Fe and C co-limitation of heterotrophic bacteria in the naturally fertilized region off the Kerguelen Islands

Abstract

Abstract. It has been univocally shown that iron (Fe) is the primary limiting nutrient for phytoplankton metabolism in high-nutrient, low-chlorophyll (HNLC) waters, yet the question of how this trace metal affects heterotrophic microbial activity is far less understood. We investigated the role of Fe for bacterial heterotrophic production and growth at three contrasting sites in the naturally Fe-fertilized region east of the Kerguelen Islands and at one site in HNLC waters during the KEOPS2 (Kerguelen Ocean and Plateau Compared Study 2) cruise in spring 2011. We performed dark incubations of natural microbial communities amended either with iron (Fe, as FeCl 3 ) or carbon (C, as trace-metal clean glucose), or a combination of both, and followed bacterial abundance and heterotrophic production for up to 7 days. Our results show that single and combined additions of Fe and C stimulated bulk and cell-specific bacterial production at the Fe-fertilized sites, while in HNLC waters only combined additions resulted in significant increases in these parameters. Bacterial abundance was enhanced in two out of the three experiments performed in Fe-fertilized waters but did not respond to Fe or C additions in HNLC waters. Our results provide evidence that both Fe and C are present at limiting concentrations for bacterial heterotrophic activity in the naturally fertilized region off the Kerguelen Islands in spring, while bacteria were co-limited by these elements in HNLC waters. These results shed new light on the role of Fe in bacterial heterotrophic metabolism in regions of the Southern Ocean that receive variable Fe inputs.

Uptake of Leucine, Chitin, and Iron by Prokaryotic Groups during Spring Phytoplankton Blooms Induced by Natural Iron Fertilization off Kerguelen Island (Southern Ocean)

“…These transporters promote the uptake of diverse substrates present at low concentrations, and they have high affinity with Fe complexes (Moeck and Coulton, 1998). In the region of the present study where Fe and carbon are limiting nutrients (Obernosterer et al, 2015), the presence of TonB-dependent transporters could provide an advantage to members of these bacterial groups. Previous studies report strain-specific differences in the cellular Fe quota under both Fe replete and Fe limited conditions (Granger and Price, 1999;Fourquez et al, 2014).…”

“…The present study took place in early spring when prokaryotic heterotrophic activity is greatly stimulated by the release of phytoplanktonderived DOM in the study region (Christaki et al, 2014), which in turn affects community composition (Landa et al, 2015). Both C and Fe were identified as growth-limiting factors (Obernosterer et al, 2015). Interestingly, the contributions of several bacterial groups at a fine phylogenetic level (>99% identity of the 16S rRNA gene) to abundance and leucine incorporation were also strongly positively correlated during the declining phase of the bloom above the Kerguelen Plateau (Obernosterer et al, 2011), concomitant with high virus-mediated prokaryotic mortality (Malits et al, 2014).…”

“…Concentrations of Chl a increased from 0.6 to 1.2 µg L −1 , accompanied by increases in prokaryotic growth rate from 0.07 to 0.12 d −1 . The unchanged low concentrations of dissolved organic carbon (mean 49 ± 1 µM; Obernosterer et al, 2015;Tremblay et al, 2015) across all sampling sites are likely a result of the rapid consumption of phytoplankton-derived DOM.…”

“…Experimental studies using bacterial strains revealed a positive effect of Fe on bacterial growth (Tortell et al, 1996;Fourquez et al, 2014). The response of natural bacterial communities to Fe addition varies among studies (see Obernosterer et al, 2015 for an overview). A common observation is the positive effect of Fe addition on heterotrophic bacteria when incubated in the light and in the presence of phytoplankton.…”

Illegal fishing bycatch overshadows climate as a driver of albatross population decline

“…Second, warmer SST may increase zooplankton by supporting phytoplankton growth. As Kerguelen surface productivity is likely iron-limited, SST may set an upper limit to the potential response of iron availability (Blain et al 2007, d'Ovidio et al 2015, Obernosterer et al 2015. While BBA in this region are not known to forage heavily on zooplankton (Cherel et al 2000(Cherel et al , 2002, changes in phytoplankton communities may indirectly enhance albatross by supporting growth of other predators, such as squid.…”