Dumas method needs complete digestion of algae coming to the end product as total nitrogen content, certainly you can not expect to get intact protein bands from the same digest.

I tried Lowry method in extracting protein from freshwater algae, just need grinding work in extraction and can be determined spectrophotometrically. I hope you might find it useful.

You can easily extract proteins with a sonicator

Alkaline extraction method has very effective and combined with any other mechanical methods are very innovative and time consuming process.

I think you are trying to make a protein rich extract and then evaluate the protein content... If so, you will need an ultra centrifuge (at least 15000xg for 10-15 mL) to extract a high content of protein to evaluate with the Dumas method and do an SDS using the same extract. Or you will have to perform a bradford or lowry method due to the low need of sample.

Dear Nuno, you are right. That´s what I´m doing. I used fresh Ulva biomass, rinsed with fresh water, freeze dried and grounded with centrifugal mill as cell disruption method. Extraction with aqueous and alkaline solution based on a protocol from Alex R. Angell. Protein contents were calculated based on the nitrogen content (N) according to the Dumas combustion method as described in the German Food Act (2005) using a Protein/Nitrogen Analyzer FP 528 (Leco). But I had problems to analyze enough protein in the supernatant. Using 20:1 v/W ratio I started to extract 50g of algae powder, which results in ~300ml aqueous and alkaline supernatant (->50ml to Leco analyzer). Trying to differ temperate and incubation time during the extraction. The measured TS in the Leco was to high.