Prochlorophytes as secondary prey for heterotrophic nanoflagellates in the deep chlorophyll maximum layer of the (sub)tropical North Atlantic

Growth rates in prochlorophytes and their loss rates to grazing were investigated by the dilution technique in the lower euphotic zone of the (sub)tropical North Atlantic. Directly above the ‘deep chlorophyll maximum’ (DCM), prochlorophyte growth rates were approximately 0.3 d–1and grazing rates 0.4 d–1; both rates were considerably lower in and directly below the DCM. Grazing was only weakly affected by filtration through 3 μm pore-size filters, indicating heterotrophic nanoflagellates as the main grazers. Discrimination between day and night revealed that the prochlorophytes above the DCM, with doubling times of 3 d or more, divided during the day whereas at greater depth division occurred during the night. In contrast, grazing was always restricted to the daylight period and grazing rate was independent of prochlorophyte abundance, growth rate or time of division. Prochlorophyte grazing rates were, however, correlated with the loss rates in the much more abundant heterotrophic bacteria in the same layers. We conclude therefore that prochlorophytes are consumed as a secondary prey by heterotrophic nanoflagellates grazing on heterotrophic bacteria.