Epiphytic biofilms on seagrass leaves can lead to extreme microenvironmental conditions for the encapsulated leaf limiting both its photosynthesis and respiration. Yet, little is known about how the biological activity of the biofilm itself changes the seagrass phyllosphere microenvironment and dynamics. We used microsensors to measure O₂ concentrations and pH gradients and calculate fluxes of O₂, CO₂ and bicarbonate (Formula presented.) around seagrass leaves (Z. marina L.) covered with artificial, inactive biofilms and natural epiphytic biofilms. A sterilized seawater-agar matrix was used to make an artificial “inactive” biofilm on seagrass leaves with the same thickness as the natural leaf epiphytic biofilm, which impeded turbulent exchange of gases but did not have microbial activity. We compared the concentration profiles and fluxes of O₂ and inorganic carbon of the “active” and “inactive” biofilm to investigate the effect of microbial activity and molecular diffusion in seagrass leaf biofilms. In light, the O₂ flux of leaves with inactive biofilm was only 31% of the leaves with active biofilm, indicating that the photosynthesis of the microbial community in the biofilm makes up the majority of O₂ production in the leaf microenvironment. During darkness, the O₂ concentration profiles and O₂ fluxes were almost identical in the “active” and “inactive” biofilms. The pH profiles showed the same trend with an increase in pH of ~1.0 in the “active” biofilms and ~0.3 pH units in the “inactive” biofilms in the light, and both showing a decrease of ~0.3 pH units in darkness compared to the bulk seawater. Our measurements thus demonstrate strong photosynthesis in the epiphyte layer driving phyllosphere basification and inorganic carbon limitation. The calculated CO₂ concentration on the leaf surface decreased to 0.09 μmol L^-1 in the epiphytic biofilm in the light compared to leaf surface CO₂ concentrations of 13.8 μmol L^-1 on bare seagrass leaves, and the CO₂ influxes were only 3.0% and 5.4% of O₂ effluxes for leaves with “active” and “inactive” biofilm, respectively. Calculations also showed that (Formula presented.) influxes in light accounted for 91-97% of the total inorganic carbon influx to the seagrass leaf, although the (Formula presented.) utilization via CO₂ concentration mechanisms is energy-consuming. Besides increasing mass transfer impedance, leaf epiphytic biofilm activity thus strongly affects the seagrass leaf microenvironment in the light by inducing higher O₂ concentration and pH, increasing CO₂ limitation and reducing the leaf photosynthetic efficiency.