The effects of changing the amount of silica in the cell wall of diatom
prey, on the production, decomposition rate and sinking velocity of fecal
pellets of the calanoid copepod,
In the marine environment, zooplankton fecal pellets constitute a main vehicle for transporting biogenic elements to the sediments, although a substantial proportion of this flux is recycled or repackaged in the water column by microbial decomposition and zooplankton coprophagy (Turner, 2002, 2015). Diatoms are among the most abundant phytoplankton, and they represent a main component in the diet of zooplankton in marine environments. Studies show that zooplankton with a diatom diet usually produce fecal pellets that sink faster than those on other diets (Feinberg and Dam, 1998). Dagg et al. (2003) reported that the contribution of fecal pellets to the flux of particulate organic carbon (POC) and biogenic silica (bSi) is higher during the spring diatom bloom than during the summer within the Antarctic Polar Front region. Similarly, Goldthwait and Steinberg (2008) reported an increase in mesozooplankton biomass and fecal production and flux inside cyclonic and mode-water eddies. However, González et al. (2007) reported a negative correlation between the vertical carbon flux of diatoms and the production of fecal material in a time-series study in the upwelling waters off Chile.
The quantity and characteristics of the fecal pellets produced by zooplankton depend on several factors. The pellet production rate is reported to be affected by the rate of ingestion and assimilation efficiency (Butler and Dam, 1994; Besiktepe and Dam, 2002). It has also been demonstrated that the type of diet can affect the characteristics of the fecal pellets produced – including size, density and sinking rates (e.g., Feinberg and Dam, 1998, and references therein). In addition, the decomposition rate of pellets varies with water temperature, as well as with both microbial and metazoan activity (Poulsen and Iversen, 2008; Svensen et al., 2012). Factors that contribute to the sinking velocity of the pellets include size, density and shape, all of which can vary dramatically both among different zooplankton species and within the same zooplankton species feeding on different types of prey (Fowler and Small, 1972; Turner, 1977; Feinberg and Dam, 1998). Turbulence in the water column, the presence or absence of a peritrophic membrane, and the production of microbial gas within a peritrophic membrane might also affect the sinking rate of pellets (Honjo and Roman, 1978; Bathmann et al., 1987). Indeed, the sinking rate and decomposition rate are the two most important parameters used, to determine whether a pellet will or will not be successfully transported into deeper water before its contents are degraded. For example, a slowly sinking pellet is more likely to decompose and become part of the recycled materials before it exits the euphotic zone (Dagg and Walser, 1986).
The cell wall (frustrule) of diatoms is composed of two silicate shells, which are believed to act as a defense mechanism to prevent ingestion by grazers (Pondaven et al., 2007); thus different levels of silicification of the frustrule might affect the grazing rate of copepods (Friedrichs et al., 2013, Liu et al., 2016). The silica content of the cell wall of diatoms is not only species-specific, but it is also affected by environmental parameters such as light, temperature, salinity, pH, nutrients and trace metals (Martin-Jézéquel et al., 2000, and references therein; Claquin et al., 2002; Vrieling et al., 2007; Herve et al., 2012; Liu et al., 2016). Although the frustule has no nutritional value for zooplankton, it is thought to provide ballast, which is especially advantageous when the fecal pellets are sinking. Hence, pellets with a high diatom biomass generally exhibit higher levels of export of POC (Armstrong et al., 2002; François et al., 2002; Klaas and Archer, 2002). Thus, the content of the zooplankton diet (and therefore the type and concentration of ballast minerals ingested) might strongly affect the sinking velocity of the fecal pellets produced, and hence the vertical flux of biogenic silica and carbon.
Most of the studies describing the production rates and characteristics of
copepod fecal pellets have focused on aspects such as food types (Feinberg
and Dam, 1998), or the different periods of phytoplankton blooms (Butler and
Dam, 1994). There are currently no reports that describe the effect of the
silica content of diatoms on the production, degradation and sinking of
fecal pellets. Liu et al. (2016) recently demonstrated that the diatom
The herbivorous copepod
The diatom
Summary of the concentration and cellular silica content of the diatom prey in each experiment.
Active adult female
In the fecal pellet production experiments, five replicate bottles
containing one copepod per bottle and two control bottles without a grazer
were used. All the bottles were filled with 100 mL freshly prepared media
consisting of 0.2
Degradation rate of the fecal pellets produced by
Note: HSi: high silica content, LSi: low silica content.
In order to obtain fresh pellets for the degradation experiments, two
plastic beakers were prepared for the high- and low-silica content prey.
Each beaker contained 7–8 copepods and 700 mL culture medium, prepared as
described for the production experiments. After 12 h of incubation
(except for experiment 3, which was incubated for 18 h), the medium
was sieved through a 40
Experiments to estimate the fecal pellet sinking rate were conducted by
obtaining fecal pellets using the degradation experiment procedure
(described above) but with an incubation time of 24 h. After collecting
all the fecal pellets from the beakers, 50 intact pellets were selected and
suspended in 260 mL 0.2
The water samples containing the fecal pellets in the 50 mL polypropylene tubes were allowed
to settle for 24 h. The upper water was then removed smoothly and the
remainder was poured into the well of a six-well plate and the number of
pellets was counted using an inverted microscope (Olympus IX51) at
100
The rate of degradation of the fecal pellets was calculated from the loss of fecal pellet equation,
described by
The rate that fecal pellets
sank was calculated from the formula reported by Bienfang et al. (1982),
which was originally used to measure the average sinking rate of
phytoplankton. Thus,
In addition, the density of the fecal pellets was calculated using the
semi-empirical equation deduced by Komar (1980), as follows:
The cellular silica content of
The cellular silica content of first- and second-generation
The grazing response of
Grazing rate
The rate of fecal pellet production varied both with the silica content and
the concentration of the prey (Fig. 3a). At a high prey concentration,
The degradation rate of fecal pellets was significantly different when the
copepods fed diatoms with different silica content (Table 2). The
degradation rate of the fecal pellets produced from the low-silica prey was
approximately 4–5-fold higher than that of the pellets generated from the
high-silica prey, irrespective of the prey concentration or the period of
degradation incubation. In addition, the degradation rate of the fecal
pellets from low prey concentration was significantly higher than ones from
high prey concentration after an incubation period of 24 h (
Production rate
The sinking rate of fecal pellets was also different for the high and low
prey concentrations (Fig. 4). At a high concentration of prey, the sinking
rates of the pellets produced by the high- and low-silica prey (i.e., 3.05
and 3.13 cm min
The grazing activity of copepods varies not only with the concentration of the prey but also with the nutritional quality of the prey. In our study, the grazing and clearance rates determined with varying food concentrations followed a similar trend to that described in the literature (e.g., Frost, 1972). In addition, the grazing activity was affected by the cellular silica content of the prey, as has been observed with other copepod species (Liu et al., 2016). Silicification has been suggested to be one of the strategies that is used by diatoms to protect them from ingestion by grazers (Pondaven et al., 2007). Friedrichs et al. (2013) examined the mechanical strength of the frustules of three diatom species and measured the feeding efficiency of copepods on these diatoms. Their results showed that the diatom species with the more weakly silicified frustules and the highest growth rate was the least stable and was fed upon the most, whereas the species with the most complex frustule exhibited the greatest stability and was fed upon the least. Within the same species of diatom, different growth rates have resulted in different amounts of silica in the frustule (Claquin et al., 2002). This results in higher copepod ingestion and clearance rates for diatoms with a low silica content when compared with those for diatoms with a higher silica content (Liu et al., 2016). The results obtained in the current study are consistent with those reported by Friedrichs et al. (2013) and Liu et al. (2016).
The sinking rate (bars) and calculated density (open dots) of the
fecal pellets generated by
Previous studies indicate that while there is a linear relationship between the ingestion rate and the total number of fecal pellets produced per unit time (Ayukai and Nishizawa, 1986; Ayukai, 1990), there is a high level of variation among different diets (Båamstedt et al., 1999, and references therein; Besiktepe and Dam, 2002). In addition, the size of fecal pellets increases as the concentration of the food increases, such that they reach a maximum size when the concentration of food is above the saturation level (Dagg and Walser, 1986; Butler and Dam, 1994). Our results confirmed these previous findings and demonstrated that the size of fecal pellets produced was only affected by the concentration of prey, and fecal pellets did not show any significant size differences when comparing prey of high and low cellular silica content. Butler and Dam (1994) reported that when sufficient food was available, the size of the fecal pellets varied with the nutritional quality (e.g., the C : N ratio) of the prey. Since diatoms with different silica content (generated by varying the light intensity) do not differ in their cellular C : N ratio (Claquin et al., 2002; Liu et al., 2016), these ratios did not affect the size of the pellets produced.
The relationship between degradation rates and surface : volume ratio
of fecal pellets from different experimental treatments. HSi and LSi are
high- and low-silica-content diatoms, respectively; high and low prey are
high and low prey concentrations, respectively; 48 and 24 h are the
incubation periods used for the degradation experiments. The error bars show
The degradation rate and sinking velocity of the fecal pellets are highly
dependent on the characteristics of the pellets, which are in turn affected
by the quality and quantity of the food ingested (Feinberg and Dam, 1998;
Turner, 2002, 2015, and references therein). For example, it is known that
the decomposition rate of the fecal pellets is affected by diet, pellet size
and the producer of the pellets (e.g., Shek and Liu, 2010), but no research
has addressed the degradation rates of fecal pellets produced by prey under
different stoichiometric conditions. Hansen et al. (1996) estimated the
degradation rate of fecal pellets produced from diets of
The sinking rate of fecal pellets is usually considered to be related to their size and density, which is in turn dependent on the concentration and composition of the prey (Bienfang, 1980; Urban et al., 1993; Feinberg and Dam, 1998). We also demonstrated that fecal pellet size, sinking rate and density were correlated with the concentration of prey (Figs. 3b, 4), especially in the low-silica diatom prey treatment. Using the ratio of ingestion rate : fecal pellet production rate ratio as an index to compare the diatom content per fecal pellet, no differences were found in pellets produced from diets of the same silica content (Fig. 6), indicating that prey concentration does not affect the package content of the fecal pellets. On the other hand, copepods were shown to pack fewer hard-shelled (i.e., high-silica) diatoms into each fecal pellet in comparison to the soft-shelled (i.e., low-silica) diatoms, although these data were not significantly different statistically (Fig. 6).
The grazing rate : fecal pellet production rate ratio of each treatment. HSi and LSi are the high- and low-silica diatom prey, respectively. The error bars show 1 standard deviation.
The fecal pellets of copepods are formed in the midgut surrounded by a
peritrophic membrane, which is believed to protect the gut wall from the
sharp edges of the prey's cell wall. Moreover, the different sizes of fecal
pellets with similar prey content per fecal pellet are thought to result
from the decrease gut passage time with the increase of food
concentration. A high prey concentration results in the food passing through
the gut more quickly and results in incomplete digestion, whereas a low prey
concentration allows the food to be kept in the intestinal tract for a
longer time and therefore digestion is relatively more complete. We showed
that the silica content of the diatom cell wall determines the density and
sinking rate of the fecal pellets when the prey concentration was low due to
complete digestion. In addition, we showed that only the low concentration
of low-silica prey group resulted in a significantly lower fecal pellet density
and sinking rate. In previous studies, the sinking rate and density of the
fecal pellets of
The L ratio (m
To compare the combined effects of sinking and degradation rates for each treatment, the reciprocal length scale, or L ratio, which is the fraction of pellet degradation per unit length traveled, was calculated (Feinberg and Dam, 1998). The product of the L ratio multiplied by the depth of the mixed layer can then be used to provide the degree of degradation of a pellet within this layer. The results from such calculations suggest that some diets might result in pellets that are substantially recycled within the epipelagic layer whereas others result in pellets that are exported out of the mixed layer in a relatively non-degraded manner. It should be pointed out, however, that the degradation rates we calculated are likely to be highly underestimated due to the absence of zooplankton activities. For example, it has been reported that copepod ingestion of entire fecal pellets (i.e., coprophagy) or only partial breakdown of fecal pellets might dramatically reduce the overall downward transport of fecal material and thus increase its retention in the epipelagic layer (Lampitt et al., 1990; Gonzalez and Smetacek, 1994; Svensen et al., 2012). For the same reason, plus the absence of turbulence in our experimental set-up, our sinking rate measurements are likely to be overestimated. Nevertheless, the L ratio provides a relative indicator of the export efficiency of the fecal pellets produced on diatom diets of different silica content and can be used for a comparison with copepod fecal pellets produced with other diets. Our results also show that pellets produced from high-silica-content diatoms are more likely to sink out of the mixed layer before being degraded, when compared with pellets from low-silica-content diatoms. On the other hand, fecal pellets produced from a low concentration of prey with low-silica content are the most likely to be degraded in the mixed layer (Table 3). Our results suggest that the grazing activity of copepods might result in organic matter being mostly recycled in the mixed layer during the fast-growth period of diatoms (e.g., at the beginning of the bloom), whereas it could accelerate the export of POC to the deep ocean by producing fast-sinking fecal pellets during the slow-growth period of diatoms (e.g., during the senescent stage of the diatom bloom).
In conclusion, the silica content of the cell wall of diatoms can affect the grazing activity of copepods and influence the rates of production, decomposition and sinking of their fecal pellets. Our findings suggest that it is not only the nutritional quality but also the digestion process of copepods that can result in the different characteristics of the pellets produced. In addition, it is a combination of both degradation and sinking rates (which are affected by the abundance and cellular silica content of the diatom prey among other physicochemical factors) that determines the efficiency of the downward export of biogenic silica and organic carbon by fecal pellets.
Financial support for this study was from the Research Grant Council of Hong Kong (661610, 661911 and 661912) and the National Key Scientific Research Projects of China (2015CB954003). Additional support was provided by the TUYF Charitable Trust (TUYF10SC08). Edited by: G. Herndl Reviewed by: two anonymous referees