Salp fecal pellets release more bioavailable iron to Southern Ocean phytoplankton than krill fecal pellets

Sebastian Böckmann,Florian Koch,Bettina Meyer,Franziska Pausch,Morten Iversen,Ryan Driscoll,Luis Miguel Laglera,Christel Hassler,Scarlett Trimborn

doi:10.1016/j.cub.2021.02.033

Abstract

Over the last decades, it has been reported that the habitat of the Southern Ocean (SO) key species Antarctic krill (Euphausia superba) has contracted to high latitudes, putatively due to reduced winter sea ice coverage, while salps as Salpa thompsoni have extended their dispersal to the former krill habitats. To date, the potential implications of this population shift on the biogeochemical cycling of the limiting micronutrient iron (Fe) and its bioavailability to SO phytoplankton has never been tested. Based on uptake of fecal pellet (FP)-released Fe by SO phytoplankton, this study highlights how efficiently krill and salps recycle Fe. To test this, we collected FPs of natural populations of salps and krill, added them to the same SO phytoplankton community, and measured the community's Fe uptake rates. Our results reveal that both FP additions yielded similar dissolved iron concentrations in the seawater. Per FP carbon added to the seawater, 4.8± 1.5 times more Fe was taken up by the same phytoplankton community from salp FP than from krill FP, suggesting that salp FP increased the Fe bioavailability, possibly through the release of ligands. With respect to the ongoing shift from krill to salps, the potential for carbon fixation of the Fe-limited SO could be strengthened in the future, representing a negative feedback to climate change.

Highlights

In vast areas of the Southern Ocean (SO), phytoplankton growth is limited by iron (Fe) availability
It has been reported that the habitat of the Southern Ocean (SO) key species Antarctic krill (Euphausia superba) has contracted to high latitudes, putatively due to reduced winter sea ice coverage, while salps as Salpa thompsoni have extended their dispersal to the former krill habitats
Our results reveal that both fecal pellet (FP) additions yielded similar dissolved iron concentrations in the seawater

Summary

Introduction

In vast areas of the Southern Ocean (SO), phytoplankton growth is limited by iron (Fe) availability. For the open ocean, upwelling and Fe recycling in the water column is of paramount importance.[5,6] Previous studies suggested that grazers[7,8] and predators[9] contribute to the recycling of Fe in the water column. In some open ocean regions, up to 50% of the soluble Fe pool is turned over on a weekly basis, mediated by biological recycling,[10] with Antarctic krill (Euphausia superba), hereafter referred to as krill, being important.[11] So far, few studies have quantified the amount of Fe released by krill[8,12,13,14] and the dominant SO salp species Salpa thompsoni,[15] hereafter referred to as salps. The actual bioavailability of this recycled Fe to SO phytoplankton has never been tested

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Results