Distribution and morphological features of aerobic anoxygenic phototrophs in the central Adriatic Sea

Aerobic anoxygenic phototrophs (AAP’s) have drawn much attention as they use carbon to sustain their metabolism and growth, but can derive a portion of their energy requirements by harvesting light using pigment bacteriochlorophyll a. Typically they account for 1-7% of total bacteria in the euphotic zones of the world’s oceans. Infrared epifluorescence microscopy was used to determine distribution of AAP’s along a trophic and vertical gradient in central Adriatic Sea during April and June 2016. AAP abundances ranged from 0.44 to 23.45 x 104 cells mL-1, with highest values on the stations with riverine influence. In general, the abundances decreased with depth. The proportion of AAP’s in total heterotrophic bacteria ranged from 6.84% in June to 13.03%. in April. Generally, AAP cell length was higher during warmer period, but the highest value was measured in colder period. Biomass contribution of AAP’s increased distinctly along the trophic gradient. In this survey AAP’s average biomass ranged from 4.38 to 31.85 µgCL-1. However, in our previous studies AAP’s were overlooked when calculating heterotrophic biomass. We observed that AAP bacteria thrive better in warmer period and also in more eutrophic areas, which suggests positive effects of temperature and nutrients as influencing factors in the Adriatic Sea. The data from our survey indicate an important contribution of AAP’s to carbon flow through microbial food web. In addition, the data allow preferably more accurate estimation of carbon flow through heterotrophic bacteria in the Adriatic Sea.