Hydrilla verticillata is a highly invasive submerged aquatic plant that does not “eat” in the traditional sense, as animals do. Like all plants, Hydrilla acquires energy through photosynthesis, converting light into chemical energy. It also absorbs necessary mineral nutrients from both the water column and the sediment to fuel its growth. This dual strategy for energy and nutrient acquisition is a primary factor in its success as an invasive species.
Photosynthesis: Hydrilla’s Primary Energy Source
The core mechanism by which Hydrilla produces energy is photosynthesis, a biochemical process that converts light energy, water, and carbon dioxide (\(\text{CO}_2\)) into glucose sugars and oxygen. These sugars provide the energy for all its metabolic functions, including growth and reproduction. The plant exhibits exceptional photosynthetic efficiency, which is a major reason for its rapid, aggressive growth.
Hydrilla can photosynthesize effectively even in low-light conditions, reportedly thriving in areas receiving as little as one percent of the surface light. This adaptation allows it to colonize deeper waters than many native species, which are quickly shaded out. Once it approaches the water surface, it forms dense, multi-layered mats that intercept nearly all available sunlight, shading and eliminating competing vegetation below.
A specific adaptation for carbon acquisition is its ability to utilize bicarbonate (\(\text{HCO}_3^-\)) ions, which are abundant in hard water environments. Most aquatic plants rely primarily on dissolved free \(\text{CO}_2\), which can become depleted during the day due to high photosynthetic demand. Hydrilla can chemically process the plentiful bicarbonate, giving it a significant competitive advantage in high-alkalinity waters where free \(\text{CO}_2\) limits other species.
Mineral Uptake: Absorbing Nutrients from Water and Sediment
While photosynthesis provides energy (sugars), Hydrilla must absorb essential mineral elements like nitrogen and phosphorus to build its biomass. It employs a flexible, dual-uptake strategy, drawing nutrients from its roots anchored in the sediment and directly through its submerged stems and leaves from the water column. This adaptability allows it to flourish in a wide range of aquatic environments, from nutrient-rich lakes to nutrient-poor reservoirs.
The roots typically draw macronutrients, especially phosphorus (P) and nitrogen (N), from the substrate. Studies show that Hydrilla grown in nutrient-rich sediments has a significantly higher crude protein content, a direct measure of its total nitrogen uptake. Furthermore, the plant can chemically alter the sediment environment to enhance nutrient acquisition, increasing the sediment’s ability to bind with and retain phosphorus.
When sediment nutrients are scarce, the plant can shift its focus to absorbing dissolved nutrients directly from the surrounding water. This flexibility is advantageous in new or disturbed habitats where sediment quality is variable. The ability to absorb ions directly through its leaves and stems means that fragments can continue to grow and establish new colonies even before rooting. This opportunistic nutrient absorption, combined with its high growth rate, allows Hydrilla to quickly dominate an ecosystem by sequestering available resources.
Energy Reserves: Tubers and Turions for Survival
The sugars and mineral nutrients acquired through photosynthesis and absorption are converted into stored energy reserves for long-term survival. Hydrilla produces two specialized, carbohydrate-rich structures for this purpose: tubers and turions. Tubers are subterranean, potato-like buds that form at the tips of underground rhizomes, typically buried a few inches deep in the sediment.
Turions are dense, compact, dormant buds that form in the leaf axils along the stem. Both structures are packed with stored carbohydrates, serving as the plant’s survival ration during unfavorable conditions, such as winter, low light, or drought. The tubers are extremely resilient and can remain viable and dormant in undisturbed sediment for up to seven years.
These energy reserves allow Hydrilla to overwinter and rapidly sprout new growth when conditions improve, often before native plants can recover. The sheer number of tubers produced—sometimes thousands per square meter—means that control efforts that remove the visible plant mass often fail to eliminate the dormant reserves below the sediment. This storage of energy and nutrients ensures the plant’s persistence and contributes significantly to its invasive success.