| | | 19.10 The importance of microbial and viral loops in carbon cycling |
In the planktonic ecosystem bacteria are the main consumers of dissolved organic carbon, and bacterial carbon production can be as much as half, or at times even higher than the primary production because carbon can be recycled several times through the microbial loop [174]. This seems to be the case in the Gulf of Riga during summer, when bacterial production was as high as primary production [470]. The carbon fixed by the phytoplankton is released by direct exudation, cell lysis and directly from the higher trophic levels (e.g. sloppy feeding). All this dissolved organic matter can be utilised by bacteria. Viral lysis of bacterioplankton is thought to make the bacteria-DOM-bacteria loop even more important [463]. Thus, there are several routes by which the carbon from the primary producers ends up in bacteria and can be cycled several times through bacteria. One important route is the lysis of the cells due to viral infection. On average more than half of the bacterial production in the Gulf of Riga was lost through viral lysis [470]. This bacterial carbon is recycled back to bacteria. Unfortunately it is not possible to estimate how much of the other plankters in the Gulf were lysed. In particular phytoplankton blooms may be attacked by viral infection.
Grazing by heterotrophic nanoflagellates and viral lysis have been recognised as two main reasons for bacterial mortality in aquatic ecosystems (e.g. [166]). Viral lysis of bacterial biomass produces dissolved organic matter to be utilised again by bacteria, while grazing can transfer bacterial carbon and other elements to higher trophic levels. In the Gulf of Riga heterotrophic nanoflagellates were estimated to consume on average one third of the bacterial production [470]. Due to respiration and sloppy feeding, only part of this carbon finally ends up to higher trophic levels.
It appears that a larger proportion of the carbon channelled into bacteria is recycled among the bacteria than is transferred to the higher trophic levels in the Gulf of Riga. Between trophic levels bacteria are thus more a sink than a link. Viral lysis of the cells seems to be very important at least among the bacteria. This results in the consumption of carbon among the bacteria and fast turnover of mineral nutrients. While the amount of carbon recycled through bacteria is as high as the primary production during summer, the bacterial production is lower during the other seasons, where bacterial production comprised 13 to 45% of primary production [470]. Thus the importance of the microbial loop in the carbon cycling of the whole plankton community is less pronounced.
| | | 19.10 The importance of microbial and viral loops in carbon cycling |