Copepod feeding in a coastal area of active tidal mixing: diel and monthly variations of grazing impacts on phytoplankton biomass

This study examined monthly feeding rates and grazing impact on phytoplankton biomass, as well as diel feeding rhythms of four key copepod species in a tidally well mixed estuary (Asan Bay, Korean Peninsula). Monthly ingestion rates estimated based on gut pigment analysis were closely associated with their peak densities, but not with phytoplankton biomass, implying high ingestion may be related to reproductive output for population growth. The three smaller copepods, Acartia hongi, Acartia pacifica and Paracalanus parvus, showed feeding preference for smaller phytoplankton (<20 μm) with higher clearance rates, whereas the larger Calanus sinicus preferred larger phytoplankton. Acartia pacifica and P. parvus showed distinct increased nocturnal feeding rates as measured with gut fluorescence, whereas A. hongi showed no significant day–night differences. Copepod diel feeding patterns were not associated with food quantity, and endogenous physiological rhythm might be hypothesized as responsible for the observed diel feeding patterns. Grazing impact on phytoplankton biomass by the four copepods in the estuary was on average 8% (range 0.2–29.8%) of the phytoplankton standing stock, similar to values reported in other coastal waters. Very high copepod abundances but low daily carbon ration (<20% for all copepods) provided by feeding on phytoplankton indicate that copepods also grazed on other non‐phytoplankton foods in Asan Bay.     

Molecular insights into the circadian clock in marine diatoms

The circadian clock is a fundamental endogenous mechanism of adaptation that coordinates the physiology and behavior of most organisms with diel variations in the external environment to maintain temporal homeostasis. Diatoms are the major primary producers in the ocean. However, little is known about the circadian clock in marine diatoms compared with other organisms. Here, we investigated circadian clock genes, their expression patterns, and responses to environmental stimuli such as light, nitrogen and phosphorus in two marine diatoms, Skeletonema costatum and Phaeodactylum tricornutum, using a combination of qRT-PCR and bioinformatic analysis. We identified 17 and 18 circadian clock genes in P. tricornutum and S. costatum, respectively. Despite significant evolutionary differences, these genes were similar to those of the higher plant Arabidopsis. We also established a molecular model for the marine diatom circadian clock comprising an input pathway, core oscillator, output pathway, and valve effector. Notably, the expression patterns of core clock genes (circadian clock associated 1 (CCA1), late elongated hypocotyl (LHY) and timing of cab 1 (TOC1)) in both species differed from those of terrestrial plants. Furthermore, the expression of these genes was influenced by variations in ambient light, nitrogen and phosphorus availability. Although marine diatoms and higher plants share common circadian clock components, their clock genes have diverged throughout evolution, likely as a result of adapting to contrasting environments.