Is there lignin in the Ulva cell wall? A multidisciplinary structural investigation to provide new insights into cell wall evolution and macroscopic complexity in the chlorophyte green seaweeds

Abstract

Seaweeds are not only an integral component of the marine ecosystem via their role in global carbon and sulfur cycles, but also have significant economic value as crops for food and fuel, with certain intertidal seaweeds like the green 'sea lettuce' Ulva (Chlorophyta) appearing as attractive bioenergy feedstocks due to their rapid growth rates and polysaccharide-rich cell wall that can comprise half its total biomass.

Seaweeds have a distinct biochemistry to traditional land plant crops. For example, they lack the phenylpropanoid pathway, a key milestone in land plant evolution that enabled the biosynthesis of the secondary cell wall biopolymer lignin which confers structural support and facilitates water transport in vascular plants.

Despite this, 'lignin-like' fractions are reported in Ulva, and lignin has been found in a coralline red seaweed (Rhodophyta). No alternative pathway for lignin biosynthesis is provided by our current metabolic knowledge, meaning we have limited understanding of how ‘lignin’ arose in seaweeds. Furthermore, the only comprehensive structural investigation into seaweed 'phycolignin' to date has been performed in the coralline reds. Consequently, the presence of an equivalent component in green seaweeds like Ulva is still debatable.

As the primary aim of this thesis, I investigate the identity of the proposed lignin-like fraction of Ulva using confocal microscopy, biochemical assays, and biophysical analysis. To accomplish this, I evaluate the use of a sequential extraction protocol described for charophyte green algae (Streptophyta) to determine which cell wall polysaccharides the proposed structure associates with.

No evidence for lignin-like structures in the Ulva cell wall was identified during this research. Instead, I propose that the previous attributions to lignin in Ulva were misidentifications on account of the limitations of quantification protocols used. Interestingly, a major structural protein component is identified with possible implications for how Ulva mitigates osmotic stress at low tide. The absence of 'phycolignin' in the Ulva cell wall contrary to the lignified coralline seaweeds demonstrates that seaweeds display diverse adaptations to intertidal habitats, and provides support to the current hypothesis that lignin arose convergently in the red lineage, with green seaweeds and land plants sharing a more recent evolutionary history.