Aquatic plants undergo photosynthesis and cellular respiration much like terrestrial plants. Oxygen dissolves into water when aquatic autotrophs release oxygen as a byproduct of photosynthesis. Dissolved oxygen can be measured directly to determine if aquatic plants undergo photosynthesis or cellular respiration in different conditions.
Finding a reliable source of aquatic plants can prove difficult because some species are only available seasonally and others are invasive. So, for this new experiment, we recommend using Java moss (Vesicularia dubyana) or Christmas moss (Vesicularia montagnei), two aquatic bryophytes that are commonly used in freshwater aquaria. As can be seen in our data, Java moss undergoes cellular respiration in the dark, leading to a reduction of dissolved oxygen, and quickly switches to photosynthesis when exposed to an LED or halogen plant light. Students can easily manipulate light levels to further explore photosynthesis and cellular respiration using this robust aquatic plant.
In this experiment, students use a Vernier Optical DO Probe or Go Direct® Optical Dissolved Oxygen Probe to measure the dissolved oxygen produced by aquatic moss. Aged tap water and a golf-ball-sized ball of moss are added to a 250 mL Nalgene® bottle. The bottle is then exposed to a high-intensity plant light or wrapped in aluminum foil to prevent light from reaching the plants. Students can see results in 15 minutes, allowing this activity to be completed in a single 45-minute class period. After data collection, students calculate and compare the rates of oxygen production (or consumption) in each condition.