Estimation of Microbial Abundances and Their Respiration Rates in a Specialized Coastal Water Body

Microbial communities in an ecosystem are crucial for having active participation in shaping the biological and biochemical cycles of this ecosystem. In this study, we estimated marine bacterial counts and the microbial respiration rates in seawater samples collected from Gorliz beach, Bilbao, Spain, using DAPI (4′,6-diamidino-2-phenylindole)-based epifluorescence microscopy and optical oxygen sensors. The microbial density was 1.6 × 10⁶ cells/mL (± 47,948 SE; Standard Error), which is consistent with values typically reported for temperate coastal regions. In addition, microbial respiration rate based on the oxygen consumption was around a mean of 0.0127 μmol O₂/L/h (± 0.0026 SE) over 14 hours of incubation in the dark. These findings suggested an abundant and meaningful active microbial community in this coastal ecosystem, highlighting the critical role of microorganisms in regulating coastal ecosystem dynamics, local oxygen budget, and carbon biogeo-cycling. Hence, this study underlines the need for sequential, spatial, and temporal investigations of microbial communities and their biochemical processes in response to environmental changes and anthropogenic activities.