Prokaryotes, Eukaryotes, and Viruses
A | A rank–abundance curve showing marine prokaryotes and eukaryotes. The most abundant organisms in the ocean, such as SAR11, are probably K-selected organisms that have slow maximum growth rates but are resistant to viral lysis and grazing. By contrast, less abundant organisms, such as Roseobacter spp. and Vibrio spp., are capable of rapid growth but are highly susceptible to viral infection and grazing. Consequently, the rarer microorganisms are more r selected, whereas the microorganisms that dominate the biomass are the most K selected. The yellow arrow represents taxa that are typically present in low abundance and periodically encounter conditions that are conducive to rapid growth, but as their abundance increases the rates of viral infection also increase, resulting in lysis of the host cells and a return to low abundance. b | A rank–abundance curve showing marine viruses. In contrast to the most abundant prokaryotes, the most abundant viruses are r selected. They are virulent, have small genome sizes and are short-lived. The population structure is probably uneven, with many of the viruses at any given time being progeny from a limited number of lytic events. The rarer, more K-selected viruses have larger genomes, decay slowly and can form stable associations with their hosts. Also included in this group are some RNA and DNA viruses that are long-lived and have low virulence. IHNV, infectious haematopoietic necrosis virus.