Abstract

Background: Little is known about interactions between filamentous heterotrophs and filamentous cyanobacteria.
Here, interactions between the filamentous heterotrophic bacteria Fibrella aestuarina (strain BUZ 2) and Fibrisoma
limi (BUZ 3) with an axenic strain of the autotrophic filamentous cyanobacterium Nostoc muscorum (SAG 25.82)
were studied in mixed cultures under nutrient rich (carbon source present in medium) and poor (carbon source
absent in medium) conditions.
Findings: F. aestuarina BUZ 2 significantly reduced the cyanobacterial population whereas F. limi BUZ 3 did not.
Physical contact between heterotrophs and autotroph was observed and the cyanobacterial cells showed some level of damage and lysis. Therefore, either contact lysis or entrapment with production of extracellular compounds
in close vicinity of host cells could be considered as potential modes of action. The supernatants from pure heterotrophic cultures did not have an effect on Nostoc cultures. However, supernatant from mixed cultures of BUZ 2 and Nostoc had a negative effect on cyanobacterial growth, indicating that the lytic compounds were only produced in the presence of Nostoc. The growth and survival of tested heterotrophs was enhanced by the presence of Nostoc or its metabolites, suggesting that the heterotrophs could utilize the autotrophs and its products as a nutrient source. However, the autotroph could withstand and out-compete the heterotrophs under nutrient poor conditions.
Conclusions: Our results suggest that the nutrients in cultivation media, which boost or reduce the number of
heterotrophs, were the important factor influencing the outcome of the interplay between filamentous heterotrophs and autotrophs. For better understanding of these interactions, additional research is needed. In particular, it is necessary to elucidate the mode of action for lysis by heterotrophs, and the possible defense mechanisms of the autotrophs.