Abstract
As important bacterivores in planktonic food webs, mixotrophic nanoflagellates cancause mortality in marine Synechococcus spp. Our previous study found that the pigmented nanoflagellate (PNF) has a significant grazing impact on Synechococcus spp. In the current study, we applied the dilution approach to test the growth and grazing rates of nanoflagellates on Synechococcus spp. We then compared the differences between experimental nutrient additions and in situ conditions in the coastal waters of the East China Sea during the summer season from July to September. The growth rates of Synechococcus spp. in the ambient environment were between 0.54 and 0.62 day−1, which were slightly higher than the 0.56 and 0.66 day−1 with nutrient enrichment in summer. In contrast, our nutrient enrichment experiments produced a marked decline approximately from 21% to 58% in the nanoflagellate grazing rate on Synechococcus spp. The reason was that the mixotrophic PNFs directly used the added nutrients and reduced their supply of nutrients from prey during the incubation experiments.
Highlights
Synechococcus spp. is a genus of cyanobacteria that lives in coastal environments and is responsible for primary production [1]
We found that PNFs have more abundance than nonpigmented nanoflagellate communities, comprising about 60% of the total nanoflagellate population at our study site
Our results show that the grazing rate of PNFs on Synechococcus was significantly lower in nutrient-enriched treatments than in the in situ condition
Summary
Synechococcus spp. is a genus of cyanobacteria that lives in coastal environments and is responsible for primary production [1]. Some previous studies demonstrate the importance of the grazing mortality of marine Synechococcus, of which daily removal rate of Synechococcus assemblage could be as high as 50% [4,5]. It is apparent that grazing is critical in the marine food webs and provides energy to the higher trophic levels. Grazing by heterotrophic nanoflagellates is showed by top-down control in the bacterial communities, pigmented nanoflagellates (PNFs) have been considered as important bacterivores in planktonic food webs in recent studies [6,7,8]. Some field studies observed that in marine and freshwater systems, PNFs outnumber the heterotrophic nanoflagellates, and have a greater grazing impact on the bacterial community [6,8,9,10]. PNFs can survive in different environments by relying on bacterivory during low-light conditions, darkness, dissolved nutrient concentration, and photosynthesis when prey concentration is low [11,12,13,14]
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