Magnetic separation of algae genetically modified for increased intracellular iron uptake

Abstract

Algae were investigated in the past as a potential source of biofuel and other useful chemical derivatives. Magnetic separation of algae by iron oxide nanoparticle binding to cells has been proposed by others for dewatering of cellular mass prior to lipid extraction. We have investigated feasibility of magnetic separation based on the presence of natural iron stores in the cell, such as the ferritin in Auxenochlorella protothecoides (A. p.) strains. The A. p. cell constructs were tested for inserted genes and for increased intracellular iron concentration by inductively coupled plasma atomic absorption (ICP-AA). They were grown in Sueoka's modified high salt media with added vitamin B1 and increasing concentration of soluble iron compound (FeCl3 EDTA, from 1× to 8× compared to baseline). The cell magnetic separation conditions were tested using a thin rectangular flow channel pressed against interpolar gaps of a permanent magnet forming a separation system of a well-defined fluid flow and magnetic fringing field geometry (up to 2.2 T and 1,000 T/m) dubbed “magnetic deposition microscopy”, or MDM. The presence of magnetic cells in suspension was detected by formation of characteristic deposition bands at the edges of the magnet interpolar gaps, amenable to optical scanning and microscopic examination. The results demonstrated increasing cellular Fe uptake with increasing Fe concentration in the culture media in wild type strain and in selected genetically-modified constructs, leading to magnetic separation without magnetic particle binding. The throughput in this study is not sufficient for an economical scale harvest.

Incorporation of Terbium into a Microalga Leads to Magnetotactic Swimmers

“…We recorded that the average amount of incorporated Tb 3+ per cell was 0.23 ± 0.0027 picogram (pg). This uptake of Tb 3+ is possibly due to detoxification mechanisms of the cells toward the nonessential/toxic Tb 3+ , as shown before for other positively charged ions like zinc or iron . In general, the uptake of positively charged ions takes place in two steps: the first is a rapid passive biosorption onto the cell wall to negatively charged functional groups, followed by a slow, energy consuming membrane transport .…”

Intrinsically Magnetic Cells: A Review on Their Natural Occurrence and Synthetic Generation

“…Simple overexpression of iron importers to increase intracellular iron concentrations is not the method of choice to magnetize cells. Free iron cations induce Fenton reactions and disturb iron homeostasis and show only little magnetic properties (Buck et al, 2015). Also the use of heme-proteins to magnetize cells seems to be limited.…”

“…Uncontrolled import of iron without a specific intracellular recipient will likely result in disturbances in iron homeostasis (Crichton et al, 2002;Konhauser et al, 2011) and an increase in damaging Fenton reactions (Imlay, 2008;Yamada et al, 2012). However, we want to mention a recent study that showed successful magnetization of algae through overexpression of iron importers (Buck et al, 2015). Although separation of magnetic algae was demonstrated, the researchers highlighted this approach as economically unfeasible, due to low magnetization of algae.…”