Crassulas underwater. How Crassula helmsii acquires carbon and colonizes new sites

A plant of the water–land boundary

Crassula helmsii is an accepted species of Crassulaceae. POWO records its native occurrence in Australia and New Zealand. Introduced regions include Great Britain, Ireland, Belgium, France, Germany and the Netherlands. Tillaea helmsii is a synonym. Geographical origin and experimental location deserve separate attention because many physiological and ecological studies concern populations outside the native range. [2-WWW]

The botanical account by Smith and Buckley describes submerged, emergent, terrestrial and free-floating forms. A single plant can produce shoots with different traits depending on their position. Internodes lengthen with greater submergence, and adventitious roots develop at nodes. Small leaves occur in opposite pairs. This variation allows the same species to be considered in several different ecological settings, from an exposed shore to vegetation beneath the water surface. [Smith and Buckley 2020]

Carbon access changes through the day

Water offers high resistance to gas diffusion. Keeley’s synthesis explains that this restricts both CO₂ supply from the surroundings and its leakage from tissues. Nocturnal carbon fixation allows use of a period when carbon availability is greater in some waterbodies. During the day, the plant can use released internal reserves. Both components of the budget matter in aquatic CAM, acquisition of new carbon and recovery of CO₂ produced by respiration. Their contributions change with conditions. [Keeley 1998]

The shared feature of CAM is storage of organic acids in cells and their subsequent use in the light. In terrestrial plants, this rhythm is often associated with nocturnal stomatal opening. Males and Griffiths emphasize that aquatic CAM lineages can also function with organs lacking stomata. This separates two levels of organization. One concerns chemical transformations within the mesophyll, the other the routes by which gas reaches cells. When describing the mechanism across environments, carbon storage and properties of the gas-exchanging surface therefore need separate identification. In a submerged crassula, a central question is how much carbon comes from the surrounding water and how much from its own respiration. [Males and Griffiths 2017]

What physiological measurements revealed

Newman and Raven investigated Crassula helmsii photosynthesis under different pH and light conditions. Daily acidity changes corresponded to changes in malic acid. The plant used CO₂, while the experiment found no photosynthetic use of bicarbonate. This matters when interpreting the carbon content of water. The total dissolved inorganic carbon pool includes different chemical forms, of which the studied species can use a particular fraction. The authors linked the benefit of CAM to an extended period of available-carbon assimilation. [Newman and Raven 1995]

Klavsen and Maberly examined the contribution of CAM in a population from an English soft-water lake. Measurements were taken in April and July, at depths of 0.5 to 2.2 m. The estimated CAM contribution to the total carbon budget ranged from 18 to 42 percent. Activity decreased with depth, in parallel with declining light, and was greater in summer. These findings concern a particular population, two sampling periods and the studied depth range. They demonstrate the practical importance of nocturnal metabolism under lake conditions. [Klavsen and Maberly 2009]

In a subsequent experiment, the same authors combined two light levels with two CO₂ concentrations. After three weeks under strong light, CAM activity was 2.8 times greater at the lower CO₂ concentration than at the higher one. Under weak light, CAM remained undetectable in both carbon treatments. The response therefore linked two resources. Strong light increased the importance of carbon supply, while CO₂ availability affected the contribution of nocturnal storage. The species’ metabolic capacity was expressed to different degrees depending on growing conditions. [Klavsen and Maberly 2010]

What aquatic-plant anatomy contributes

Jones’s anatomical comparison included Crassula helmsii maintained in waterlogged greenhouse substrate. Its leaves had a single-layered epidermis, a thin cuticle and intercellular spaces. Air spaces also occurred in the stems. This organization provides information about internal routes for gas movement. Cells contained chloroplasts, and the vascular system connected different parts of the organ. In the sampled material, the stems retained primary structure. The aquatic growth form was therefore associated with a different tissue arrangement from that of the strongly developed, shrubby jade plant. [Jones 2011]

Stomata were identified on both sides of these leaf blades. This observation is particularly relevant to a plant capable of living at the boundary between water and air. Stomatal presence describes the leaf’s equipment, while function depends on current conditions. Microscopy and gas-exchange experiments answer complementary questions. A section reveals the distribution of cells and empty spaces. Physiological measurement determines the direction and magnitude of carbon flow. Combining both kinds of information connects tissue organization with the budget of the entire organ. Comparisons also need to retain whether material came from submerged, emergent or waterlogged-substrate plants. [Jones 2011] [Males and Griffiths 2017]

Another crassula responds to emergence

Keeley studied Crassula aquatica in reconstructed seasonal pools representing a Californian plant community. Tracking labelled carbon revealed its nocturnal fixation and storage in organic acids by submerged foliage. In the light, carbon moved into subsequent metabolic products. After transition to the terrestrial stage, foliage displayed C3 photosynthesis and daily acidity fluctuations disappeared. This result applies to Crassula aquatica in that experimental system. It shows how a change in leaf position relative to the water surface can accompany a change in the organization of carbon metabolism. [Keeley 1999]

A shoot fragment starts a new colonization

Carbon physiology forms part of the wider biology of Crassula helmsii. Coughlan and colleagues tested regrowth of shoot sections from three invasive aquatic plants. All produced new shoots and roots from fragments 10 mm long. Whether the fragment came from an apical or middle shoot section also mattered. In the Crassula helmsii treatment standardized by node number, mid-stem sections failed to resume growth. Fragment length is therefore one of several parameters describing material capable of regeneration. [Coughlan et al. 2022]

Settlement also depends on plants already occupying the substrate. Van der Loop and colleagues compared bare soil with turfs of Littorella uniflora or Pilularia globulifera in a greenhouse. These species reduced crassula establishment and growth. Adding waterfowl droppings increased its biomass particularly on exposed, nutrient-poor substrate. The experiment therefore connected expansion with space and resource availability. The plant’s physiological equipment operated within a neighbourhood that changed the conditions for using those resources. [van der Loop et al. 2020]

Water-level changes alter competition

Van Doorn and colleagues combined two soil types with submerged and exposed conditions. When carbon availability in the water was restricted, Crassula helmsii coexisted with Littorella uniflora and other plants without becoming dominant. It could dominate on exposed, buffered substrate with moderate nutrient availability. Development was poor on acidic organic soil. The authors identified aluminium toxicity as a probable explanation for the latter response. The result emphasizes the combined importance of soil chemistry and carbon access in the surrounding environment. [van Doorn et al. 2024]

Fieldwork by van Kleef and colleagues complements this picture. Surveys in the Netherlands and England linked changes in crassula cover with hydrology, resources and neighbouring vegetation. A sequence of exceptionally dry years favoured increased cover in some habitats. The authors associated this with weakened competition and transition from carbon limitation underwater to access to air. At the same time, some native plant communities retained their capacity to constrain expansion. The observed direction of change depended on the condition of the entire habitat. [van Kleef et al. 2024]

Choosing the appropriate explanatory scale

Aquatic crassulas connect descriptions of cells with descriptions of environments. Organic acids provide a temporary carbon pool, chloroplasts participate in subsequent processing, and the leaf surface and intercellular spaces help define gas-transport routes. Each level requires an appropriate kind of observation. CAM activity is measured at a specified time and under defined conditions, whereas anatomy describes the spatial organization of tissues. This approach preserves the importance of both the plant’s daily rhythm and its changing position relative to water. [Jones 2011] [Males and Griffiths 2017]