Respiration, Production, and Growth Efficiency of Marine Pelagic Fungal Isolates

Abstract

Despite recent studies suggesting that marine fungi are ubiquitous in oceanic systems and involved in organic matter degradation, their role in the carbon cycle of the oceans is still not characterized and fungal respiration and production are understudied. This study focused on determining fungal growth efficiencies and its susceptibility to temperature differences and nutrient concentration. Hence, respiration and biomass production of three fungal isolates (Rhodotorula mucilaginosa, Rhodotorula sphaerocarpa, Sakaguchia dacryoidea) were measured in laboratory experiments at two temperatures and two nutrient concentrations. We found that fungal respiration and production rates differed among species, temperature, and nutrient concentration. Fungal respiration and production were higher at higher temperatures, but higher fungal growth efficiencies were observed at lower temperatures. Nutrient concentration affected fungal respiration, production, and growth efficiency, but its influence differed among species. Altogether, this study provides the first growth efficiency estimates of pelagic fungi, providing novel insights into the role of fungi as source/sink of carbon during organic matter remineralization. Further research is now needed to unravel the role of pelagic fungi in the marine carbon cycle, a topic that gains even more importance in times of increasing CO₂ concentrations and global warming.

Keywords: growth efficiency; marine fungi; production; respiration.

Figure 1

Growth curves of Rhodotorula mucilaginosa, Rhodotorula sphaerocarpa, and Sakaguchia dacryoidea in the high nutrient media; measured via optical density (OD; 660 nm). Dashed lines represent the growth at 15 °C and continuous lines the growth at 5 °C. The black dots indicate the sampling points during the exponential growth phase.

Figure 2

Growth curves of Rhodotorula mucilaginosa, Rhodotorula sphaerocarpa, and Sakaguchia dacryoidea in the low nutrient media; measured via optical density (OD; 660 nm). The black dots indicate the sampling points during the exponential growth phase.

Figure 3

Production rates of the three fungal isolates Rhodotorula mucilaginosa (RM), Rhodotorula sphaerocarpa (RS), and Sakaguchia dacryoidea (SD), bars represent the mean production, and the error bars the standard error (SE). (a) Production rates in the high nutrient media at 5 °C (blue bars) and 15 °C (red bars; due to technical issues no measurements were performed in the R. mucilaginosa culture). (b) Production rates in the low nutrient media at 15 °C.

Figure 4

Respiration rates of the three fungal isolates Rhodotorula mucilaginosa (RM), Rhodotorula sphaerocarpa (RS), and Sakaguchia dacryoidea (SD) with the respective statistical test (ANOVA or Student’s t-test). Red letters indicate which rates differed significantly from each other, as determined by a TukeyHSD post-hoc test. (a) Respiration rates in the high nutrient media at 5 °C. (b) Respiration rates in the high nutrient media at 15 °C (due to technical issues no measurements were performed in the R. mucilaginosa culture). (c) Respiration rates in the low nutrient media at 15 °C.

Figure 5

Respiration rates of the fungal cultures in the high nutrient media at 15 °C (E1_15) and at 5 °C (E1_5) and in the low nutrient media (E2) with the respective statistical test (ANOVA). Red letters indicate which treatments differed significantly from each other, as determined by a TukeyHSD post-hoc test. (a) Respiration rates of Rhodotorula sphaerocarpa. (b) Respiration rates of Sakaguchia dacryoidea.

Figure 6

Cell-specific respiration rates of the three fungal isolates Rhodotorula mucilaginosa (RM), Rhodotorula sphaerocarpa (RS), and Sakaguchia dacryoidea (SD) with the respective statistical test. Red letters indicate which rates differed significantly from each other, as determined by a TukeyHSD post-hoc test. (a) Cell-specific respiration rates in the high nutrient media at 5 °C. (b) Cell-specific respiration in the high nutrient media at 15 °C (due to technical issues no measurements were performed in the R. mucilaginosa culture). (c) Cell-specific respiration rates in the low nutrient media at 15 °C.

Figure 7

Cell-specific respiration rates of the fungal cultures in the high nutrient media at 15 °C (E1_15) and at 5 °C (E1_5) and in the low nutrient media (E2) with the respective statistical test. Red letters indicate which treatments differed significantly from each other, as determined by a TukeyHSD post-hoc test. (a) Respiration rates of Rhodotorula sphaerocarpa. (b) Respiration rates of Sakaguchia dacryoidea.