-NEW),
diffusively-supplied regenerated DFe (Fe-REG), and a control with no
addition of DFe (Fe-NO). After 6 days, 3.5 (Fe-NO, Fe-NEW) to 5-fold
(Fe-REG) increases in Chl a were observed despite
~2.5-fold range between treatments of initial DFe.
Marked differences were also evident in the proportion of in vitro DFe
derived from recycling to sustain phytoplankton growth (Fe-REG, 30%
recycled c.f. 70% Fe-NEW, 50% Fe-NO). This trend supports the concept
that DFe/ligands released from subsurface particles are more
bioavailable than new DFe collected at the same depth. This additional
recycling may be mediated by bacteria. Indeed, by day 6 bacterial
production (BP) was comparable between Fe-NO and Fe-NEW
but~2 fold higher in Fe-REG. Interestingly, a
preferential response of phytoplankton (haptophyte-dominated) relative
to bacteria was also found in Fe-REG. In contrast, in Fe-NEW and Fe-NO
the proportion of diatoms increased. Hence, different modes of Fe/ligand
supply modify BP and Fe bioavailability to phytoplankton that may drive
distinctive floristic shifts and biogeochemical signatures.