Picophytoplankton community from tropical marine biofilms

Although the importance of picophytoplankton (< 3 μm) as the major primary producers is well established in the oceanic and coastal marine environment, occurrence of this community in the marine biofilms is not explored. Considering their small size and high abundance which is next to bacteria, the initial heterotrophic colonizers, it was hypothesized that picophytoplankton are the first autotrophic colonizers in marine biofilms. In this study biofilms were developed by immersing glass slides in the tropical waters of the Dona Paula Bay up to a period of 192 h. The presence of picophytoplankton and the temporal variations in community structure was investigated flow cytometrically after every 24 h. Biofilms were also developed in the laboratory by immersing the glass slides in fresh natural seawater and the sequence of appearance of the microorganisms was assessed with high frequency samplings. Field observations showed the presence of three groups of picophytoplankton, two prokaryotes, Prochlorococcus-like organisms (PRO), Synechococcus (SYN) and the picoeukaryotes (PEUK). Nanoeukaryotes, which are mostly represented by diatoms were also monitored. In the total biofilm community, prokaryotes were dominant throughout the study period wherein contribution of SYN was highest (50%) in the earlier stages which were later overtaken by PRO-like cells. The contribution of PEUK and nanoeukaryotes was always below 20%. Picophytoplankton contribution to the total photosynthetic biomass was > 60% in the initial period of biofilm formation, both in terms of numbers and biomass with PEUK as the major contributors. However, after 2 days of incubation, their contribution to total chlorophyll declined thus revealing that although their numbers were increasing, picophytoplankton were succeeded by nanoeukaryotes in terms of biomass. Laboratory experiments revealed that heterotrophic bacteria and picophytoplankton appeared within 5 min in the biofilms which were followed by the nanoeukaryotes after 5 h. This study shows that picophytoplankton are the pioneer autotrophic colonizers in the tropical marine biofilms, suggesting an essential role in the biofilm food web dynamics, especially in the initial stages.

► Flow cytometric study of picophytoplankton in natural tropical marine biofilms.
► Picophytoplankton contribution to the total biofilm photosynthetic biomass was > 60%.
► Bacteria and picophytoplankton appeared within 5 min on the substrata.
► Picophytoplankton are the pioneer autotrophic members in the tropical marine biofilms.