Is methanol a better choice of polar solvent compared to isopropanol to extract lipids?

Hello all!

So I am looking to see if anyone knows why there is such a big difference in the chloroform: methanol methods (FLE and BDE) and chloroform: isopropanol methods (HIPA & CIPA) used in my study for Chlorella vulgaris but not the other two species (Nannochloropsis oceanica and Phaeodactylum tricornutum).

See my attached graph to see my results. The figure shows Total lipid extract as percentage of sample dry weight for each of the three species tested (Nannochloropsis oceanicaCCAP 849/10, Chlorella vulgaris CCAP 211/21A and Phaeodactylum tricornutum UTEX646) comparing the Folch (FLE), Bligh and Dyer (BDE), Hot Isopropanol Extraction (HIPA) and Cold Isopropanol Extraction (CIPA). Error bars indicate ±SD (n=3-4). Letters indicate significant differences (p≤0.05) between methods within each of the species. Letter pairings start at ‘A’ within each species, and letter pairings across different species are not relevant.

For your information, the species seem to have been harvested during the exponential phase which may explain total lipid extracts are low for some. The two chloroform: methanol extractions are modified versions of the Folch (2:1) and Bligh & Dyer (1:1) and HIPA and CIPA represent a hot and cold isopropanol extraction (chloroform: isopropanol (1:1)).

In each extraction, the biomass was freeze-dried and once solvents were added the samples were homogenised for 1 minute. Instead of water, a KCl solution was added. I understand Nannochloropsis and Chlorella both have thick cell walls so it could be that not all the cell walls were disrupted during this homogenisation step however I don't think this is the reason why there is such differences in the values obtained using the methods on Chlorella but not Nannochloropsis.

I understand methanol is a more polar solvent than isopropanol however, I don't think this is why there is such a difference because if that was the case then you would expect similar results to be obtained from the Nannochloropsis too.

I have also looked at the lipid classes in these samples and there isn't any bias towards polar or non-polar lipids. Its just that the isopropanol methods have extracted much less lipids than the chloroform: methanol methods in Chlorella.

If anyone has any ideas please put them forward, they would be greatly appreciated.

Thanks!