From marine protists to diatoms
Mereschkowski studied in St. Petersburg and investigated organisms from the White Sea and Naples, including ciliates, sponges and other marine forms. Later comparisons of diatoms from the Black Sea, Mediterranean and California directed his attention to pigmented cell structures and their evolutionary significance.
References: [1]
The plastid proposal of 1905
He treated plastids as possessing a measure of independence and proposed that they descended from cyanobacteria, then called blue-green algae, living inside another cell. Some features of plant cells could thus reflect the association of different lineages rather than differentiation within a single ancestral organism.
Two protoplasmic lineages in 1910
During his Kazan period he developed a scheme in which two protoplasmic lineages combined through symbiosis to form complex organisms. Physiological traits organized his classification. “Protoplasm” belonged to contemporary ideas about living matter and is not simply equivalent to chemically characterized cytoplasm in modern cell biology.
References: [2]
Historical proposals and modern evidence
His plastid argument became important background to later work, but the wider scheme included errors. He grouped fungi with bacteria and proposed a bacterial assembly into the nucleus while overlooking mitochondria. Modern endosymbiotic research investigates some of the same questions without adopting his complete classification or origin model.
References: [2]