What does cyanobacteria look for in a good moss host?

For mosses, nitrogen is essential for growth, like any other plant. In ecosystems away from pollution like the subarctic tundra, they can get this element in partnership with cyanobacteria that can colonise them. But not all mosses are equal. Nitrogen fixation can vary significantly between mosses, and why has not been clear. So Xin Liu and Kathrin Rousk examined moss traits to find out why some mosses are more attractive to cyanobacteria than others. If they could associate bacterial activity with a set of traits, they could make predictions about nitrogen fixation in all sorts of mosses.

While they are definitely plants, mosses upend many ideas you’re taught about plants as a child. An important feature is they’re non-vascular. Typically you’d think of a plant having roots, a stem and some leaves. Tubes from the roots would carry water and nutrients from the soil to the rest of the plant. Mosses lack these tubes and also have a different body plan. They don’t have roots as such. They have rhizoids, whose job is to anchor the moss to a surface, but these rhizoids don’t bring water or nutrients inside for the rest of the plant to use. Instead, the mosses soak these up a bit like a sponge.

Liu and Rousk knew from earlier research that water helped increase nitrogen fixation for mosses but didn’t know why. One reason could be that water allows cyanobacteria to move into the mosses to colonise them. Another possibility is that the cyanobacteria have no trouble occupying mosses but need the water for their own activity. So the mosses that best move water over their surfaces might have the best nitrogen fixation.

There might be other features that affect bacterial colonisation, say Liu and Rousk. For example, leaf shape will alter how mosses can protect bacteria. Or maybe it’s the chemical properties of leaves that matter. Could the pH of the water on the leaves matter? Or is it the plant defences of the mosses that also inhibit the cyanobacteria, with some plants friendlier to their guests than others?

To determine if there were traits that mattered to cyanobacteria, Lin and Rousk examined four different moss species with varying levels of nitrogen fixation. They then measured the mosses to see what mattered. In their article, Liu and Rousk write: “To accomplish this, we measured acetylene reduction rate as a measure of N₂ fixation activity, and assessed cyanobacterial colonization and abundance at shoot and colony (group of shoots) level and linked this to water balance traits (maximum water content, water absorption rate and water loss rate of moss colonies), chemical traits (pH, total phenols), colony structural traits (frequency of shoots and height) and morphological traits (shoot length, frequency of leaves, leaf area, and so on) at shoot as well as at leaf level of the four moss species collected in the subarctic region.”

The botanists found that the hydration rate of the moss host, not water content per se, controlled cyanobacterial colonisation. The higher the hydration rate, the more cyanobacteria a moss could host. Mosses with larger leaves had slower hydration rates, so they had lower colonisation. Mosses with more leaves had more colonisation. Liu and Rousk argue that mosses trade-off quantity and size for leaves, so the mosses with more leaves also have smaller leaves, affecting the hydration rate.

The chemical results were interesting. Lin and Rousk write: “In our experiment, phenols affected cyanobacterial activity negatively, but not cyanobacterial colonization. This fits with previous findings showing that moss phenol content did not correlate with cyanobacterial colonization.” They also found that the range of pH of the most colonised mosses was broadly the same as the least occupied mosses, indicating the acidity of the leaves was not a significant issue.

The botanists also found that paraphyllia were colonized by cyanobacteria. Paraphyllia are hair-like structures on moss stems found between leaves. Liu and Rousk found that cyanobacteria colonised around 20% to 30% of paraphyllia.

Liu and Rousk conclude: “Given that mosses are a key source of N to ecosystems where they dominate the ground cover, uncovering the relation between moss traits and cyanobacterial colonization will ultimately result in a better estimation of the amount of N input – as dependent on moss traits – and can provide new perspectives and information for understanding the relationship between mosses and cyanobacteria.”

READ THE ARTICLE

Liu, X. and Rousk, K. (2021) “The moss traits that rule cyanobacterial colonization,” Annals of Botany. https://doi.org/10.1093/aob/mcab127