Abstract
Our current knowledge on the microbial component of zooplankton diet is limited, and it is generally assumed that bacteria-sized prey is not directly consumed by most mesozooplankton grazers in the marine food webs. We questioned this assumption and conducted field and laboratory studies to examine picocyanobacteria contribution to the diets of Baltic Sea zooplankton, including copepods. First, qPCR targeting ITS-1 rDNA sequence of the picocyanobacteria Synechococcus spp. was used to examine picocyanobacterial DNA occurrence in the guts of Baltic zooplankton (copepods, cladocerans and rotifers). All field-collected zooplankton were found to consume picocyanobacteria in substantial quantities. In terms of Synechococcus quantity, the individual gut content was highest in cladocerans, whereas biomass-specific gut content was highest in rotifers and copepod nauplii. Moreover, the gut content in copepods was positively related to the picocyanobacteria abundance and negatively to the total phytoplankton abundance in the water column at the time of sampling. This indicates that increased availability of picocyanobacteria resulted in the increased intake of this prey and that copepods may rely more on picoplankton when food in the preferred size range declines. Second, a feeding experiments with a laboratory reared copepod Acartia tonsa fed a mixture of the picocyanobacterium Synechococcus bacillaris and microalga Rhodomonas salina confirmed that copepods ingested Synechococcus, even when the alternative food was plentiful. Finally, palatability of the picocyanobacteria for A. tonsa was demonstrated using uptake of 13C by the copepods as a proxy for carbon uptake in feeding experiment with 13C-labeled S. bacillaris. These findings suggest that, if abundant, picoplankton may become an important component of mesozooplankton diet, which needs to be accounted for in food web models and productivity assessments.
Highlights
The smallest photosynthetic organisms include autotrophic picoplankton, a diverse group united by size,2 mm
It has been suggested that ‘‘picocyanobacteria are in a size range suitable for utilization by nauplii and early copepodite stages as well as rotifers’’ [2], and some field and experimental studies indicate that ingestion of unicellular picoautotrophs by copepod species does occur [17,18]
For size specific GC (ssGC), a different pattern was observed, with the values decreasing with the body size and differing significantly between the zooplankton groups: copepods vs. microzooplankton (t17 = 12.507, p,0.0001), copepods vs. cladocerans (t6 = 5.60, p,0.0014), and cladocerans vs. microzooplankton (t7 = 3.402, p,0.0145; Figure 1)
Summary
The smallest photosynthetic organisms include autotrophic picoplankton, a diverse group united by size ,2 mm This group contributes as much as 40% of global ocean primary productivity and is mainly composed by picocyanobacteria [1,2]. Appendicularians [10], cladocerans [11,12], rotifers [12], and bivalve larvae [13] are known to substantially feed on picoplankton, but not copepods These most important grazers in marine systems do not feed efficiently on particles of this size as shown by feeding experiments with algal cultures [14] and natural phytoplankton assemblages [15]. At low concentrations of preferred phytoplankton species, zooplankton may switch to more abundant suboptimal prey [20]
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