Effect of Diatom Silica Content on Copepod Grazing, Growth and Reproduction

Hongbin Liu,Mianrun Chen,Paul J Harrison,Feng Zhu

doi:10.3389/fmars.2016.00089

Abstract

Diatoms are often a major food source for zooplankton and contribute significantly to vertical POC flux through sinking of dead cells, aggregates and zooplankton fecal pellets. The silica content of diatoms varies among different species and within a species growing under different environmental conditions and physiological status. However, to-date there has been no investigation of the effect of diatom silica content on zooplankton grazing, growth and reproduction. We conducted a series of experiments using the diatom Thalassiosira weissflogii with different silica content achieved by growth under high and low light and these cells were fed to a copepod, Parvocalanus crassirostris. Our results show that this copepod strongly preferred cells with low silica content over high silica-containing cells, with the ingestion rate on low silica diatoms being 2-3 times higher than that on high silica diatoms. Fecal pellet production rate was significantly higher for copepods feeding on highly silicified cells. Furthermore, copepod growth rate (measured as an increase in wet weight), egg production rate and hatching success were all severely compromised under a high silica diatom diet. Females of P. crassirostris feeding on a low silica diatom diet produced an average 90 eggs during a one day incubation, while those fed with high silica diatoms produced only 11 eggs per day. Similarly, the hatching success during a 3-day period was 82 ± 17% and 23 ± 36% for the low and high silica diatom treatments, respectively, with zero success observed in ~65% of the females feeding on high Si diatoms. Our findings have important ecological implications for the biological pump and may alter our previous view of the role of diatoms in planktonic food webs and the role of the degree of silicification in controlling amount of POC flux to deeper waters.

Highlights

Carbon uptake by marine phytoplankton, and its export as organic matter to the interior of the ocean, lowers the partial pressure of carbon dioxide in the upper ocean and facilitates the downward diffusive flux of atmospheric CO2
N, and P-limited continuous cultures of Thalassiosira pseudonana, Claquin et al (2002) demonstrated that a decrease in growth rate resulted in a significant increase in the time spent in the period of cell wall synthesis and mitosis (G2+M), when Si uptake is usually high
Claquin et al (2002) found that regardless of the type of limitation, biogenic silica increased per cell and per cell surface area with decreasing growth rate, indicating that silicification was not coupled to the metabolism of N or P

Summary

Introduction

Carbon uptake by marine phytoplankton, and its export as organic matter to the interior of the ocean (i.e., the “biological pump”), lowers the partial pressure of carbon dioxide (pCO2) in the upper ocean and facilitates the downward diffusive flux of atmospheric CO2. Diatoms tend to dominate phytoplankton communities in wellmixed coastal and upwelling regions, as well as along the seaice edge, where sufficient light, inorganic nitrogen, phosphorus, silicate, and trace elements are available to sustain their growth (Armbrust, 2009). They form spring blooms in the temperate and polar seas and after artificial or natural iron inputs in high nitrate low chlorophyll (HNLC) regions (Ducklow and Harris, 1993; Boyd et al, 2007). Pigment and microscopic analyses of samples after the passing of the Hawaiian Lee cyclone revealed that the bloom within the eddy was dominated by large diatoms and they accounted for almost 85% of the total phytoplankton biomass (Brown et al, 2008; Rii et al, 2008)

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