Ocean Acidification means more fun for fungi

When one thinks of ocean acidification; it’s rare that we think of marine organisms thriving.  However, a paper by Krause et al.,(2016) suggests that a group of previously neglected marine fungi, prefer low pH levels, and therefore may play an important role in our increasingly acidic oceans. Due to the ability of fungi to degrade complex substrates (lignocellulose and calcareous structure), they are important decomposers in the ocean and are able to fulfil a niche which previously had been thought unfulfilled.

Water was collected from Helgoland Roads Station on the 24^th of April 2011, and the 3^rd of May 2012. Sea water samples were either incubated at the in situ pH (8.10 in 2011 and 8.26 in 2012) or adjusted to pH 7.82 and 7.67.  20 replicates were incubated for each pH treatment (microcosm), and incubated in the dark for a period of 4 weeks, at the temperature of the in-situ samples (7 °C for 2011 & 8 °C for 2012), with Jars mixed daily by inversion. Fungal abundance (cfu) was determined at 7 °C to 8 °C (mean of the upper 10% of temperatures recorded from the study site from 2000-2010). These temperatures were chosen to determine whether different fungal groups (cold-adapted versus warm-adapted) were present and reacted differently to pH. Samples were filtered through nitrocellulose filters, and placed onto Wickerham’s YM agar, prepared with sea water from the sampling site. All filamentous and yeast-like colonies were counted by eye. F-ARISA analysis was used (culture-independent), and mechanical lysis was performed. To determine whether the different incubation temperatures selected for different fungal populations, DNA from bulk cfu filters with mycel was extracted and analysed using F-ARISA. Amplification using specific primers and reverse primers was used, labelled with infrared dye, and analysed based on the Jaccard coefficient. To avoid bias from the lack of data from 2011, statistical analysis was conducted separately for 2011 & 2012.  The surface pH at Helgoland was determined by sampling 5 times a week from September 2011-2012, with samples taken between 06:00 and 10:00h. Samples were immediately taken to the lab and measured.

In the sea water samples, 88 +-cfu 1^-1 In Spring 2011, and 34+-5cfu 1^-1 in spring 2012 were observed. Higher abundances in 2011 may be explained by the higher overall phytoplankton abundance on the sampling days (8.5x10⁶ cells 1^-1 versus 2.4 x10⁶ cells 1^-1 in 2012).  Fungal spores from terrestrial origin, likely introduced by water runoffs or wind, were found on agar plates, but were not actively growing within the sea water. During the experiment, fungal number increased greatly, indicating active growth. A strong influence of pH was also observed. Up to 1.2 x 10³ cfu 1^-1 at pH in-situ, 1.6 x 10⁴ cfu 1^-1 at pH 7.82 and up to 9.0 x 10⁵ cfu 1^-1 at pH 7.67. Factorial ANOVA’s showed that pH significantly influenced cfu 1^-1in both 2011 and 2012. Temperature also had a significant effect on community structure on bulk cfu filters with mycelium, and the pH effect on abundance for both groups was comparable, because, for both temperatures, cfu 1^-1 were always significantly higher at 7.82 and 7.67 than in-situ. Cfu 1^-1 were 8 to 9 times higher at pH 7.82 and 34 times higher at pH 7.67, compared to in-situ. Results from this experiment indicate that even moderate acidification may lead to an increase in fungal abundance. Different fungal communities were present between the 2 years; with the community structures of both significantly influenced by pH – the most significant being between pH 7.82 and in-situ.

The authors emphasise that microbes already experience large natural fluctuations in pH, due to depth of phytoplankton blooms, thus, it is important to take into account the natural variability of the study site. An average pH_NBS of >8.1 was determined, but higher values were observed which correlated with the spring blooming events, and values <8.0 do not occur at present. Therefore, it is probable that fungal responses may differ in regions which experience lower pH values. None the less, the results indicate that pH observed are of a general nature, and although different fungal communities developed in the 2 year study, with different temperatures hinting at the presence of fungi occupying different fundamental niches, the direct pH effect on fungal numbers remained consistent (The authors have made plans to identify the fungi)

The paper concludes with a section named  ‘Ecological implications’, summarising how OA may lead to: increased importance of fungi in microbial food webs, increased nutrient availability, rising threats of marine fungal parasites and pathogens, and an increase in the abundance of fungi in our oceans. The paper highlights how marine fungi have, up until now, been neglected – and the authors suggest a strong role of fungi in the microbial loop, which is likely to increase with increasing acidity. However, the authors are aware of the limitations of their work, and suggest that future research into the role of fungi in marine environments is “urgently needed”, with their own plans already made to identify fungal species found within samples in a future paper.

Reviewed paper:  Krause, E., Wichels, A., Giménez, L. and Gerdts, G., 2013. Marine fungi may benefit from ocean acidification. Aquatic Microbial Ecology, 69(1), pp.59-67.