BioProject

Copepods are important suppliers of bioreactive compounds for marine bacteria through fecal pellet production, sloppy feeding and excretion. However, the interaction between copepods and bacteria is poorly understood in the marine environment. Herein, we investigated the nitrogen and phosphorus compounds excreted by copepods fed by two size fractions of natural food (<20 µm and 20-150 µm) in the upwelling zone of central south of Chile in late summer and spring. We then assessed the biogeochemical response of the bacterial community and their structure, in terms of total and active cells, to the addition of these dissolved compounds. Results revealed that copepods actively excreted nitrogen and phosphorus compounds, mainly in the form of ammonium and dissolved organic phosphorus (DOP), reaching excretion rates up to 2.6 µmol L-1 h-1 and 0.05 µmol L-1 h-1, respectively, in spring. The high accumulation of excreted nitrogen and phosphorus was associated in both periods with the 20-150 µm size fraction of food, which coincided with the main food item available for copepods in the field. Variation in the abundance (contribution to total bacterial sequences) of bacteria taxa was characterized by a decrease of Bacteroidetes and Cyanobacteria, and an increase of Proteobacteria, mainly Alphaproteobacteria and Gammaproteobacteria depending on treatment. Some orders, such as Flavobacteriales presented a rapid response to the products excreted by copepods, increasing their abundance between 3 to 10% in the first four hours of the experiment evolution. Moreover, the specific bacterial activity (16S rRNA: rDNA) suggested a differential response associated with the two size fractions of food provided to copepods, Alphaproteobacteria was stimulated in the treatment enrichment with excretion products by copepods fed with <20 µm size fraction, but inhibited at the 20-150 µm size fraction in March. In December, Alphaproteobacteria was active in both treatments, but in the treatment enriched with excretion products by copepods fed with <20 µm size fraction was higher than the treatment enriched with excretion products by copepods fed with 20-150 µm size fraction. Our findings indicate that the different size fractions of food provided to copepods have an impact on the main products excreted by copepods and showed that bacterial community can respond in a short period of time (<4 h) to excreted organic and inorganic substrates.